Add plugin and key-cache for ExternalJWTSigner integration

cmd/kube-apiserver/app/options/completion.go

@@ -17,6 +17,7 @@ limitations under the License.
 package options
 
 import (
+	"context"
 	"fmt"
 	"net"
 	"strings"
@@ -45,27 +46,27 @@ type CompletedOptions struct {
 
 // Complete set default ServerRunOptions.
 // Should be called after kube-apiserver flags parsed.
-func (opts *ServerRunOptions) Complete() (CompletedOptions, error) {
-	if opts == nil {
+func (s *ServerRunOptions) Complete(ctx context.Context) (CompletedOptions, error) {
+	if s == nil {
 		return CompletedOptions{completedOptions: &completedOptions{}}, nil
 	}
 
-	// process opts.ServiceClusterIPRange from list to Primary and Secondary
+	// process s.ServiceClusterIPRange from list to Primary and Secondary
 	// we process secondary only if provided by user
-	apiServerServiceIP, primaryServiceIPRange, secondaryServiceIPRange, err := getServiceIPAndRanges(opts.ServiceClusterIPRanges)
+	apiServerServiceIP, primaryServiceIPRange, secondaryServiceIPRange, err := getServiceIPAndRanges(s.ServiceClusterIPRanges)
 	if err != nil {
 		return CompletedOptions{}, err
 	}
-	controlplane, err := opts.Options.Complete([]string{"kubernetes.default.svc", "kubernetes.default", "kubernetes"}, []net.IP{apiServerServiceIP})
+	controlplane, err := s.Options.Complete(ctx, []string{"kubernetes.default.svc", "kubernetes.default", "kubernetes"}, []net.IP{apiServerServiceIP})
 	if err != nil {
 		return CompletedOptions{}, err
 	}
 
 	completed := completedOptions{
 		CompletedOptions: controlplane,
-		CloudProvider:    opts.CloudProvider,
+		CloudProvider:    s.CloudProvider,
 
-		Extra: opts.Extra,
+		Extra: s.Extra,
 	}
 
 	completed.PrimaryServiceClusterIPRange = primaryServiceIPRange

cmd/kube-apiserver/app/server.go

@@ -67,6 +67,7 @@ func NewAPIServerCommand() *cobra.Command {
 	_, featureGate := featuregate.DefaultComponentGlobalsRegistry.ComponentGlobalsOrRegister(
 		featuregate.DefaultKubeComponent, utilversion.DefaultBuildEffectiveVersion(), utilfeature.DefaultMutableFeatureGate)
 	s := options.NewServerRunOptions()
+	ctx := genericapiserver.SetupSignalContext()
 
 	cmd := &cobra.Command{
 		Use: "kube-apiserver",
@@ -97,7 +98,7 @@ cluster's shared state through which all other components interact.`,
 			cliflag.PrintFlags(fs)
 
 			// set default options
-			completedOptions, err := s.Complete()
+			completedOptions, err := s.Complete(ctx)
 			if err != nil {
 				return err
 			}
@@ -108,7 +109,7 @@ cluster's shared state through which all other components interact.`,
 			}
 			// add feature enablement metrics
 			featureGate.AddMetrics()
-			return Run(cmd.Context(), completedOptions)
+			return Run(ctx, completedOptions)
 		},
 		Args: func(cmd *cobra.Command, args []string) error {
 			for _, arg := range args {
@@ -119,7 +120,7 @@ cluster's shared state through which all other components interact.`,
 			return nil
 		},
 	}
-	cmd.SetContext(genericapiserver.SetupSignalContext())
+	cmd.SetContext(ctx)
 
 	fs := cmd.Flags()
 	namedFlagSets := s.Flags()

cmd/kube-apiserver/app/testing/testserver.go

@@ -390,7 +390,7 @@ func StartTestServer(t ktesting.TB, instanceOptions *TestServerInstanceOptions,
 	s.Authentication.ServiceAccounts.Issuers = []string{"https://foo.bar.example.com"}
 	s.Authentication.ServiceAccounts.KeyFiles = []string{saSigningKeyFile.Name()}
 
-	completedOptions, err := s.Complete()
+	completedOptions, err := s.Complete(tCtx)
 	if err != nil {
 		return result, fmt.Errorf("failed to set default ServerRunOptions: %v", err)
 	}

go.mod

@@ -103,6 +103,7 @@ require (
 	k8s.io/csi-translation-lib v0.0.0
 	k8s.io/dynamic-resource-allocation v0.0.0
 	k8s.io/endpointslice v0.0.0
+	k8s.io/externaljwt v0.0.0
 	k8s.io/klog/v2 v2.130.1
 	k8s.io/kms v0.0.0
 	k8s.io/kube-aggregator v0.0.0
@@ -239,6 +240,7 @@ replace (
 	k8s.io/csi-translation-lib => ./staging/src/k8s.io/csi-translation-lib
 	k8s.io/dynamic-resource-allocation => ./staging/src/k8s.io/dynamic-resource-allocation
 	k8s.io/endpointslice => ./staging/src/k8s.io/endpointslice
+	k8s.io/externaljwt => ./staging/src/k8s.io/externaljwt
 	k8s.io/kms => ./staging/src/k8s.io/kms
 	k8s.io/kube-aggregator => ./staging/src/k8s.io/kube-aggregator
 	k8s.io/kube-controller-manager => ./staging/src/k8s.io/kube-controller-manager

go.work

@@ -23,6 +23,7 @@ use (
 	./staging/src/k8s.io/csi-translation-lib
 	./staging/src/k8s.io/dynamic-resource-allocation
 	./staging/src/k8s.io/endpointslice
+	./staging/src/k8s.io/externaljwt
 	./staging/src/k8s.io/kms
 	./staging/src/k8s.io/kube-aggregator
 	./staging/src/k8s.io/kube-controller-manager

hack/unwanted-dependencies.json

@@ -111,6 +111,7 @@
         "k8s.io/client-go",
         "k8s.io/code-generator",
         "k8s.io/cri-api",
+        "k8s.io/externaljwt",
         "k8s.io/kms",
         "k8s.io/kube-aggregator",
         "k8s.io/kubelet",

hack/update-codegen.sh

@@ -785,6 +785,8 @@ function codegen::protobindings() {
 
         "staging/src/k8s.io/kubelet/pkg/apis/pluginregistration"
         "pkg/kubelet/pluginmanager/pluginwatcher/example_plugin_apis"
+
+        "staging/src/k8s.io/externaljwt/apis"
     )
 
     kube::log::status "Generating protobuf bindings for ${#apis[@]} targets"

pkg/apis/authentication/validation/validation.go

@@ -19,19 +19,18 @@ limitations under the License.
 package validation
 
 import (
-	"time"
-
 	"k8s.io/apimachinery/pkg/util/validation/field"
 	"k8s.io/kubernetes/pkg/apis/authentication"
 )
 
+const MinTokenAgeSec = 10 * 60 // 10 minutes
+
 // ValidateTokenRequest validates a TokenRequest.
 func ValidateTokenRequest(tr *authentication.TokenRequest) field.ErrorList {
 	allErrs := field.ErrorList{}
 	specPath := field.NewPath("spec")
 
-	const min = 10 * time.Minute
-	if tr.Spec.ExpirationSeconds < int64(min.Seconds()) {
+	if tr.Spec.ExpirationSeconds < MinTokenAgeSec {
 		allErrs = append(allErrs, field.Invalid(specPath.Child("expirationSeconds"), tr.Spec.ExpirationSeconds, "may not specify a duration less than 10 minutes"))
 	}
 	if tr.Spec.ExpirationSeconds > 1<<32 {

pkg/controller/serviceaccount/tokens_controller.go

@@ -409,7 +409,9 @@ func (e *TokensController) generateTokenIfNeeded(logger klog.Logger, serviceAcco
 
 	// Generate the token
 	if needsToken {
-		token, err := e.token.GenerateToken(serviceaccount.LegacyClaims(*serviceAccount, *liveSecret))
+		c, pc := serviceaccount.LegacyClaims(*serviceAccount, *liveSecret)
+		// TODO: need to plumb context if using external signer ever becomes a posibility.
+		token, err := e.token.GenerateToken(context.TODO(), c, pc)
 		if err != nil {
 			return false, err
 		}

pkg/controller/serviceaccount/tokens_controller_test.go

@@ -17,6 +17,7 @@ limitations under the License.
 package serviceaccount
 
 import (
+	"context"
 	"reflect"
 	"testing"
 	"time"
@@ -40,7 +41,7 @@ type testGenerator struct {
 	Err   error
 }
 
-func (t *testGenerator) GenerateToken(sc *jwt.Claims, pc interface{}) (string, error) {
+func (t *testGenerator) GenerateToken(ctx context.Context, sc *jwt.Claims, pc interface{}) (string, error) {
 	return t.Token, t.Err
 }
 

pkg/controlplane/apiserver/apis.go

@@ -52,6 +52,7 @@ func (c *CompletedConfig) NewCoreGenericConfig() *corerest.GenericConfig {
 		LoopbackClientConfig:        c.Generic.LoopbackClientConfig,
 		ServiceAccountIssuer:        c.Extra.ServiceAccountIssuer,
 		ExtendExpiration:            c.Extra.ExtendExpiration,
+		IsTokenSignerExternal:       c.Extra.IsTokenSignerExternal,
 		ServiceAccountMaxExpiration: c.Extra.ServiceAccountMaxExpiration,
 		APIAudiences:                c.Generic.Authentication.APIAudiences,
 		Informers:                   c.Extra.VersionedInformers,

pkg/controlplane/apiserver/config.go

@@ -92,6 +92,7 @@ type Extra struct {
 	ServiceAccountIssuer        serviceaccount.TokenGenerator
 	ServiceAccountMaxExpiration time.Duration
 	ExtendExpiration            bool
+	IsTokenSignerExternal       bool
 
 	// ServiceAccountIssuerDiscovery
 	ServiceAccountIssuerURL        string
@@ -300,6 +301,7 @@ func CreateConfig(
 			ServiceAccountIssuer:        opts.ServiceAccountIssuer,
 			ServiceAccountMaxExpiration: opts.ServiceAccountTokenMaxExpiration,
 			ExtendExpiration:            opts.Authentication.ServiceAccounts.ExtendExpiration,
+			IsTokenSignerExternal:       opts.Authentication.ServiceAccounts.IsTokenSignerExternal,
 
 			VersionedInformers: versionedInformers,
 		},

pkg/controlplane/apiserver/config_test.go

@@ -17,6 +17,7 @@ limitations under the License.
 package apiserver
 
 import (
+	"context"
 	"net"
 	"testing"
 
@@ -45,7 +46,7 @@ func TestBuildGenericConfig(t *testing.T) {
 	s.BindPort = ln.Addr().(*net.TCPAddr).Port
 	opts.SecureServing = s
 
-	completedOptions, err := opts.Complete(nil, nil)
+	completedOptions, err := opts.Complete(context.TODO(), nil, nil)
 	if err != nil {
 		t.Fatalf("Failed to complete apiserver options: %v", err)
 	}

pkg/controlplane/apiserver/options/options.go

@@ -18,6 +18,7 @@ limitations under the License.
 package options
 
 import (
+	"context"
 	"fmt"
 	"net"
 	"os"
@@ -36,9 +37,11 @@ import (
 	"k8s.io/klog/v2"
 	netutil "k8s.io/utils/net"
 
+	"k8s.io/kubernetes/pkg/apis/authentication/validation"
 	_ "k8s.io/kubernetes/pkg/features"
 	kubeoptions "k8s.io/kubernetes/pkg/kubeapiserver/options"
 	"k8s.io/kubernetes/pkg/serviceaccount"
+	"k8s.io/kubernetes/pkg/serviceaccount/externaljwt/plugin"
 )
 
 // Options define the flags and validation for a generic controlplane. If the
@@ -85,6 +88,8 @@ type Options struct {
 	ShowHiddenMetricsForVersion string
 
 	SystemNamespaces []string
+
+	ServiceAccountSigningEndpoint string
 }
 
 // completedServerRunOptions is a private wrapper that enforces a call of Complete() before Run can be invoked.
@@ -191,9 +196,12 @@ func (s *Options) AddFlags(fss *cliflag.NamedFlagSets) {
 
 	fs.StringVar(&s.ServiceAccountSigningKeyFile, "service-account-signing-key-file", s.ServiceAccountSigningKeyFile, ""+
 		"Path to the file that contains the current private key of the service account token issuer. The issuer will sign issued ID tokens with this private key.")
+
+	fs.StringVar(&s.ServiceAccountSigningEndpoint, "service-account-signing-endpoint", s.ServiceAccountSigningEndpoint, ""+
+		"Path to socket where a external JWT signer is listening. This flag is mutually exclusive with --service-account-signing-key-file and --service-account-key-file. Requires enabling feature gate (ExternalServiceAccountTokenSigner)")
 }
 
-func (o *Options) Complete(alternateDNS []string, alternateIPs []net.IP) (CompletedOptions, error) {
+func (o *Options) Complete(ctx context.Context, alternateDNS []string, alternateIPs []net.IP) (CompletedOptions, error) {
 	if o == nil {
 		return CompletedOptions{completedOptions: &completedOptions{}}, nil
 	}
@@ -233,36 +241,9 @@ func (o *Options) Complete(alternateDNS []string, alternateIPs []net.IP) (Comple
 	// adjust authentication for completed authorization
 	completed.Authentication.ApplyAuthorization(completed.Authorization)
 
-	// verify and adjust ServiceAccountTokenMaxExpiration
-	if completed.Authentication.ServiceAccounts.MaxExpiration != 0 {
-		lowBound := time.Hour
-		upBound := time.Duration(1<<32) * time.Second
-		if completed.Authentication.ServiceAccounts.MaxExpiration < lowBound ||
-			completed.Authentication.ServiceAccounts.MaxExpiration > upBound {
-			return CompletedOptions{}, fmt.Errorf("the service-account-max-token-expiration must be between 1 hour and 2^32 seconds")
-		}
-		if completed.Authentication.ServiceAccounts.ExtendExpiration {
-			if completed.Authentication.ServiceAccounts.MaxExpiration < serviceaccount.WarnOnlyBoundTokenExpirationSeconds*time.Second {
-				klog.Warningf("service-account-extend-token-expiration is true, in order to correctly trigger safe transition logic, service-account-max-token-expiration must be set longer than %d seconds (currently %s)", serviceaccount.WarnOnlyBoundTokenExpirationSeconds, completed.Authentication.ServiceAccounts.MaxExpiration)
-			}
-			if completed.Authentication.ServiceAccounts.MaxExpiration < serviceaccount.ExpirationExtensionSeconds*time.Second {
-				klog.Warningf("service-account-extend-token-expiration is true, enabling tokens valid up to %d seconds, which is longer than service-account-max-token-expiration set to %s seconds", serviceaccount.ExpirationExtensionSeconds, completed.Authentication.ServiceAccounts.MaxExpiration)
-			}
-		}
-	}
-	completed.ServiceAccountTokenMaxExpiration = completed.Authentication.ServiceAccounts.MaxExpiration
-
-	if len(completed.Authentication.ServiceAccounts.Issuers) != 0 && completed.Authentication.ServiceAccounts.Issuers[0] != "" {
-		if completed.ServiceAccountSigningKeyFile != "" {
-			sk, err := keyutil.PrivateKeyFromFile(completed.ServiceAccountSigningKeyFile)
+	err := o.completeServiceAccountOptions(ctx, &completed)
 	if err != nil {
-				return CompletedOptions{}, fmt.Errorf("failed to parse service-account-issuer-key-file: %w", err)
-			}
-			completed.ServiceAccountIssuer, err = serviceaccount.JWTTokenGenerator(completed.Authentication.ServiceAccounts.Issuers[0], sk)
-			if err != nil {
-				return CompletedOptions{}, fmt.Errorf("failed to build token generator: %w", err)
-			}
-		}
+		return CompletedOptions{}, err
 	}
 
 	for key, value := range completed.APIEnablement.RuntimeConfig {
@@ -281,6 +262,72 @@ func (o *Options) Complete(alternateDNS []string, alternateIPs []net.IP) (Comple
 	}, nil
 }
 
+func (o *Options) completeServiceAccountOptions(ctx context.Context, completed *completedOptions) error {
+	transitionWarningFmt := "service-account-extend-token-expiration is true, in order to correctly trigger safe transition logic, service-account-max-token-expiration must be set longer than %d seconds (currently %s)"
+	expExtensionWarningFmt := "service-account-extend-token-expiration is true, enabling tokens valid up to %d seconds, which is longer than service-account-max-token-expiration set to %s"
+	// verify service-account-max-token-expiration
+	if completed.Authentication.ServiceAccounts.MaxExpiration != 0 {
+		lowBound := time.Hour
+		upBound := time.Duration(1<<32) * time.Second
+		if completed.Authentication.ServiceAccounts.MaxExpiration < lowBound ||
+			completed.Authentication.ServiceAccounts.MaxExpiration > upBound {
+			return fmt.Errorf("the service-account-max-token-expiration must be between 1 hour and 2^32 seconds")
+		}
+	}
+
+	if len(completed.Authentication.ServiceAccounts.Issuers) != 0 && completed.Authentication.ServiceAccounts.Issuers[0] != "" {
+		switch {
+		case completed.ServiceAccountSigningEndpoint != "" && completed.ServiceAccountSigningKeyFile != "":
+			return fmt.Errorf("service-account-signing-key-file and service-account-signing-endpoint are mutually exclusive and cannot be set at the same time")
+		case completed.ServiceAccountSigningKeyFile != "":
+			sk, err := keyutil.PrivateKeyFromFile(completed.ServiceAccountSigningKeyFile)
+			if err != nil {
+				return fmt.Errorf("failed to parse service-account-issuer-key-file: %w", err)
+			}
+			completed.ServiceAccountIssuer, err = serviceaccount.JWTTokenGenerator(completed.Authentication.ServiceAccounts.Issuers[0], sk)
+			if err != nil {
+				return fmt.Errorf("failed to build token generator: %w", err)
+			}
+		case completed.ServiceAccountSigningEndpoint != "":
+			plugin, cache, err := plugin.New(ctx, completed.Authentication.ServiceAccounts.Issuers[0], completed.ServiceAccountSigningEndpoint, 60*time.Second, false)
+			if err != nil {
+				return fmt.Errorf("while setting up external-jwt-signer: %w", err)
+			}
+			timedContext, cancel := context.WithTimeout(ctx, 10*time.Second)
+			defer cancel()
+			metadata, err := plugin.GetServiceMetadata(timedContext)
+			if err != nil {
+				return fmt.Errorf("while setting up external-jwt-signer: %w", err)
+			}
+			if metadata.MaxTokenExpirationSeconds < validation.MinTokenAgeSec {
+				return fmt.Errorf("max token life supported by external-jwt-signer (%ds) is less than acceptable (min %ds)", metadata.MaxTokenExpirationSeconds, validation.MinTokenAgeSec)
+			}
+			if completed.Authentication.ServiceAccounts.MaxExpiration != 0 {
+				return fmt.Errorf("service-account-max-token-expiration and service-account-signing-endpoint are mutually exclusive and cannot be set at the same time")
+			}
+			transitionWarningFmt = "service-account-extend-token-expiration is true, in order to correctly trigger safe transition logic, token lifetime supported by external-jwt-signer must be longer than %d seconds (currently %s)"
+			expExtensionWarningFmt = "service-account-extend-token-expiration is true, tokens validity will be caped at the smaller of %d seconds and maximum token lifetime supported by external-jwt-signer (%s)"
+			completed.ServiceAccountIssuer = plugin
+			completed.Authentication.ServiceAccounts.ExternalPublicKeysGetter = cache
+			completed.Authentication.ServiceAccounts.MaxExpiration = time.Duration(metadata.MaxTokenExpirationSeconds) * time.Second
+			completed.Authentication.ServiceAccounts.IsTokenSignerExternal = true
+		}
+	}
+
+	// Set Max expiration and warn on conflicting configuration.
+	if completed.Authentication.ServiceAccounts.ExtendExpiration && completed.Authentication.ServiceAccounts.MaxExpiration != 0 {
+		if completed.Authentication.ServiceAccounts.MaxExpiration < serviceaccount.WarnOnlyBoundTokenExpirationSeconds*time.Second {
+			klog.Warningf(transitionWarningFmt, serviceaccount.WarnOnlyBoundTokenExpirationSeconds, completed.Authentication.ServiceAccounts.MaxExpiration)
+		}
+		if completed.Authentication.ServiceAccounts.MaxExpiration < serviceaccount.ExpirationExtensionSeconds*time.Second {
+			klog.Warningf(expExtensionWarningFmt, serviceaccount.ExpirationExtensionSeconds, completed.Authentication.ServiceAccounts.MaxExpiration)
+		}
+	}
+	completed.ServiceAccountTokenMaxExpiration = completed.Authentication.ServiceAccounts.MaxExpiration
+
+	return nil
+}
+
 // ServiceIPRange checks if the serviceClusterIPRange flag is nil, raising a warning if so and
 // setting service ip range to the default value in kubeoptions.DefaultServiceIPCIDR
 // for now until the default is removed per the deprecation timeline guidelines.

pkg/controlplane/apiserver/options/options_test.go

@@ -17,6 +17,13 @@ limitations under the License.
 package options
 
 import (
+	"context"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/pem"
+	"fmt"
+	"os"
 	"reflect"
 	"testing"
 	"time"
@@ -39,6 +46,7 @@ import (
 	"k8s.io/component-base/metrics"
 	utilversion "k8s.io/component-base/version"
 	kubeoptions "k8s.io/kubernetes/pkg/kubeapiserver/options"
+	v1alpha1testing "k8s.io/kubernetes/pkg/serviceaccount/externaljwt/plugin/testing/v1alpha1"
 	netutils "k8s.io/utils/net"
 )
 
@@ -308,3 +316,198 @@ func TestAddFlags(t *testing.T) {
 		t.Errorf("Got emulation version %s, wanted %s", testEffectiveVersion.EmulationVersion().String(), "1.31")
 	}
 }
+
+func TestCompleteForServiceAccount(t *testing.T) {
+
+	key, err := rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		panic("Error while generating first RSA key")
+	}
+
+	// Marshal the private key into PEM format
+	privateKeyPEM := &pem.Block{
+		Type:  "RSA PRIVATE KEY",
+		Bytes: x509.MarshalPKCS1PrivateKey(key),
+	}
+
+	// Open a file to write the private key
+	privateKeyFile, err := os.Create("private_key.pem")
+	if err != nil {
+		t.Fatalf("Failed to create private key file: %v", err)
+	}
+	t.Cleanup(func() {
+		_ = privateKeyFile.Close()
+		_ = os.Remove("private_key.pem")
+	})
+
+	// Write the PEM-encoded private key to the file
+	if err := pem.Encode(privateKeyFile, privateKeyPEM); err != nil {
+		t.Fatalf("Failed to encode private key: %v", err)
+	}
+
+	// create and start mock signer.
+	socketPath := "@mock-external-jwt-signer.sock"
+	mockSigner := v1alpha1testing.NewMockSigner(t, socketPath)
+	defer mockSigner.CleanUp()
+
+	testCases := []struct {
+		desc                     string
+		issuers                  []string
+		signingEndpoint          string
+		signingKeyFiles          string
+		maxExpiration            time.Duration
+		externalMaxExpirationSec int64
+		fetchError               error
+		metadataError            error
+
+		wantError                      error
+		expectedMaxtokenExp            time.Duration
+		expectedIsExternalSigner       bool
+		externalPublicKeyGetterPresent bool
+	}{
+		{
+			desc: "no endpoint or key file",
+			issuers: []string{
+				"iss",
+			},
+			signingEndpoint: socketPath,
+			signingKeyFiles: "private_key.pem",
+			maxExpiration:   time.Second * 3600,
+
+			wantError: fmt.Errorf("service-account-signing-key-file and service-account-signing-endpoint are mutually exclusive and cannot be set at the same time"),
+		},
+		{
+			desc: "max token expiration breaching accepteable values",
+			issuers: []string{
+				"iss",
+			},
+			signingEndpoint: socketPath,
+			signingKeyFiles: "private_key.pem",
+			maxExpiration:   time.Second * 10,
+
+			wantError: fmt.Errorf("the service-account-max-token-expiration must be between 1 hour and 2^32 seconds"),
+		},
+		{
+			desc: "path to a signing key provided",
+			issuers: []string{
+				"iss",
+			},
+			signingEndpoint: "",
+			signingKeyFiles: "private_key.pem",
+			maxExpiration:   time.Second * 3600,
+
+			expectedIsExternalSigner:       false,
+			externalPublicKeyGetterPresent: false,
+			expectedMaxtokenExp:            time.Second * 3600,
+		},
+		{
+			desc: "signing endpoint provided",
+			issuers: []string{
+				"iss",
+			},
+			signingEndpoint:          socketPath,
+			signingKeyFiles:          "",
+			maxExpiration:            0,
+			externalMaxExpirationSec: 600, // 10m
+
+			expectedIsExternalSigner:       true,
+			externalPublicKeyGetterPresent: true,
+			expectedMaxtokenExp:            time.Second * 600, // 10m
+		},
+		{
+			desc: "signing endpoint provided and max token expiration set",
+			issuers: []string{
+				"iss",
+			},
+			signingEndpoint:          socketPath,
+			signingKeyFiles:          "",
+			maxExpiration:            time.Second * 3600,
+			externalMaxExpirationSec: 600, // 10m
+
+			wantError: fmt.Errorf("service-account-max-token-expiration and service-account-signing-endpoint are mutually exclusive and cannot be set at the same time"),
+		},
+		{
+			desc: "signing endpoint provided but return smaller than accaptable max token exp",
+			issuers: []string{
+				"iss",
+			},
+			signingEndpoint:          socketPath,
+			signingKeyFiles:          "",
+			maxExpiration:            0,
+			externalMaxExpirationSec: 300, // 5m
+
+			wantError: fmt.Errorf("max token life supported by external-jwt-signer (300s) is less than acceptable (min 600s)"),
+		},
+		{
+			desc: "signing endpoint provided and error when getting metadata",
+			issuers: []string{
+				"iss",
+			},
+			signingEndpoint:          socketPath,
+			signingKeyFiles:          "",
+			maxExpiration:            0,
+			externalMaxExpirationSec: 900, // 15m
+			metadataError:            fmt.Errorf("metadata error"),
+
+			wantError: fmt.Errorf("while setting up external-jwt-signer: rpc error: code = Unknown desc = metadata error"),
+		},
+		{
+			desc: "signing endpoint provided and error when creating plugin (during initial fetch)",
+			issuers: []string{
+				"iss",
+			},
+			signingEndpoint:          socketPath,
+			signingKeyFiles:          "",
+			maxExpiration:            0,
+			externalMaxExpirationSec: 900, // 15m
+			fetchError:               fmt.Errorf("keys fetch error"),
+
+			wantError: fmt.Errorf("while setting up external-jwt-signer: while initially filling key cache: while performing initial cache fill: while fetching token verification keys: while getting externally supported jwt signing keys: rpc error: code = Unknown desc = keys fetch error"),
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.desc, func(t *testing.T) {
+			options := NewOptions()
+			options.ServiceAccountSigningEndpoint = tc.signingEndpoint
+			options.ServiceAccountSigningKeyFile = tc.signingKeyFiles
+			options.Authentication = &kubeoptions.BuiltInAuthenticationOptions{
+				ServiceAccounts: &kubeoptions.ServiceAccountAuthenticationOptions{
+					Issuers:       tc.issuers,
+					MaxExpiration: tc.maxExpiration,
+				},
+			}
+
+			_ = mockSigner.Reset()
+			mockSigner.MaxTokenExpirationSeconds = tc.externalMaxExpirationSec
+			mockSigner.MetadataError = tc.metadataError
+			mockSigner.FetchError = tc.fetchError
+
+			co := completedOptions{
+				Options: *options,
+			}
+
+			err := options.completeServiceAccountOptions(context.Background(), &co)
+
+			if tc.wantError != nil {
+				if err == nil || tc.wantError.Error() != err.Error() {
+					t.Errorf("Expected error: %v, got: %v", tc.wantError, err)
+				}
+				return
+			}
+
+			if err != nil {
+				t.Errorf("Didn't expect any error but got: %v", err)
+			}
+			if tc.externalPublicKeyGetterPresent != (co.Authentication.ServiceAccounts.ExternalPublicKeysGetter != nil) {
+				t.Errorf("Unexpected value of ExternalPublicKeysGetter: %v", co.Authentication.ServiceAccounts.ExternalPublicKeysGetter)
+			}
+			if tc.expectedIsExternalSigner != co.Authentication.ServiceAccounts.IsTokenSignerExternal {
+				t.Errorf("Expected IsTokenSignerExternal %v, found %v", tc.expectedIsExternalSigner, co.Authentication.ServiceAccounts.IsTokenSignerExternal)
+			}
+			if tc.expectedMaxtokenExp.Seconds() != co.Authentication.ServiceAccounts.MaxExpiration.Seconds() {
+				t.Errorf("Expected MaxExpiration to be %v, found %v", tc.expectedMaxtokenExp, co.Authentication.ServiceAccounts.MaxExpiration)
+			}
+		})
+	}
+}

pkg/controlplane/apiserver/options/validation.go

@@ -19,6 +19,7 @@ package options
 import (
 	"errors"
 	"fmt"
+	"regexp"
 	"strings"
 
 	apiextensionsapiserver "k8s.io/apiextensions-apiserver/pkg/apiserver"
@@ -103,6 +104,29 @@ func validateUnknownVersionInteroperabilityProxyFlags(options *Options) []error
 	return err
 }
 
+var pathOrSocket = regexp.MustCompile(`(^(/[^/ ]*)+/?$)|(^@([a-zA-Z0-9_-]+\.)*[a-zA-Z0-9_-]+$)`)
+
+func validateServiceAccountTokenSigningConfig(options *Options) []error {
+	if len(options.ServiceAccountSigningEndpoint) == 0 {
+		return nil
+	}
+
+	errors := []error{}
+
+	if len(options.ServiceAccountSigningKeyFile) != 0 || len(options.Authentication.ServiceAccounts.KeyFiles) != 0 {
+		errors = append(errors, fmt.Errorf("can't set `--service-account-signing-key-file` and/or `--service-account-key-file` with `--service-account-signing-endpoint` (They are mutually exclusive)"))
+	}
+	if !utilfeature.DefaultFeatureGate.Enabled(features.ExternalServiceAccountTokenSigner) {
+		errors = append(errors, fmt.Errorf("setting `--service-account-signing-endpoint` requires enabling ExternalServiceAccountTokenSigner feature gate"))
+	}
+	// Check if ServiceAccountSigningEndpoint is a linux file path or an abstract socket name.
+	if !pathOrSocket.MatchString(options.ServiceAccountSigningEndpoint) {
+		errors = append(errors, fmt.Errorf("invalid value %q passed for `--service-account-signing-endpoint`, should be a valid location on the filesystem or must be prefixed with @ to name UDS in abstract namespace", options.ServiceAccountSigningEndpoint))
+	}
+
+	return errors
+}
+
 // Validate checks Options and return a slice of found errs.
 func (s *Options) Validate() []error {
 	var errs []error
@@ -121,6 +145,7 @@ func (s *Options) Validate() []error {
 	errs = append(errs, validateUnknownVersionInteroperabilityProxyFeature()...)
 	errs = append(errs, validateUnknownVersionInteroperabilityProxyFlags(s)...)
 	errs = append(errs, validateNodeSelectorAuthorizationFeature()...)
+	errs = append(errs, validateServiceAccountTokenSigningConfig(s)...)
 
 	return errs
 }

pkg/controlplane/apiserver/options/validation_test.go

@@ -17,6 +17,8 @@ limitations under the License.
 package options
 
 import (
+	"fmt"
+	"reflect"
 	"strings"
 	"testing"
 
@@ -262,3 +264,186 @@ func TestValidateOptions(t *testing.T) {
 		})
 	}
 }
+
+func TestValidateServcieAccountTokenSigningConfig(t *testing.T) {
+	tests := []struct {
+		name           string
+		featureEnabled bool
+		options        *Options
+		expectedErrors []error
+	}{
+		{
+			name:           "Signing keys file provided while external signer endpoint is provided",
+			featureEnabled: true,
+			expectedErrors: []error{
+				fmt.Errorf("can't set `--service-account-signing-key-file` and/or `--service-account-key-file` with `--service-account-signing-endpoint` (They are mutually exclusive)"),
+			},
+			options: &Options{
+				ServiceAccountSigningEndpoint: "@ebc.eng.hij",
+				ServiceAccountSigningKeyFile:  "/abc/efg",
+			},
+		},
+		{
+			name:           "Verification keys file provided while external signer endpoint is provided",
+			featureEnabled: true,
+			expectedErrors: []error{
+				fmt.Errorf("can't set `--service-account-signing-key-file` and/or `--service-account-key-file` with `--service-account-signing-endpoint` (They are mutually exclusive)"),
+			},
+			options: &Options{
+				ServiceAccountSigningEndpoint: "@ebc.eng.hij",
+				Authentication: &kubeoptions.BuiltInAuthenticationOptions{
+					ServiceAccounts: &kubeoptions.ServiceAccountAuthenticationOptions{
+						KeyFiles: []string{
+							"abc",
+							"efg",
+						},
+					},
+				},
+			},
+		},
+		{
+			name:           "Verification key  and signing key file provided while external signer endpoint is provided",
+			featureEnabled: true,
+			expectedErrors: []error{
+				fmt.Errorf("can't set `--service-account-signing-key-file` and/or `--service-account-key-file` with `--service-account-signing-endpoint` (They are mutually exclusive)"),
+			},
+			options: &Options{
+				ServiceAccountSigningEndpoint: "@ebc.eng.hij",
+				ServiceAccountSigningKeyFile:  "/abc/efg",
+				Authentication: &kubeoptions.BuiltInAuthenticationOptions{
+					ServiceAccounts: &kubeoptions.ServiceAccountAuthenticationOptions{
+						KeyFiles: []string{
+							"/abc/efg",
+							"/abc/xyz",
+						},
+					},
+				},
+			},
+		},
+		{
+			name:           "feature disabled and external signer endpoint is provided",
+			featureEnabled: false,
+			expectedErrors: []error{
+				fmt.Errorf("setting `--service-account-signing-endpoint` requires enabling ExternalServiceAccountTokenSigner feature gate"),
+			},
+			options: &Options{
+				ServiceAccountSigningEndpoint: "@ebc.eng.hij",
+			},
+		},
+		{
+			name:           "invalid external signer endpoint provided - 1",
+			featureEnabled: true,
+			expectedErrors: []error{
+				fmt.Errorf("invalid value \"abc\" passed for `--service-account-signing-endpoint`, should be a valid location on the filesystem or must be prefixed with @ to name UDS in abstract namespace"),
+			},
+			options: &Options{
+				ServiceAccountSigningEndpoint: "abc",
+			},
+		},
+		{
+			name:           "invalid external signer endpoint provided - 2",
+			featureEnabled: true,
+			expectedErrors: []error{
+				fmt.Errorf("invalid value \"@abc@\" passed for `--service-account-signing-endpoint`, should be a valid location on the filesystem or must be prefixed with @ to name UDS in abstract namespace"),
+			},
+			options: &Options{
+				ServiceAccountSigningEndpoint: "@abc@",
+			},
+		},
+		{
+			name:           "invalid external signer endpoint provided - 3",
+			featureEnabled: true,
+			expectedErrors: []error{
+				fmt.Errorf("invalid value \"@abc.abc  .ae\" passed for `--service-account-signing-endpoint`, should be a valid location on the filesystem or must be prefixed with @ to name UDS in abstract namespace"),
+			},
+			options: &Options{
+				ServiceAccountSigningEndpoint: "@abc.abc  .ae",
+			},
+		},
+		{
+			name:           "invalid external signer endpoint provided - 4",
+			featureEnabled: true,
+			expectedErrors: []error{
+				fmt.Errorf("invalid value \"/@e_adnb/xyz /efg\" passed for `--service-account-signing-endpoint`, should be a valid location on the filesystem or must be prefixed with @ to name UDS in abstract namespace"),
+			},
+			options: &Options{
+				ServiceAccountSigningEndpoint: "/@e_adnb/xyz /efg",
+			},
+		},
+		{
+			name:           "invalid external signer endpoint provided - 5",
+			featureEnabled: true,
+			expectedErrors: []error{
+				fmt.Errorf("invalid value \"/e /xyz /efg\" passed for `--service-account-signing-endpoint`, should be a valid location on the filesystem or must be prefixed with @ to name UDS in abstract namespace"),
+			},
+			options: &Options{
+				ServiceAccountSigningEndpoint: "/e /xyz /efg",
+			},
+		},
+		{
+			name:           "valid external signer endpoint provided - 1",
+			featureEnabled: true,
+			expectedErrors: []error{},
+			options: &Options{
+				ServiceAccountSigningEndpoint: "/e/an_b-d/efg",
+			},
+		},
+		{
+			name:           "valid external signer endpoint provided - 2",
+			featureEnabled: true,
+			expectedErrors: []error{},
+			options: &Options{
+				ServiceAccountSigningEndpoint: "@ebc.sock",
+			},
+		},
+		{
+			name:           "valid external signer endpoint provided - 3",
+			featureEnabled: true,
+			expectedErrors: []error{},
+			options: &Options{
+				ServiceAccountSigningEndpoint: "@ebc.eng.hij",
+			},
+		},
+		{
+			name:           "All errors at once",
+			featureEnabled: false,
+			expectedErrors: []error{
+				fmt.Errorf("can't set `--service-account-signing-key-file` and/or `--service-account-key-file` with `--service-account-signing-endpoint` (They are mutually exclusive)"),
+				fmt.Errorf("setting `--service-account-signing-endpoint` requires enabling ExternalServiceAccountTokenSigner feature gate"),
+				fmt.Errorf("invalid value \"/e /xyz /efg\" passed for `--service-account-signing-endpoint`, should be a valid location on the filesystem or must be prefixed with @ to name UDS in abstract namespace"),
+			},
+			options: &Options{
+				ServiceAccountSigningEndpoint: "/e /xyz /efg",
+				ServiceAccountSigningKeyFile:  "/abc/efg",
+				Authentication: &kubeoptions.BuiltInAuthenticationOptions{
+					ServiceAccounts: &kubeoptions.ServiceAccountAuthenticationOptions{
+						KeyFiles: []string{
+							"/abc/xyz",
+						},
+					},
+				},
+			},
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+
+			if test.options.Authentication == nil {
+				test.options.Authentication = &kubeoptions.BuiltInAuthenticationOptions{
+					ServiceAccounts: &kubeoptions.ServiceAccountAuthenticationOptions{
+						KeyFiles: []string{},
+					},
+				}
+			}
+
+			if test.featureEnabled {
+				featuregatetesting.SetFeatureGateDuringTest(t, utilfeature.DefaultFeatureGate, features.ExternalServiceAccountTokenSigner, true)
+			}
+			errs := validateServiceAccountTokenSigningConfig(test.options)
+			if !reflect.DeepEqual(errs, test.expectedErrors) {
+				t.Errorf("Expected errors message: %v \n but got: %v", test.expectedErrors, errs)
+			}
+		})
+	}
+}

pkg/controlplane/apiserver/samples/generic/server/server.go

@@ -83,7 +83,9 @@ APIs.`,
 			}
 			cliflag.PrintFlags(fs)
 
-			completedOptions, err := s.Complete([]string{}, []net.IP{})
+			ctx := genericapiserver.SetupSignalContext()
+
+			completedOptions, err := s.Complete(ctx, []string{}, []net.IP{})
 			if err != nil {
 				return err
 			}
@@ -94,7 +96,7 @@ APIs.`,
 
 			// add feature enablement metrics
 			utilfeature.DefaultMutableFeatureGate.AddMetrics()
-			ctx := genericapiserver.SetupSignalContext()
+
 			return Run(ctx, completedOptions)
 		},
 		Args: func(cmd *cobra.Command, args []string) error {

pkg/controlplane/apiserver/samples/generic/server/testing/testserver.go

@@ -164,7 +164,7 @@ func StartTestServer(t ktesting.TB, instanceOptions *TestServerInstanceOptions,
 	o.Authentication.ServiceAccounts.Issuers = []string{"https://foo.bar.example.com"}
 	o.Authentication.ServiceAccounts.KeyFiles = []string{saSigningKeyFile.Name()}
 
-	completedOptions, err := o.Complete(nil, nil)
+	completedOptions, err := o.Complete(tCtx, nil, nil)
 	if err != nil {
 		return result, fmt.Errorf("failed to set default ServerRunOptions: %w", err)
 	}

pkg/features/kube_features.go

@@ -819,6 +819,13 @@ const (
 	// instead of changing each file on the volumes recursively.
 	// Enables the SELinuxChangePolicy field in PodSecurityContext before SELinuxMount featgure gate is enabled.
 	SELinuxChangePolicy featuregate.Feature = "SELinuxChangePolicy"
+
+	// owner: @HarshalNeelkamal
+	// alpha: v1.32
+	//
+	// Enables external service account JWT signing and key management.
+	// If enabled, it allows passing --service-account-signing-endpoint flag to configure external signer.
+	ExternalServiceAccountTokenSigner featuregate.Feature = "ExternalServiceAccountTokenSigner"
 )
 
 func init() {

pkg/features/versioned_kube_features.go

@@ -195,6 +195,10 @@ var defaultVersionedKubernetesFeatureGates = map[featuregate.Feature]featuregate
 		{Version: version.MustParse("1.20"), Default: true, PreRelease: featuregate.GA}, // lock to default and remove after v1.22 based on KEP #1972 update
 	},
 
+	ExternalServiceAccountTokenSigner: {
+		{Version: version.MustParse("1.32"), Default: false, PreRelease: featuregate.Alpha},
+	},
+
 	genericfeatures.AdmissionWebhookMatchConditions: {
 		{Version: version.MustParse("1.27"), Default: false, PreRelease: featuregate.Alpha},
 		{Version: version.MustParse("1.28"), Default: true, PreRelease: featuregate.Beta},

pkg/kubeapiserver/options/authentication.go

@@ -133,9 +133,13 @@ type ServiceAccountAuthenticationOptions struct {
 	JWKSURI               string
 	MaxExpiration         time.Duration
 	ExtendExpiration      bool
+	IsTokenSignerExternal bool
 	// OptionalTokenGetter is a function that returns a service account token getter.
 	// If not set, the default token getter will be used.
 	OptionalTokenGetter func(factory informers.SharedInformerFactory) serviceaccount.ServiceAccountTokenGetter
+	// ExternalPublicKeysGetter gets set if `--service-account-signing-endpoint` is passed.
+	// ExternalPublicKeysGetter is mutually exclusive with KeyFiles.
+	ExternalPublicKeysGetter serviceaccount.PublicKeysGetter
 }
 
 // TokenFileAuthenticationOptions contains token file authentication options for API Server
@@ -270,8 +274,8 @@ func (o *BuiltInAuthenticationOptions) Validate() []error {
 		if len(o.ServiceAccounts.Issuers) == 0 {
 			allErrors = append(allErrors, errors.New("service-account-issuer is a required flag"))
 		}
-		if len(o.ServiceAccounts.KeyFiles) == 0 {
-			allErrors = append(allErrors, errors.New("service-account-key-file is a required flag"))
+		if len(o.ServiceAccounts.KeyFiles) == 0 && o.ServiceAccounts.ExternalPublicKeysGetter == nil {
+			allErrors = append(allErrors, errors.New("either `--service-account-key-file` or `--service-account-signing-endpoint` must be set"))
 		}
 
 		// Validate the JWKS URI when it is explicitly set.
@@ -592,7 +596,11 @@ func (o *BuiltInAuthenticationOptions) ToAuthenticationConfig() (kubeauthenticat
 		if len(o.ServiceAccounts.Issuers) != 0 && len(o.APIAudiences) == 0 {
 			ret.APIAudiences = authenticator.Audiences(o.ServiceAccounts.Issuers)
 		}
-		if len(o.ServiceAccounts.KeyFiles) > 0 {
+
+		switch {
+		case len(o.ServiceAccounts.KeyFiles) > 0 && o.ServiceAccounts.ExternalPublicKeysGetter != nil:
+			return kubeauthenticator.Config{}, fmt.Errorf("cannot set mutually exclusive flags `--service-account-key-file` and `--service-account-signing-endpoint` at the same time")
+		case len(o.ServiceAccounts.KeyFiles) > 0:
 			allPublicKeys := []interface{}{}
 			for _, keyfile := range o.ServiceAccounts.KeyFiles {
 				publicKeys, err := keyutil.PublicKeysFromFile(keyfile)
@@ -606,7 +614,10 @@ func (o *BuiltInAuthenticationOptions) ToAuthenticationConfig() (kubeauthenticat
 				return kubeauthenticator.Config{}, fmt.Errorf("failed to set up public service account keys: %w", err)
 			}
 			ret.ServiceAccountPublicKeysGetter = keysGetter
+		case o.ServiceAccounts.ExternalPublicKeysGetter != nil:
+			ret.ServiceAccountPublicKeysGetter = o.ServiceAccounts.ExternalPublicKeysGetter
 		}
+
 		ret.ServiceAccountIssuers = o.ServiceAccounts.Issuers
 		ret.ServiceAccountLookup = o.ServiceAccounts.Lookup
 	}

pkg/kubeapiserver/options/authentication_test.go

@@ -18,6 +18,11 @@ package options
 
 import (
 	"context"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/pem"
+	"fmt"
 	"os"
 	"reflect"
 	"strings"
@@ -45,6 +50,7 @@ import (
 	openapicommon "k8s.io/kube-openapi/pkg/common"
 	kubeauthenticator "k8s.io/kubernetes/pkg/kubeapiserver/authenticator"
 	"k8s.io/kubernetes/pkg/serviceaccount"
+
 	"k8s.io/utils/pointer"
 )
 
@@ -102,7 +108,7 @@ func TestAuthenticationValidate(t *testing.T) {
 			testSA: &ServiceAccountAuthenticationOptions{
 				Issuers: []string{"http://foo.bar.com"},
 			},
-			expectErr: "service-account-key-file is a required flag",
+			expectErr: "either `--service-account-key-file` or `--service-account-signing-endpoint` must be set",
 		},
 		{
 			name: "test when ServiceAccounts doesn't have issuer",
@@ -1511,3 +1517,171 @@ func errString(err error) string {
 	}
 	return err.Error()
 }
+
+func TestToAuthenticationConfigForServiceAccount(t *testing.T) {
+
+	dummyExternalGetter := &dummyPublicKeyGetter{}
+	keyFileName := "public_key.pem"
+
+	key1, err := rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		panic("Error while generating first RSA key")
+	}
+	pubKey1Bytes, err := x509.MarshalPKIXPublicKey(&key1.PublicKey)
+	if err != nil {
+		panic("Error while marshaling first public key")
+	}
+
+	publicKeyBlock := &pem.Block{
+		Type:  "PUBLIC KEY",
+		Bytes: pubKey1Bytes,
+	}
+
+	publicKeyFile, err := os.Create(keyFileName)
+	if err != nil {
+		fmt.Println("Error creating public key file:", err)
+		return
+	}
+	t.Cleanup(func() {
+		// An open file cannot be removed on Windows. Close it first.
+		if err := publicKeyFile.Close(); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.Remove(publicKeyFile.Name()); err != nil {
+			t.Fatal(err)
+		}
+	})
+
+	if err := pem.Encode(publicKeyFile, publicKeyBlock); err != nil {
+		fmt.Println("Error encoding public key:", err)
+		return
+	}
+
+	testCases := []struct {
+		desc                   string
+		options                *BuiltInAuthenticationOptions
+		expectConfig           kubeauthenticator.Config
+		expectedErr            error
+		expectedExternalGetter bool
+		expectedStaticGetter   bool
+	}{
+		{
+			desc: "neither key file nor external getter configured",
+			options: &BuiltInAuthenticationOptions{
+				ServiceAccounts: &ServiceAccountAuthenticationOptions{
+					Lookup:                   true,
+					Issuers:                  []string{"http://foo.bar.com"},
+					KeyFiles:                 []string{},
+					ExternalPublicKeysGetter: nil,
+				},
+			},
+			expectConfig: kubeauthenticator.Config{
+				APIAudiences:          authenticator.Audiences{"http://foo.bar.com"},
+				ServiceAccountLookup:  true,
+				ServiceAccountIssuers: []string{"http://foo.bar.com"},
+			},
+			expectedErr: nil,
+		},
+		{
+			desc: "both key file and external getter configured",
+			options: &BuiltInAuthenticationOptions{
+				ServiceAccounts: &ServiceAccountAuthenticationOptions{
+					Lookup:                   true,
+					Issuers:                  []string{"http://foo.bar.com"},
+					KeyFiles:                 []string{keyFileName},
+					ExternalPublicKeysGetter: dummyExternalGetter,
+				},
+			},
+			expectConfig: kubeauthenticator.Config{
+				APIAudiences:          authenticator.Audiences{"http://foo.bar.com"},
+				ServiceAccountLookup:  true,
+				ServiceAccountIssuers: []string{"http://foo.bar.com"},
+			},
+			expectedErr: fmt.Errorf("cannot set mutually exclusive flags `--service-account-key-file` and `--service-account-signing-endpoint` at the same time"),
+		},
+		{
+			desc: "external getter configured",
+			options: &BuiltInAuthenticationOptions{
+				ServiceAccounts: &ServiceAccountAuthenticationOptions{
+					Lookup:                   true,
+					Issuers:                  []string{"http://foo.bar.com"},
+					KeyFiles:                 []string{},
+					ExternalPublicKeysGetter: dummyExternalGetter,
+				},
+			},
+			expectConfig: kubeauthenticator.Config{
+				APIAudiences:          authenticator.Audiences{"http://foo.bar.com"},
+				ServiceAccountLookup:  true,
+				ServiceAccountIssuers: []string{"http://foo.bar.com"},
+			},
+			expectedErr:            nil,
+			expectedExternalGetter: true,
+		},
+		{
+			desc: "external getter configured",
+			options: &BuiltInAuthenticationOptions{
+				ServiceAccounts: &ServiceAccountAuthenticationOptions{
+					Lookup:                   true,
+					Issuers:                  []string{"http://foo.bar.com"},
+					KeyFiles:                 []string{keyFileName},
+					ExternalPublicKeysGetter: nil,
+				},
+			},
+			expectConfig: kubeauthenticator.Config{
+				APIAudiences:          authenticator.Audiences{"http://foo.bar.com"},
+				ServiceAccountLookup:  true,
+				ServiceAccountIssuers: []string{"http://foo.bar.com"},
+			},
+			expectedErr:          nil,
+			expectedStaticGetter: true,
+		},
+	}
+
+	for _, tc := range testCases {
+		resultConfig, err := tc.options.ToAuthenticationConfig()
+		if tc.expectedErr != nil {
+			if err == nil || tc.expectedErr.Error() != err.Error() {
+				t.Fatalf("Expected error: %v and got: %v", tc.expectedErr, err)
+			}
+			return
+		}
+
+		// make out of scope fields nil
+		resultConfig.AuthenticationConfig = nil
+
+		if tc.expectedExternalGetter {
+			if resultConfig.ServiceAccountPublicKeysGetter == nil {
+				t.Fatalf("Expected external getter but none")
+			} else if resultConfig.ServiceAccountPublicKeysGetter != dummyExternalGetter {
+				t.Fatalf("Expected external getter but found someting else")
+			}
+			resultConfig.ServiceAccountPublicKeysGetter = nil
+		}
+
+		if tc.expectedStaticGetter {
+			if resultConfig.ServiceAccountPublicKeysGetter == nil {
+				t.Fatalf("Expected static getter but none")
+			} else if resultConfig.ServiceAccountPublicKeysGetter == dummyExternalGetter {
+				t.Fatalf("Expected static getter but found external getter")
+			}
+			resultConfig.ServiceAccountPublicKeysGetter = nil
+		}
+
+		if !reflect.DeepEqual(resultConfig, tc.expectConfig) {
+			t.Error(cmp.Diff(resultConfig, tc.expectConfig))
+		}
+	}
+}
+
+type dummyPublicKeyGetter struct {
+}
+
+func (d *dummyPublicKeyGetter) AddListener(listener serviceaccount.Listener) {}
+
+func (d *dummyPublicKeyGetter) GetCacheAgeMaxSeconds() int {
+	return 10
+}
+
+func (d *dummyPublicKeyGetter) GetPublicKeys(ctx context.Context, keyIDHint string) []serviceaccount.PublicKey {
+	return []serviceaccount.PublicKey{}
+}

pkg/registry/core/rest/storage_core.go

@@ -223,7 +223,7 @@ func (p *legacyProvider) NewRESTStorage(apiResourceConfigSource serverstorage.AP
 			utilfeature.DefaultFeatureGate.Enabled(features.ServiceAccountTokenPodNodeInfo) {
 			nodeGetter = nodeStorage.Node.Store
 		}
-		serviceAccountStorage, err = serviceaccountstore.NewREST(restOptionsGetter, p.ServiceAccountIssuer, p.APIAudiences, p.ServiceAccountMaxExpiration, podStorage.Pod.Store, storage["secrets"].(rest.Getter), nodeGetter, p.ExtendExpiration)
+		serviceAccountStorage, err = serviceaccountstore.NewREST(restOptionsGetter, p.ServiceAccountIssuer, p.APIAudiences, p.ServiceAccountMaxExpiration, podStorage.Pod.Store, storage["secrets"].(rest.Getter), nodeGetter, p.ExtendExpiration, p.IsTokenSignerExternal)
 		if err != nil {
 			return genericapiserver.APIGroupInfo{}, err
 		}

pkg/registry/core/rest/storage_core_generic.go

@@ -57,6 +57,7 @@ type GenericConfig struct {
 	ServiceAccountIssuer        serviceaccount.TokenGenerator
 	ServiceAccountMaxExpiration time.Duration
 	ExtendExpiration            bool
+	IsTokenSignerExternal       bool
 
 	APIAudiences authenticator.Audiences
 
@@ -102,9 +103,9 @@ func (c *GenericConfig) NewRESTStorage(apiResourceConfigSource serverstorage.API
 
 	var serviceAccountStorage *serviceaccountstore.REST
 	if c.ServiceAccountIssuer != nil {
-		serviceAccountStorage, err = serviceaccountstore.NewREST(restOptionsGetter, c.ServiceAccountIssuer, c.APIAudiences, c.ServiceAccountMaxExpiration, newNotFoundGetter(schema.GroupResource{Resource: "pods"}), secretStorage.Store, newNotFoundGetter(schema.GroupResource{Resource: "nodes"}), c.ExtendExpiration)
+		serviceAccountStorage, err = serviceaccountstore.NewREST(restOptionsGetter, c.ServiceAccountIssuer, c.APIAudiences, c.ServiceAccountMaxExpiration, newNotFoundGetter(schema.GroupResource{Resource: "pods"}), secretStorage.Store, newNotFoundGetter(schema.GroupResource{Resource: "nodes"}), c.ExtendExpiration, c.IsTokenSignerExternal)
 	} else {
-		serviceAccountStorage, err = serviceaccountstore.NewREST(restOptionsGetter, nil, nil, 0, newNotFoundGetter(schema.GroupResource{Resource: "pods"}), newNotFoundGetter(schema.GroupResource{Resource: "secrets"}), newNotFoundGetter(schema.GroupResource{Resource: "nodes"}), false)
+		serviceAccountStorage, err = serviceaccountstore.NewREST(restOptionsGetter, nil, nil, 0, newNotFoundGetter(schema.GroupResource{Resource: "pods"}), newNotFoundGetter(schema.GroupResource{Resource: "secrets"}), newNotFoundGetter(schema.GroupResource{Resource: "nodes"}), false, c.IsTokenSignerExternal)
 	}
 	if err != nil {
 		return genericapiserver.APIGroupInfo{}, err

pkg/registry/core/serviceaccount/storage/storage.go

@@ -39,7 +39,7 @@ type REST struct {
 }
 
 // NewREST returns a RESTStorage object that will work against service accounts.
-func NewREST(optsGetter generic.RESTOptionsGetter, issuer token.TokenGenerator, auds authenticator.Audiences, max time.Duration, podStorage, secretStorage, nodeStorage rest.Getter, extendExpiration bool) (*REST, error) {
+func NewREST(optsGetter generic.RESTOptionsGetter, issuer token.TokenGenerator, auds authenticator.Audiences, max time.Duration, podStorage, secretStorage, nodeStorage rest.Getter, extendExpiration bool, isTokenSignerExternal bool) (*REST, error) {
 	store := &genericregistry.Store{
 		NewFunc:                   func() runtime.Object { return &api.ServiceAccount{} },
 		NewListFunc:               func() runtime.Object { return &api.ServiceAccountList{} },
@@ -70,6 +70,7 @@ func NewREST(optsGetter generic.RESTOptionsGetter, issuer token.TokenGenerator,
 			audsSet:               sets.NewString(auds...),
 			maxExpirationSeconds:  int64(max.Seconds()),
 			extendExpiration:      extendExpiration,
+			isTokenSignerExternal: isTokenSignerExternal,
 		}
 	}
 

pkg/registry/core/serviceaccount/storage/storage_test.go

@@ -27,6 +27,7 @@ import (
 	"k8s.io/apimachinery/pkg/labels"
 	"k8s.io/apimachinery/pkg/runtime"
 	"k8s.io/apiserver/pkg/audit"
+	"k8s.io/apiserver/pkg/authentication/authenticator"
 	"k8s.io/apiserver/pkg/endpoints/request"
 	"k8s.io/apiserver/pkg/registry/generic"
 	genericregistrytest "k8s.io/apiserver/pkg/registry/generic/testing"
@@ -42,6 +43,10 @@ import (
 )
 
 func newStorage(t *testing.T) (*REST, *etcd3testing.EtcdTestServer) {
+	return newTokenStorage(t, fakeTokenGenerator{"fake"}, nil, panicGetter{}, panicGetter{}, nil)
+}
+
+func newTokenStorage(t *testing.T, issuer token.TokenGenerator, auds authenticator.Audiences, podStorage, secretStorage, nodeStorage rest.Getter) (*REST, *etcd3testing.EtcdTestServer) {
 	etcdStorage, server := registrytest.NewEtcdStorage(t, "")
 	restOptions := generic.RESTOptions{
 		StorageConfig:           etcdStorage,
@@ -50,7 +55,7 @@ func newStorage(t *testing.T) (*REST, *etcd3testing.EtcdTestServer) {
 		ResourcePrefix:          "serviceaccounts",
 	}
 	// set issuer, podStore and secretStore to allow the token endpoint to be initialised
-	rest, err := NewREST(restOptions, fakeTokenGenerator{"fake"}, nil, 0, panicGetter{}, panicGetter{}, nil, false)
+	rest, err := NewREST(restOptions, issuer, auds, 0, podStorage, secretStorage, nodeStorage, false, false)
 	if err != nil {
 		t.Fatalf("unexpected error from REST storage: %v", err)
 	}
@@ -62,7 +67,7 @@ type fakeTokenGenerator struct {
 	staticToken string
 }
 
-func (f fakeTokenGenerator) GenerateToken(claims *jwt.Claims, privateClaims interface{}) (string, error) {
+func (f fakeTokenGenerator) GenerateToken(ctx context.Context, claims *jwt.Claims, privateClaims interface{}) (string, error) {
 	return f.staticToken, nil
 }
 

pkg/registry/core/serviceaccount/storage/token.go

@@ -65,6 +65,7 @@ type TokenREST struct {
 	audsSet               sets.String
 	maxExpirationSeconds  int64
 	extendExpiration      bool
+	isTokenSignerExternal bool
 }
 
 var _ = rest.NamedCreater(&TokenREST{})
@@ -217,16 +218,22 @@ func (r *TokenREST) Create(ctx context.Context, name string, obj runtime.Object,
 	exp := req.Spec.ExpirationSeconds
 	if r.extendExpiration && pod != nil && req.Spec.ExpirationSeconds == token.WarnOnlyBoundTokenExpirationSeconds && r.isKubeAudiences(req.Spec.Audiences) {
 		warnAfter = exp
+		// If token issuer is external-jwt-signer, then choose the smaller of
+		// ExpirationExtensionSeconds and max token lifetime supported by external signer.
+		if r.isTokenSignerExternal {
+			exp = min(r.maxExpirationSeconds, token.ExpirationExtensionSeconds)
+		} else {
 			exp = token.ExpirationExtensionSeconds
 		}
+	}
 
 	sc, pc, err := token.Claims(*svcacct, pod, secret, node, exp, warnAfter, req.Spec.Audiences)
 	if err != nil {
 		return nil, err
 	}
-	tokdata, err := r.issuer.GenerateToken(sc, pc)
+	tokdata, err := r.issuer.GenerateToken(ctx, sc, pc)
 	if err != nil {
-		return nil, fmt.Errorf("failed to generate token: %v", err)
+		return nil, errors.NewInternalError(fmt.Errorf("failed to generate token: %v", err))
 	}
 
 	// populate status

pkg/registry/core/serviceaccount/storage/token_test.go

@@ -0,0 +1,202 @@
+/*
+Copyright 2024 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package storage
+
+import (
+	"context"
+	"encoding/base64"
+	"encoding/json"
+	"strings"
+	"testing"
+	"time"
+
+	"gopkg.in/square/go-jose.v2/jwt"
+
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/runtime"
+	"k8s.io/apiserver/pkg/authentication/authenticator"
+	"k8s.io/apiserver/pkg/endpoints/request"
+	"k8s.io/apiserver/pkg/registry/rest"
+	utilfeature "k8s.io/apiserver/pkg/util/feature"
+	featuregatetesting "k8s.io/component-base/featuregate/testing"
+	authenticationapi "k8s.io/kubernetes/pkg/apis/authentication"
+	api "k8s.io/kubernetes/pkg/apis/core"
+	"k8s.io/kubernetes/pkg/features"
+	token "k8s.io/kubernetes/pkg/serviceaccount"
+)
+
+func TestCreate_Token_WithExpiryCap(t *testing.T) {
+
+	testcases := []struct {
+		desc                 string
+		extendExpiration     bool
+		maxExpirationSeconds int
+		expectedTokenAgeSec  int
+		isExternal           bool
+	}{
+		{
+			desc:                 "maxExpirationSeconds honoured",
+			extendExpiration:     true,
+			maxExpirationSeconds: 5 * 60 * 60, // 5h
+			expectedTokenAgeSec:  5 * 60 * 60, // 5h
+			isExternal:           true,
+		},
+		{
+			desc:                 "ExpirationExtensionSeconds used for exp",
+			extendExpiration:     true,
+			maxExpirationSeconds: 2 * 365 * 24 * 60 * 60,           // 2 years
+			expectedTokenAgeSec:  token.ExpirationExtensionSeconds, // 1y
+			isExternal:           true,
+		},
+		{
+			desc:                 "ExpirationExtensionSeconds used for exp",
+			extendExpiration:     true,
+			maxExpirationSeconds: 5 * 60 * 60,                      // 5h
+			expectedTokenAgeSec:  token.ExpirationExtensionSeconds, // 1y
+			isExternal:           false,
+		},
+		{
+			desc:                 "requested time use with extension disabled",
+			extendExpiration:     false,
+			maxExpirationSeconds: 5 * 60 * 60, // 5h
+			expectedTokenAgeSec:  3607,        // 1h
+			isExternal:           true,
+		},
+		{
+			desc:                 "maxExpirationSeconds honoured with extension disabled",
+			extendExpiration:     false,
+			maxExpirationSeconds: 30 * 60, // 30m
+			expectedTokenAgeSec:  30 * 60, // 30m
+			isExternal:           true,
+		},
+	}
+
+	// Create a test service account
+	serviceAccount := validNewServiceAccount("foo")
+
+	// Create a new pod
+	pod := &api.Pod{
+		ObjectMeta: metav1.ObjectMeta{
+			Name:      "test",
+			Namespace: serviceAccount.Namespace,
+		},
+		Spec: api.PodSpec{
+			ServiceAccountName: serviceAccount.Name,
+		},
+	}
+	podGetter := &objectGetter{obj: pod}
+	aud := authenticator.Audiences{
+		"aud-1",
+		"aud-2",
+	}
+
+	for _, tc := range testcases {
+		t.Run(tc.desc, func(t *testing.T) {
+			storage, server := newTokenStorage(t, testTokenGenerator{"fake"}, aud, podGetter, panicGetter{}, nil)
+			defer server.Terminate(t)
+			defer storage.Store.DestroyFunc()
+
+			ctx := context.Background()
+			// add the namespace to the context as it is required
+			ctx = request.WithNamespace(ctx, serviceAccount.Namespace)
+
+			// Enable ExternalServiceAccountTokenSigner feature
+			featuregatetesting.SetFeatureGateDuringTest(t, utilfeature.DefaultFeatureGate, features.ExternalServiceAccountTokenSigner, true)
+
+			// record namespace in the store.
+			_, err := storage.Store.Create(ctx, serviceAccount, rest.ValidateAllObjectFunc, &metav1.CreateOptions{})
+			if err != nil {
+				t.Fatalf("failed creating test service account: %v", err)
+			}
+
+			// add the namespace to the context as it is required
+			ctx = request.WithNamespace(ctx, serviceAccount.Namespace)
+			storage.Token.extendExpiration = tc.extendExpiration
+			storage.Token.maxExpirationSeconds = int64(tc.maxExpirationSeconds)
+			storage.Token.isTokenSignerExternal = tc.isExternal
+
+			tokenReqTimeStamp := time.Now()
+			out, err := storage.Token.Create(ctx, serviceAccount.Name, &authenticationapi.TokenRequest{
+				ObjectMeta: metav1.ObjectMeta{
+					Name:      serviceAccount.Name,
+					Namespace: serviceAccount.Namespace,
+				},
+				Spec: authenticationapi.TokenRequestSpec{
+					ExpirationSeconds: 3607,
+					BoundObjectRef: &authenticationapi.BoundObjectReference{
+						Name:       pod.Name,
+						Kind:       "Pod",
+						APIVersion: "v1",
+					},
+					Audiences: aud,
+				},
+			}, rest.ValidateAllObjectFunc, &metav1.CreateOptions{})
+			if err != nil {
+				t.Fatalf("failed calling /token endpoint for service account: %v", err)
+			}
+
+			tokenReq := out.(*authenticationapi.TokenRequest)
+			payload := strings.Split(tokenReq.Status.Token, ".")[1]
+			claims, err := base64.RawURLEncoding.DecodeString(payload)
+			if err != nil {
+				t.Fatalf("failed when decoding payload: %v", err)
+			}
+			structuredClaim := jwt.Claims{}
+			err = json.Unmarshal(claims, &structuredClaim)
+			if err != nil {
+				t.Fatalf("Error unmarshalling Claims: %v", err)
+			}
+			structuredClaim.Expiry.Time()
+			upperBound := tokenReqTimeStamp.Add(time.Duration(tc.expectedTokenAgeSec+10) * time.Second)
+			lowerBound := tokenReqTimeStamp.Add(time.Duration(tc.expectedTokenAgeSec-10) * time.Second)
+
+			// check for token expiration with a toleration of +/-10s after tokenReqTimeStamp to make for latencies.
+			if structuredClaim.Expiry.Time().After(upperBound) ||
+				structuredClaim.Expiry.Time().Before(lowerBound) {
+				t.Fatalf("expected token expiration to be between %v to %v\n was %v", upperBound, lowerBound, structuredClaim.Expiry.Time())
+			}
+
+		})
+
+	}
+}
+
+type objectGetter struct {
+	obj runtime.Object
+	err error
+}
+
+func (f objectGetter) Get(ctx context.Context, name string, options *metav1.GetOptions) (runtime.Object, error) {
+	return f.obj, f.err
+}
+
+var _ rest.Getter = objectGetter{}
+
+// A basic fake token generator which always returns a static string
+type testTokenGenerator struct {
+	staticToken string
+}
+
+func (f testTokenGenerator) GenerateToken(ctx context.Context, claims *jwt.Claims, privateClaims interface{}) (string, error) {
+	c, err := json.Marshal(claims)
+	if err != nil {
+		return "", err
+	}
+	return f.staticToken + "." + base64.RawURLEncoding.EncodeToString(c) + "." + f.staticToken, nil
+}
+
+var _ token.TokenGenerator = testTokenGenerator{}

pkg/serviceaccount/externaljwt/metrics/metrics.go

@@ -0,0 +1,148 @@
+/*
+Copyright 2024 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package metrics
+
+import (
+	"context"
+	"errors"
+	"sync"
+	"time"
+
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
+
+	"k8s.io/component-base/metrics"
+	"k8s.io/component-base/metrics/legacyregistry"
+)
+
+const (
+	namespace = "apiserver"
+	subsystem = "externaljwt"
+)
+
+var (
+	lastKeyFetchTimeStamp = metrics.NewGaugeVec(
+		&metrics.GaugeOpts{
+			Namespace:      namespace,
+			Subsystem:      subsystem,
+			Name:           "fetch_keys_success_timestamp",
+			Help:           "Unix Timestamp in seconds of the last successful FetchKeys request",
+			StabilityLevel: metrics.ALPHA,
+		},
+		nil,
+	)
+
+	dataTimeStamp = metrics.NewGaugeVec(
+		&metrics.GaugeOpts{
+			Namespace:      namespace,
+			Subsystem:      subsystem,
+			Name:           "fetch_keys_data_timestamp",
+			Help:           "Unix Timestamp in seconds of the last successful FetchKeys data_timestamp value returned by the external signer",
+			StabilityLevel: metrics.ALPHA,
+		},
+		nil,
+	)
+
+	totalKeyFetch = metrics.NewCounterVec(
+		&metrics.CounterOpts{
+			Namespace:      namespace,
+			Subsystem:      subsystem,
+			Name:           "fetch_keys_request_total",
+			Help:           "Total attempts at syncing supported JWKs",
+			StabilityLevel: metrics.ALPHA,
+		},
+		[]string{"code"},
+	)
+
+	tokenGenReqTotal = metrics.NewCounterVec(
+		&metrics.CounterOpts{
+			Namespace:      namespace,
+			Subsystem:      subsystem,
+			Name:           "sign_request_total",
+			Help:           "Total attempts at signing JWT",
+			StabilityLevel: metrics.ALPHA,
+		},
+		[]string{"code"},
+	)
+
+	requestDurationSeconds = metrics.NewHistogramVec(
+		&metrics.HistogramOpts{
+			Namespace:      namespace,
+			Subsystem:      subsystem,
+			Name:           "request_duration_seconds",
+			Help:           "Request duration and time for calls to external-jwt-signer",
+			Buckets:        []float64{.001, .005, .01, .025, .05, .1, .25, .5, 1, 2.5, 5, 10, 30, 60},
+			StabilityLevel: metrics.ALPHA,
+		},
+		[]string{"method", "code"},
+	)
+)
+
+var registerMetrics sync.Once
+
+func RegisterMetrics() {
+	registerMetrics.Do(func() {
+		legacyregistry.MustRegister(lastKeyFetchTimeStamp)
+		legacyregistry.MustRegister(dataTimeStamp)
+		legacyregistry.MustRegister(totalKeyFetch)
+		legacyregistry.MustRegister(tokenGenReqTotal)
+		legacyregistry.MustRegister(requestDurationSeconds)
+	})
+}
+
+func RecordFetchKeysAttempt(err error) {
+	totalKeyFetch.WithLabelValues(getErrorCode(err)).Inc()
+	if err == nil {
+		lastKeyFetchTimeStamp.WithLabelValues().SetToCurrentTime()
+	}
+}
+
+func RecordTokenGenAttempt(err error) {
+	tokenGenReqTotal.WithLabelValues(getErrorCode(err)).Inc()
+}
+
+func RecordKeyDataTimeStamp(timestamp int64) {
+	dataTimeStamp.WithLabelValues().Set(float64(timestamp))
+}
+
+type gRPCError interface {
+	GRPCStatus() *status.Status
+}
+
+func getErrorCode(err error) string {
+	if err == nil {
+		return codes.OK.String()
+	}
+
+	// handle errors wrapped with fmt.Errorf and similar
+	var s gRPCError
+	if errors.As(err, &s) {
+		return s.GRPCStatus().Code().String()
+	}
+
+	// This is not gRPC error. The operation must have failed before gRPC
+	// method was called, otherwise we would get gRPC error.
+	return "unknown-non-grpc"
+}
+
+func OuboundRequestMetricsInterceptor(ctx context.Context, method string, req, reply any, cc *grpc.ClientConn, invoker grpc.UnaryInvoker, opts ...grpc.CallOption) error {
+	start := time.Now()
+	err := invoker(ctx, method, req, reply, cc, opts...)
+	requestDurationSeconds.WithLabelValues(method, getErrorCode(err)).Observe(time.Since(start).Seconds())
+	return err
+}

pkg/serviceaccount/externaljwt/metrics/metrics_test.go

@@ -0,0 +1,244 @@
+/*
+Copyright 2024 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package metrics
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+	"time"
+
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
+
+	"k8s.io/component-base/metrics/legacyregistry"
+	"k8s.io/component-base/metrics/testutil"
+)
+
+func TestFetchMetrics(t *testing.T) {
+	testCases := []struct {
+		desc    string
+		metrics []string
+		want    string
+		emit    func()
+	}{
+		{
+			desc: "basic test",
+			metrics: []string{
+				"apiserver_externaljwt_fetch_keys_request_total",
+			},
+			emit: func() {
+				RecordFetchKeysAttempt(status.New(codes.Internal, "error ocured").Err())
+			},
+			want: fmt.Sprintf(`
+			# HELP apiserver_externaljwt_fetch_keys_request_total [ALPHA] Total attempts at syncing supported JWKs
+			# TYPE apiserver_externaljwt_fetch_keys_request_total counter
+			apiserver_externaljwt_fetch_keys_request_total{code="%s"} 1
+			`, codes.Internal.String()),
+		},
+		{
+			desc: "wrapped error code",
+			metrics: []string{
+				"apiserver_externaljwt_fetch_keys_request_total",
+			},
+			emit: func() {
+				RecordFetchKeysAttempt(fmt.Errorf("some error %w", status.New(codes.Canceled, "error ocured").Err()))
+			},
+			want: fmt.Sprintf(`
+			# HELP apiserver_externaljwt_fetch_keys_request_total [ALPHA] Total attempts at syncing supported JWKs
+			# TYPE apiserver_externaljwt_fetch_keys_request_total counter
+			apiserver_externaljwt_fetch_keys_request_total{code="%s"} 1
+			apiserver_externaljwt_fetch_keys_request_total{code="%s"} 1
+			`, codes.Internal.String(), codes.Canceled.String()),
+		},
+		{
+			desc: "success count appears",
+			metrics: []string{
+				"apiserver_externaljwt_fetch_keys_request_total",
+			},
+			emit: func() {
+				RecordFetchKeysAttempt(nil)
+			},
+			want: fmt.Sprintf(`
+			# HELP apiserver_externaljwt_fetch_keys_request_total [ALPHA] Total attempts at syncing supported JWKs
+			# TYPE apiserver_externaljwt_fetch_keys_request_total counter
+			apiserver_externaljwt_fetch_keys_request_total{code="%s"} 1
+			apiserver_externaljwt_fetch_keys_request_total{code="%s"} 1
+			apiserver_externaljwt_fetch_keys_request_total{code="%s"} 1
+			`, codes.Internal.String(), codes.Canceled.String(), codes.OK.String()),
+		},
+		{
+			desc: "success count increments",
+			metrics: []string{
+				"apiserver_externaljwt_fetch_keys_request_total",
+			},
+			emit: func() {
+				RecordFetchKeysAttempt(nil)
+			},
+			want: fmt.Sprintf(`
+			# HELP apiserver_externaljwt_fetch_keys_request_total [ALPHA] Total attempts at syncing supported JWKs
+			# TYPE apiserver_externaljwt_fetch_keys_request_total counter
+			apiserver_externaljwt_fetch_keys_request_total{code="%s"} 1
+			apiserver_externaljwt_fetch_keys_request_total{code="%s"} 1
+			apiserver_externaljwt_fetch_keys_request_total{code="%s"} 2
+			`, codes.Internal.String(), codes.Canceled.String(), codes.OK.String()),
+		},
+	}
+
+	RegisterMetrics()
+
+	for _, tt := range testCases {
+		t.Run(tt.desc, func(t *testing.T) {
+			tt.emit()
+			if err := testutil.GatherAndCompare(legacyregistry.DefaultGatherer, strings.NewReader(tt.want), tt.metrics...); err != nil {
+				t.Fatal(err)
+			}
+		})
+	}
+}
+
+func TestTokenGenMetrics(t *testing.T) {
+	testCases := []struct {
+		desc    string
+		metrics []string
+		want    string
+		emit    func()
+	}{
+		{
+			desc: "basic test",
+			metrics: []string{
+				"apiserver_externaljwt_sign_request_total",
+			},
+			emit: func() {
+				RecordTokenGenAttempt(fmt.Errorf("some error %w", status.New(codes.Internal, "error ocured").Err()))
+			},
+			want: fmt.Sprintf(`
+			# HELP apiserver_externaljwt_sign_request_total [ALPHA] Total attempts at signing JWT
+			# TYPE apiserver_externaljwt_sign_request_total counter
+			apiserver_externaljwt_sign_request_total{code="%s"} 1
+			`, codes.Internal.String()),
+		},
+		{
+			desc: "wrapped error code",
+			metrics: []string{
+				"apiserver_externaljwt_sign_request_total",
+			},
+			emit: func() {
+				RecordTokenGenAttempt(fmt.Errorf("some error %w", fmt.Errorf("some error %w", status.New(codes.Canceled, "error ocured").Err())))
+			},
+			want: fmt.Sprintf(`
+			# HELP apiserver_externaljwt_sign_request_total [ALPHA] Total attempts at signing JWT
+			# TYPE apiserver_externaljwt_sign_request_total counter
+			apiserver_externaljwt_sign_request_total{code="%s"} 1
+			apiserver_externaljwt_sign_request_total{code="%s"} 1
+			`, codes.Internal.String(), codes.Canceled.String()),
+		},
+		{
+			desc: "success count appears",
+			metrics: []string{
+				"apiserver_externaljwt_sign_request_total",
+			},
+			emit: func() {
+				RecordTokenGenAttempt(nil)
+			},
+			want: fmt.Sprintf(`
+			# HELP apiserver_externaljwt_sign_request_total [ALPHA] Total attempts at signing JWT
+			# TYPE apiserver_externaljwt_sign_request_total counter
+			apiserver_externaljwt_sign_request_total{code="%s"} 1
+			apiserver_externaljwt_sign_request_total{code="%s"} 1
+			apiserver_externaljwt_sign_request_total{code="%s"} 1
+			`, codes.Internal.String(), codes.Canceled.String(), codes.OK.String()),
+		},
+		{
+			desc: "success count increments",
+			metrics: []string{
+				"apiserver_externaljwt_sign_request_total",
+			},
+			emit: func() {
+				RecordTokenGenAttempt(nil)
+			},
+			want: fmt.Sprintf(`
+			# HELP apiserver_externaljwt_sign_request_total [ALPHA] Total attempts at signing JWT
+			# TYPE apiserver_externaljwt_sign_request_total counter
+			apiserver_externaljwt_sign_request_total{code="%s"} 1
+			apiserver_externaljwt_sign_request_total{code="%s"} 1
+			apiserver_externaljwt_sign_request_total{code="%s"} 2
+			`, codes.Internal.String(), codes.Canceled.String(), codes.OK.String()),
+		},
+	}
+
+	RegisterMetrics()
+
+	for _, tt := range testCases {
+		t.Run(tt.desc, func(t *testing.T) {
+			tt.emit()
+			if err := testutil.GatherAndCompare(legacyregistry.DefaultGatherer, strings.NewReader(tt.want), tt.metrics...); err != nil {
+				t.Fatal(err)
+			}
+		})
+	}
+}
+
+func TestRecordKeyDataTimeStamp(t *testing.T) {
+
+	dataTimeStamp1 := time.Now().Unix()
+	dataTimeStamp2 := time.Now().Add(time.Second * 1200).Unix()
+
+	testCases := []struct {
+		desc    string
+		metrics []string
+		want    int64
+		emit    func()
+	}{
+		{
+			desc: "basic test",
+			metrics: []string{
+				"fetch_keys_data_timestamp",
+			},
+			emit: func() {
+				RecordKeyDataTimeStamp(dataTimeStamp1)
+			},
+			want: dataTimeStamp1,
+		},
+		{
+			desc: "update to a new value",
+			metrics: []string{
+				"fetch_keys_data_timestamp",
+			},
+			emit: func() {
+				RecordKeyDataTimeStamp(dataTimeStamp1)
+				RecordKeyDataTimeStamp(dataTimeStamp2)
+			},
+			want: dataTimeStamp2,
+		},
+	}
+
+	RegisterMetrics()
+
+	for _, tt := range testCases {
+		t.Run(tt.desc, func(t *testing.T) {
+			tt.emit()
+			actualValue, err := testutil.GetGaugeMetricValue(dataTimeStamp.WithLabelValues())
+			if err != nil {
+				t.Errorf("error when getting gauge value for dataTimeStamp: %v", err)
+			}
+			if actualValue != float64(tt.want) {
+				t.Errorf("Expected dataTimeStamp to be %v, got %v", tt.want, actualValue)
+			}
+		})
+	}
+}

pkg/serviceaccount/externaljwt/plugin/keycache.go

@@ -0,0 +1,236 @@
+/*
+Copyright 2024 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package plugin
+
+import (
+	"context"
+	"crypto/x509"
+	"fmt"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"golang.org/x/sync/singleflight"
+	"k8s.io/klog/v2"
+	"k8s.io/kubernetes/pkg/serviceaccount"
+
+	externaljwtv1alpha1 "k8s.io/externaljwt/apis/v1alpha1"
+	externaljwtmetrics "k8s.io/kubernetes/pkg/serviceaccount/externaljwt/metrics"
+)
+
+const fallbackRefreshDuration = 10 * time.Second
+
+type keyCache struct {
+	client externaljwtv1alpha1.ExternalJWTSignerClient
+
+	syncGroup     singleflight.Group
+	listenersLock sync.Mutex
+	listeners     []serviceaccount.Listener
+
+	verificationKeys atomic.Pointer[VerificationKeys]
+}
+
+// newKeyCache constructs an implementation of KeyCache.
+func newKeyCache(client externaljwtv1alpha1.ExternalJWTSignerClient) *keyCache {
+	cache := &keyCache{
+		client: client,
+	}
+	cache.verificationKeys.Store(&VerificationKeys{})
+	return cache
+}
+
+// InitialFill can be used to perform an initial fetch for keys get the
+// refresh interval as recommended by external signer.
+func (p *keyCache) initialFill(ctx context.Context) error {
+	if _, err := p.syncKeys(ctx); err != nil {
+		return fmt.Errorf("while performing initial cache fill: %w", err)
+	}
+	return nil
+}
+
+func (p *keyCache) scheduleSync(ctx context.Context, keySyncTimeout time.Duration) {
+	timer := time.NewTimer(p.verificationKeys.Load().NextRefreshHint.Sub(time.Now()))
+	defer timer.Stop()
+
+	var lastDataTimestamp time.Time
+	for {
+		select {
+		case <-ctx.Done():
+			klog.InfoS("Key cache shutting down")
+			return
+		case <-timer.C:
+		}
+
+		timedCtx, cancel := context.WithTimeout(ctx, keySyncTimeout)
+		dataTimestamp, err := p.syncKeys(timedCtx)
+		if err != nil {
+			klog.Errorf("when syncing supported public keys(Stale set of keys will be supported): %v", err)
+			timer.Reset(fallbackRefreshDuration)
+		} else {
+			timer.Reset(p.verificationKeys.Load().NextRefreshHint.Sub(time.Now()))
+			if lastDataTimestamp.IsZero() || !dataTimestamp.Equal(lastDataTimestamp) {
+				lastDataTimestamp = dataTimestamp
+				p.broadcastUpdate()
+			}
+		}
+		cancel()
+	}
+}
+
+func (p *keyCache) AddListener(listener serviceaccount.Listener) {
+	p.listenersLock.Lock()
+	defer p.listenersLock.Unlock()
+
+	p.listeners = append(p.listeners, listener)
+}
+
+func (p *keyCache) GetCacheAgeMaxSeconds() int {
+	val := int(p.verificationKeys.Load().NextRefreshHint.Sub(time.Now()).Seconds())
+	if val < 0 {
+		return 0
+	}
+	return val
+}
+
+// GetPublicKeys returns the public key corresponding to requested keyID.
+// Getter is expected to return All keys for keyID ""
+func (p *keyCache) GetPublicKeys(ctx context.Context, keyID string) []serviceaccount.PublicKey {
+	pubKeys, ok := p.findKeyForKeyID(keyID)
+	if ok {
+		return pubKeys
+	}
+
+	// If we didn't find it, trigger a sync.
+	if _, err := p.syncKeys(ctx); err != nil {
+		klog.ErrorS(err, "Error while syncing keys")
+		return []serviceaccount.PublicKey{}
+	}
+
+	pubKeys, ok = p.findKeyForKeyID(keyID)
+	if ok {
+		return pubKeys
+	}
+
+	// If we still didn't find it, then it's an unknown keyID.
+	klog.Errorf("Key id %q not found after refresh", keyID)
+	return []serviceaccount.PublicKey{}
+}
+
+func (p *keyCache) findKeyForKeyID(keyID string) ([]serviceaccount.PublicKey, bool) {
+	if len(p.verificationKeys.Load().Keys) == 0 {
+		klog.Error("No keys currently in cache. Initial fill has not completed")
+		return nil, false
+	}
+
+	if keyID == "" {
+		return p.verificationKeys.Load().Keys, true
+	}
+
+	keysToReturn := []serviceaccount.PublicKey{}
+	for _, key := range p.verificationKeys.Load().Keys {
+		if key.KeyID == keyID {
+			keysToReturn = append(keysToReturn, key)
+		}
+	}
+
+	return keysToReturn, len(keysToReturn) > 0
+}
+
+// sync supported external keys.
+// completely re-writes the set of supported keys.
+func (p *keyCache) syncKeys(ctx context.Context) (time.Time, error) {
+	val, err, _ := p.syncGroup.Do("", func() (any, error) {
+		newPublicKeys, err := p.getTokenVerificationKeys(ctx)
+		externaljwtmetrics.RecordFetchKeysAttempt(err)
+		if err != nil {
+			return nil, fmt.Errorf("while fetching token verification keys: %w", err)
+		}
+
+		p.verificationKeys.Store(newPublicKeys)
+
+		externaljwtmetrics.RecordKeyDataTimeStamp(newPublicKeys.DataTimestamp.Unix())
+
+		return newPublicKeys, nil
+	})
+	if err != nil {
+		return time.Time{}, err
+	}
+
+	vk := val.(*VerificationKeys)
+
+	return vk.DataTimestamp, nil
+}
+
+func (p *keyCache) broadcastUpdate() {
+	p.listenersLock.Lock()
+	defer p.listenersLock.Unlock()
+
+	for _, l := range p.listeners {
+		l.Enqueue()
+	}
+}
+
+// GetTokenVerificationKeys returns a map of supported external keyIDs to keys
+// the keys are PKIX-serialized. It calls external-jwt-signer with a timeout of keySyncTimeoutSec.
+func (p *keyCache) getTokenVerificationKeys(ctx context.Context) (*VerificationKeys, error) {
+	req := &externaljwtv1alpha1.FetchKeysRequest{}
+	resp, err := p.client.FetchKeys(ctx, req)
+	if err != nil {
+		return nil, fmt.Errorf("while getting externally supported jwt signing keys: %w", err)
+	}
+	// Validate the refresh hint.
+	if resp.RefreshHintSeconds <= 0 {
+		return nil, fmt.Errorf("found invalid refresh hint (%ds)", resp.RefreshHintSeconds)
+	}
+
+	if len(resp.Keys) == 0 {
+		return nil, fmt.Errorf("found no keys")
+	}
+	if err := resp.DataTimestamp.CheckValid(); err != nil {
+		return nil, fmt.Errorf("invalid data timestamp: %w", err)
+	}
+	keys := make([]serviceaccount.PublicKey, 0, len(resp.Keys))
+	for _, protoKey := range resp.Keys {
+		if protoKey == nil {
+			return nil, fmt.Errorf("found nil public key")
+		}
+		if len(protoKey.KeyId) == 0 || len(protoKey.KeyId) > 1024 {
+			return nil, fmt.Errorf("found invalid public key id %q", protoKey.KeyId)
+		}
+		if len(protoKey.Key) == 0 {
+			return nil, fmt.Errorf("found empty public key")
+		}
+		parsedPublicKey, err := x509.ParsePKIXPublicKey(protoKey.Key)
+		if err != nil {
+			return nil, fmt.Errorf("while parsing external public keys: %w", err)
+		}
+
+		keys = append(keys, serviceaccount.PublicKey{
+			KeyID:                    protoKey.KeyId,
+			PublicKey:                parsedPublicKey,
+			ExcludeFromOIDCDiscovery: protoKey.ExcludeFromOidcDiscovery,
+		})
+	}
+
+	vk := &VerificationKeys{
+		Keys:            keys,
+		DataTimestamp:   resp.DataTimestamp.AsTime(),
+		NextRefreshHint: time.Now().Add(time.Duration(resp.RefreshHintSeconds) * time.Second),
+	}
+
+	return vk, nil
+}

pkg/serviceaccount/externaljwt/plugin/keycache_test.go

@@ -0,0 +1,407 @@
+/*
+Copyright 2024 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package plugin
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"os"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/google/go-cmp/cmp/cmpopts"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/credentials/insecure"
+	"google.golang.org/protobuf/types/known/timestamppb"
+
+	externaljwtv1alpha1 "k8s.io/externaljwt/apis/v1alpha1"
+	"k8s.io/kubernetes/pkg/serviceaccount"
+)
+
+func TestExternalPublicKeyGetter(t *testing.T) {
+
+	invalidKid := string(make([]byte, 1025))
+	testCases := []struct {
+		desc                  string
+		expectedErr           error
+		supportedKeys         map[string]supportedKeyT
+		wantVerificationKeys  *VerificationKeys
+		refreshHintSec        int
+		dataTimeStamp         *timestamppb.Timestamp
+		supportedKeysOverride []*externaljwtv1alpha1.Key
+	}{
+		{
+			desc: "single key in signer",
+			supportedKeys: map[string]supportedKeyT{
+				"key-1": {
+					key: &rsaKey1.PublicKey,
+				},
+			},
+			wantVerificationKeys: &VerificationKeys{
+				Keys: []serviceaccount.PublicKey{
+					{
+						KeyID:                    "key-1",
+						PublicKey:                &rsaKey1.PublicKey,
+						ExcludeFromOIDCDiscovery: false,
+					},
+				},
+			},
+			refreshHintSec: 20,
+			dataTimeStamp:  timestamppb.New(time.Time{}),
+		},
+		{
+			desc: "multiple keys in signer",
+			supportedKeys: map[string]supportedKeyT{
+				"key-1": {
+					key: &rsaKey1.PublicKey,
+				},
+				"key-2": {
+					key:             &rsaKey2.PublicKey,
+					excludeFromOidc: true,
+				},
+			},
+			wantVerificationKeys: &VerificationKeys{
+				Keys: []serviceaccount.PublicKey{
+					{
+						KeyID:                    "key-1",
+						PublicKey:                &rsaKey1.PublicKey,
+						ExcludeFromOIDCDiscovery: false,
+					},
+					{
+						KeyID:                    "key-2",
+						PublicKey:                &rsaKey2.PublicKey,
+						ExcludeFromOIDCDiscovery: true,
+					},
+				},
+			},
+			refreshHintSec: 10,
+			dataTimeStamp:  timestamppb.New(time.Time{}),
+		},
+		{
+			desc: "empty kid",
+			supportedKeys: map[string]supportedKeyT{
+				"": {
+					key: &rsaKey1.PublicKey,
+				},
+				"key-2": {
+					key:             &rsaKey2.PublicKey,
+					excludeFromOidc: true,
+				},
+			},
+			expectedErr:    fmt.Errorf("found invalid public key id %q", ""),
+			refreshHintSec: 10,
+			dataTimeStamp:  timestamppb.New(time.Time{}),
+		},
+		{
+			desc: "kid longer than 1024",
+			supportedKeys: map[string]supportedKeyT{
+				invalidKid: {
+					key: &rsaKey1.PublicKey,
+				},
+				"key-2": {
+					key:             &rsaKey2.PublicKey,
+					excludeFromOidc: true,
+				},
+			},
+			expectedErr:    fmt.Errorf("found invalid public key id %q", invalidKid),
+			refreshHintSec: 10,
+			dataTimeStamp:  timestamppb.New(time.Time{}),
+		},
+		{
+			desc:           "no keys",
+			supportedKeys:  map[string]supportedKeyT{},
+			expectedErr:    fmt.Errorf("found no keys"),
+			refreshHintSec: 10,
+			dataTimeStamp:  timestamppb.New(time.Time{}),
+		},
+		{
+			desc: "invalid data timestamp",
+			supportedKeys: map[string]supportedKeyT{
+				"key-2": {
+					key:             &rsaKey2.PublicKey,
+					excludeFromOidc: true,
+				},
+			},
+			expectedErr:    fmt.Errorf("invalid data timestamp"),
+			refreshHintSec: 10,
+			dataTimeStamp:  nil,
+		},
+		{
+			desc:           "empty public key",
+			expectedErr:    fmt.Errorf("found empty public key"),
+			refreshHintSec: 10,
+			dataTimeStamp:  timestamppb.New(time.Time{}),
+			supportedKeys: map[string]supportedKeyT{
+				"key-2": {
+					key:             &rsaKey2.PublicKey,
+					excludeFromOidc: true,
+				},
+			},
+			supportedKeysOverride: []*externaljwtv1alpha1.Key{
+				{
+					KeyId: "kid",
+					Key:   nil,
+				},
+			},
+		},
+	}
+
+	for i, tc := range testCases {
+		t.Run(tc.desc, func(t *testing.T) {
+			ctx := context.Background()
+
+			sockname := fmt.Sprintf("@test-external-public-key-getter-%d.sock", i)
+			t.Cleanup(func() { _ = os.Remove(sockname) })
+
+			addr := &net.UnixAddr{Name: sockname, Net: "unix"}
+			listener, err := net.ListenUnix(addr.Network(), addr)
+			if err != nil {
+				t.Fatalf("Failed to start fake backend: %v", err)
+			}
+
+			grpcServer := grpc.NewServer()
+
+			backend := &dummyExtrnalSigner{
+				supportedKeys:      tc.supportedKeys,
+				refreshHintSeconds: tc.refreshHintSec,
+			}
+			backend.DataTimeStamp = tc.dataTimeStamp
+			backend.SupportedKeysOverride = tc.supportedKeysOverride
+			externaljwtv1alpha1.RegisterExternalJWTSignerServer(grpcServer, backend)
+
+			defer grpcServer.Stop()
+			go func() {
+				if err := grpcServer.Serve(listener); err != nil {
+					panic(fmt.Errorf("error returned from grpcServer: %w", err))
+				}
+			}()
+
+			clientConn, err := grpc.DialContext(
+				ctx,
+				sockname,
+				grpc.WithContextDialer(func(ctx context.Context, path string) (net.Conn, error) {
+					return (&net.Dialer{}).DialContext(ctx, "unix", path)
+				}),
+				grpc.WithTransportCredentials(insecure.NewCredentials()),
+			)
+			if err != nil {
+				t.Fatalf("Failed to dial buffconn client: %v", err)
+			}
+			defer func() {
+				_ = clientConn.Close()
+			}()
+
+			plugin := newPlugin("iss", clientConn, true)
+
+			signingKeys, err := plugin.keyCache.getTokenVerificationKeys(ctx)
+			if err != nil {
+				if tc.expectedErr == nil {
+					t.Fatalf("error getting supported keys: %v", err)
+				}
+				if !strings.Contains(err.Error(), tc.expectedErr.Error()) {
+					t.Fatalf("want error: %v, got error: %v", tc.expectedErr, err)
+					return
+				}
+			}
+
+			if tc.expectedErr == nil {
+				if diff := cmp.Diff(signingKeys.Keys, tc.wantVerificationKeys.Keys, cmpopts.SortSlices(sortPublicKeySlice)); diff != "" {
+					t.Fatalf("Bad result from GetTokenSigningKeys; diff (-got +want)\n%s", diff)
+				}
+				expectedRefreshHintSec := time.Now().Add(time.Duration(tc.refreshHintSec) * time.Second)
+				difference := signingKeys.NextRefreshHint.Sub(expectedRefreshHintSec).Seconds()
+				if difference > 1 || difference < -1 { // tolerate 1 sec of skew for test
+					t.Fatalf("refreshHint not as expected; got: %v want: %v", signingKeys.NextRefreshHint, expectedRefreshHintSec)
+				}
+			}
+		})
+	}
+}
+
+func TestInitialFill(t *testing.T) {
+	ctx := context.Background()
+
+	sockname := "@test-initial-fill.sock"
+	t.Cleanup(func() { _ = os.Remove(sockname) })
+
+	addr := &net.UnixAddr{Name: sockname, Net: "unix"}
+	listener, err := net.ListenUnix(addr.Network(), addr)
+	if err != nil {
+		t.Fatalf("Failed to start fake backend: %v", err)
+	}
+
+	grpcServer := grpc.NewServer()
+
+	supportedKeys := map[string]supportedKeyT{
+		"key-1": {
+			key: &rsaKey1.PublicKey,
+		},
+	}
+	wantPubKeys := []serviceaccount.PublicKey{
+		{
+			KeyID:                    "key-1",
+			PublicKey:                &rsaKey1.PublicKey,
+			ExcludeFromOIDCDiscovery: false,
+		},
+	}
+
+	backend := &dummyExtrnalSigner{
+		supportedKeys:      supportedKeys,
+		refreshHintSeconds: 10,
+		DataTimeStamp:      timestamppb.New(time.Time{}),
+	}
+	externaljwtv1alpha1.RegisterExternalJWTSignerServer(grpcServer, backend)
+
+	defer grpcServer.Stop()
+	go func() {
+		if err := grpcServer.Serve(listener); err != nil {
+			panic(fmt.Errorf("error returned from grpcServer: %w", err))
+		}
+	}()
+
+	clientConn, err := grpc.DialContext(
+		ctx,
+		sockname,
+		grpc.WithContextDialer(func(ctx context.Context, path string) (net.Conn, error) {
+			return (&net.Dialer{}).DialContext(ctx, "unix", path)
+		}),
+		grpc.WithTransportCredentials(insecure.NewCredentials()),
+	)
+	if err != nil {
+		t.Fatalf("Failed to dial buffconn client: %v", err)
+	}
+	defer func() { _ = clientConn.Close() }()
+
+	plugin := newPlugin("iss", clientConn, true)
+
+	if err := plugin.keyCache.initialFill(ctx); err != nil {
+		t.Fatalf("Error during InitialFill: %v", err)
+	}
+
+	gotPubKeys := plugin.keyCache.GetPublicKeys(ctx, "")
+	if diff := cmp.Diff(gotPubKeys, wantPubKeys); diff != "" {
+		t.Fatalf("Bad public keys; diff (-got +want)\n%s", diff)
+	}
+}
+
+func TestReflectChanges(t *testing.T) {
+	ctx := context.Background()
+
+	sockname := "@test-reflect-changes.sock"
+	t.Cleanup(func() { _ = os.Remove(sockname) })
+
+	addr := &net.UnixAddr{Name: sockname, Net: "unix"}
+	listener, err := net.ListenUnix(addr.Network(), addr)
+	if err != nil {
+		t.Fatalf("Failed to start fake backend: %v", err)
+	}
+
+	grpcServer := grpc.NewServer()
+
+	supportedKeysT1 := map[string]supportedKeyT{
+		"key-1": {
+			key: &rsaKey1.PublicKey,
+		},
+	}
+	wantPubKeysT1 := []serviceaccount.PublicKey{
+		{
+			KeyID:                    "key-1",
+			PublicKey:                &rsaKey1.PublicKey,
+			ExcludeFromOIDCDiscovery: false,
+		},
+	}
+
+	backend := &dummyExtrnalSigner{
+		supportedKeys:      supportedKeysT1,
+		refreshHintSeconds: 10,
+		DataTimeStamp:      timestamppb.New(time.Time{}),
+	}
+	externaljwtv1alpha1.RegisterExternalJWTSignerServer(grpcServer, backend)
+
+	defer grpcServer.Stop()
+	go func() {
+		if err := grpcServer.Serve(listener); err != nil {
+			panic(fmt.Errorf("error returned from grpcServer: %w", err))
+		}
+	}()
+
+	clientConn, err := grpc.DialContext(
+		ctx,
+		sockname,
+		grpc.WithContextDialer(func(ctx context.Context, path string) (net.Conn, error) {
+			return (&net.Dialer{}).DialContext(ctx, "unix", path)
+		}),
+		grpc.WithTransportCredentials(insecure.NewCredentials()),
+	)
+	if err != nil {
+		t.Fatalf("Failed to dial buffconn client: %v", err)
+	}
+	defer func() { _ = clientConn.Close() }()
+
+	plugin := newPlugin("iss", clientConn, true)
+
+	if err := plugin.keyCache.initialFill(ctx); err != nil {
+		t.Fatalf("Error during InitialFill: %v", err)
+	}
+
+	gotPubKeysT1 := plugin.keyCache.GetPublicKeys(ctx, "")
+	if diff := cmp.Diff(gotPubKeysT1, wantPubKeysT1, cmpopts.SortSlices(sortPublicKeySlice)); diff != "" {
+		t.Fatalf("Bad public keys; diff (-got +want)\n%s", diff)
+	}
+
+	if _, err := plugin.keyCache.syncKeys(ctx); err != nil {
+		t.Fatalf("Error while calling syncKeys: %v", err)
+	}
+
+	supportedKeysT2 := map[string]supportedKeyT{
+		"key-1": {
+			key:             &rsaKey1.PublicKey,
+			excludeFromOidc: true,
+		},
+		"key-2": {
+			key: &rsaKey2.PublicKey,
+		},
+	}
+	wantPubKeysT2 := []serviceaccount.PublicKey{
+		{
+			KeyID:                    "key-1",
+			PublicKey:                &rsaKey1.PublicKey,
+			ExcludeFromOIDCDiscovery: true,
+		},
+		{
+			KeyID:                    "key-2",
+			PublicKey:                &rsaKey2.PublicKey,
+			ExcludeFromOIDCDiscovery: false,
+		},
+	}
+
+	backend.keyLock.Lock()
+	backend.supportedKeys = supportedKeysT2
+	backend.keyLock.Unlock()
+
+	if _, err := plugin.keyCache.syncKeys(ctx); err != nil {
+		t.Fatalf("Error while calling syncKeys: %v", err)
+	}
+
+	gotPubKeysT2 := plugin.keyCache.GetPublicKeys(ctx, "")
+	if diff := cmp.Diff(gotPubKeysT2, wantPubKeysT2, cmpopts.SortSlices(sortPublicKeySlice)); diff != "" {
+		t.Fatalf("Bad public keys; diff (-got +want)\n%s", diff)
+	}
+}

pkg/serviceaccount/externaljwt/plugin/plugin.go

@@ -0,0 +1,218 @@
+/*
+Copyright 2024 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package plugin
+
+import (
+	"bytes"
+	"context"
+	"encoding/base64"
+	"encoding/json"
+	"fmt"
+	"net"
+	"time"
+
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/credentials/insecure"
+	jose "gopkg.in/square/go-jose.v2"
+	"gopkg.in/square/go-jose.v2/jwt"
+
+	externaljwtv1alpha1 "k8s.io/externaljwt/apis/v1alpha1"
+	"k8s.io/kubernetes/pkg/serviceaccount"
+	externaljwtmetrics "k8s.io/kubernetes/pkg/serviceaccount/externaljwt/metrics"
+)
+
+func init() {
+	externaljwtmetrics.RegisterMetrics()
+}
+
+type VerificationKeys struct {
+	Keys            []serviceaccount.PublicKey
+	DataTimestamp   time.Time
+	NextRefreshHint time.Time
+}
+
+// New calls external signer to fill out supported keys.
+// It also starts a periodic sync of external keys.
+// In order for the key cache and external signing to work correctly, pass a context that will live as
+// long as the dependent process; is used to maintain the lifetime of the connection to external signer.
+func New(ctx context.Context, issuer, socketPath string, keySyncTimeout time.Duration, allowSigningWithNonOIDCKeys bool) (*Plugin, *keyCache, error) {
+	conn, err := grpc.Dial(
+		socketPath,
+		grpc.WithTransportCredentials(insecure.NewCredentials()),
+		grpc.WithAuthority("localhost"),
+		grpc.WithDefaultCallOptions(grpc.WaitForReady(true)),
+		grpc.WithContextDialer(func(ctx context.Context, path string) (net.Conn, error) {
+			return (&net.Dialer{}).DialContext(ctx, "unix", path)
+		}),
+		grpc.WithChainUnaryInterceptor(externaljwtmetrics.OuboundRequestMetricsInterceptor),
+	)
+	if err != nil {
+		return nil, nil, fmt.Errorf("while dialing grpc socket at %q: %w", socketPath, err)
+	}
+
+	plugin := newPlugin(issuer, conn, allowSigningWithNonOIDCKeys)
+
+	initialFillCtx, cancel := context.WithTimeout(ctx, keySyncTimeout)
+	defer cancel()
+
+	if err := plugin.keyCache.initialFill(initialFillCtx); err != nil {
+		return nil, nil, fmt.Errorf("while initially filling key cache: %w", err)
+	}
+	go plugin.keyCache.scheduleSync(ctx, keySyncTimeout)
+
+	go func() {
+		<-ctx.Done()
+		_ = conn.Close()
+	}()
+
+	return plugin, plugin.keyCache, nil
+}
+
+// enables plugging in an external jwt signer.
+type Plugin struct {
+	iss                         string
+	client                      externaljwtv1alpha1.ExternalJWTSignerClient
+	keyCache                    *keyCache
+	allowSigningWithNonOIDCKeys bool
+}
+
+// newPlugin constructs an implementation of external JWT signer plugin.
+func newPlugin(iss string, conn *grpc.ClientConn, allowSigningWithNonOIDCKeys bool) *Plugin {
+	client := externaljwtv1alpha1.NewExternalJWTSignerClient(conn)
+	plugin := &Plugin{
+		iss:                         iss,
+		client:                      client,
+		allowSigningWithNonOIDCKeys: allowSigningWithNonOIDCKeys,
+		keyCache:                    newKeyCache(client),
+	}
+	return plugin
+}
+
+// GenerateToken creates a service account token with the provided claims by
+// calling out to the external signer binary.
+func (p *Plugin) GenerateToken(ctx context.Context, claims *jwt.Claims, privateClaims interface{}) (string, error) {
+	jwt, err := p.signAndAssembleJWT(ctx, claims, privateClaims)
+	externaljwtmetrics.RecordTokenGenAttempt(err)
+	return jwt, err
+}
+
+func (p *Plugin) signAndAssembleJWT(ctx context.Context, claims *jwt.Claims, privateClaims interface{}) (string, error) {
+	payload, err := mergeClaims(p.iss, claims, privateClaims)
+	if err != nil {
+		return "", fmt.Errorf("while merging claims: %w", err)
+	}
+
+	payloadBase64 := base64.RawURLEncoding.EncodeToString(payload)
+
+	request := &externaljwtv1alpha1.SignJWTRequest{
+		Claims: payloadBase64,
+	}
+
+	response, err := p.client.Sign(ctx, request)
+	if err != nil {
+		return "", fmt.Errorf("while signing jwt: %w", err)
+	}
+
+	if err := p.validateJWTHeader(ctx, response); err != nil {
+		return "", fmt.Errorf("while validating header: %w", err)
+	}
+
+	if len(response.Signature) == 0 {
+		return "", fmt.Errorf("empty signature returned")
+	}
+
+	return response.Header + "." + payloadBase64 + "." + response.Signature, nil
+}
+
+// GetServiceMetadata returns metadata associated with externalJWTSigner
+// It Includes details like max token lifetime supported by externalJWTSigner, etc.
+func (p *Plugin) GetServiceMetadata(ctx context.Context) (*externaljwtv1alpha1.MetadataResponse, error) {
+	req := &externaljwtv1alpha1.MetadataRequest{}
+	return p.client.Metadata(ctx, req)
+}
+
+func (p *Plugin) validateJWTHeader(ctx context.Context, response *externaljwtv1alpha1.SignJWTResponse) error {
+	jsonBytes, err := base64.RawURLEncoding.DecodeString(response.Header)
+	if err != nil {
+		return fmt.Errorf("while unwrapping header: %w", err)
+	}
+
+	decoder := json.NewDecoder(bytes.NewBuffer(jsonBytes))
+	decoder.DisallowUnknownFields()
+
+	header := &struct {
+		Algorithm string `json:"alg,omitempty"`
+		KeyID     string `json:"kid,omitempty"`
+		Type      string `json:"typ,omitempty"`
+	}{}
+
+	if err := decoder.Decode(header); err != nil {
+		return fmt.Errorf("while parsing header JSON: %w", err)
+	}
+
+	if header.Type != "JWT" {
+		return fmt.Errorf("bad type")
+	}
+	if len(header.KeyID) == 0 {
+		return fmt.Errorf("key id missing")
+	}
+	if len(header.KeyID) > 1024 {
+		return fmt.Errorf("key id longer than 1 kb")
+	}
+	switch header.Algorithm {
+	// IMPORTANT: If this function is updated to support additional algorithms,
+	// JWTTokenGenerator, signerFromRSAPrivateKey, signerFromECDSAPrivateKey in
+	// kubernetes/pkg/serviceaccount/jwt.go must also be updated to support the same Algorithms.
+	case "RS256", "ES256", "ES384", "ES512":
+		// OK
+	default:
+		return fmt.Errorf("bad signing algorithm %q", header.Algorithm)
+	}
+
+	if !p.allowSigningWithNonOIDCKeys {
+		publicKeys := p.keyCache.GetPublicKeys(ctx, header.KeyID)
+		for _, key := range publicKeys {
+			// Such keys shall only be used for validating formerly issued tokens.
+			if key.ExcludeFromOIDCDiscovery {
+				return fmt.Errorf("key used for signing JWT (kid: %s) is excluded from OIDC discovery docs", header.KeyID)
+			}
+		}
+	}
+
+	return nil
+}
+
+func mergeClaims(iss string, claims *jwt.Claims, privateClaims interface{}) ([]byte, error) {
+	var out []byte
+	signer := payloadGrabber(func(payload []byte) { out = payload })
+	_, err := serviceaccount.GenerateToken(signer, iss, claims, privateClaims)
+	if len(out) == 0 {
+		return nil, fmt.Errorf("failed to marshal: %w", err)
+	}
+	return out, nil // error is safe to ignore as long as we have the payload bytes
+}
+
+var _ jose.Signer = payloadGrabber(nil)
+
+type payloadGrabber func(payload []byte)
+
+func (p payloadGrabber) Sign(payload []byte) (*jose.JSONWebSignature, error) {
+	p(payload)
+	return nil, jose.ErrUnprotectedNonce // return some error to stop after we have the payload
+}
+
+func (p payloadGrabber) Options() jose.SignerOptions { return jose.SignerOptions{} }

pkg/serviceaccount/externaljwt/plugin/plugin_test.go

@@ -0,0 +1,450 @@
+/*
+Copyright 2024 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package plugin
+
+import (
+	"context"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/base64"
+	"encoding/json"
+	"fmt"
+	"net"
+	"os"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/google/go-cmp/cmp"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/credentials/insecure"
+	"google.golang.org/protobuf/types/known/timestamppb"
+	"gopkg.in/square/go-jose.v2/jwt"
+
+	"k8s.io/kubernetes/pkg/serviceaccount"
+
+	externaljwtv1alpha1 "k8s.io/externaljwt/apis/v1alpha1"
+)
+
+var (
+	rsaKey1 *rsa.PrivateKey
+	rsaKey2 *rsa.PrivateKey
+)
+
+func init() {
+	var err error
+
+	rsaKey1, err = rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		panic("Error while generating first RSA key")
+	}
+
+	rsaKey2, err = rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		panic("Error while generating second RSA key")
+	}
+}
+
+func TestExternalTokenGenerator(t *testing.T) {
+	testCases := []struct {
+		desc string
+
+		publicClaims  jwt.Claims
+		privateClaims privateClaimsT
+
+		iss                         string
+		backendSetKeyID             string
+		backendSetAlgorithm         string
+		supportedKeys               map[string]supportedKeyT
+		allowSigningWithNonOIDCKeys bool
+
+		wantClaims unifiedClaimsT
+		wantErr    error
+	}{
+		{
+			desc: "correct token with correct claims returned",
+			publicClaims: jwt.Claims{
+				Subject: "some-subject",
+				Audience: jwt.Audience{
+					"some-audience-1",
+					"some-audience-2",
+				},
+				ID: "id-1",
+			},
+			privateClaims: privateClaimsT{
+				Kubernetes: kubernetesT{
+					Namespace: "foo",
+					Svcacct: refT{
+						Name: "default",
+						UID:  "abcdef",
+					},
+				},
+			},
+			iss:                 "some-issuer",
+			backendSetKeyID:     "key-id-1",
+			backendSetAlgorithm: "RS256",
+			supportedKeys: map[string]supportedKeyT{
+				"key-id-1": {
+					key: &rsaKey1.PublicKey,
+				},
+			},
+
+			wantClaims: unifiedClaimsT{
+				Issuer:  "some-issuer",
+				Subject: "some-subject",
+				Audience: jwt.Audience{
+					"some-audience-1",
+					"some-audience-2",
+				},
+				ID: "id-1",
+				Kubernetes: kubernetesT{
+					Namespace: "foo",
+					Svcacct: refT{
+						Name: "default",
+						UID:  "abcdef",
+					},
+				},
+			},
+		},
+		{
+			desc: "correct token with correct claims signed by key that's excluded from OIDC",
+			publicClaims: jwt.Claims{
+				Subject: "some-subject",
+				Audience: jwt.Audience{
+					"some-audience-1",
+					"some-audience-2",
+				},
+				ID: "id-1",
+			},
+			privateClaims: privateClaimsT{
+				Kubernetes: kubernetesT{
+					Namespace: "foo",
+					Svcacct: refT{
+						Name: "default",
+						UID:  "abcdef",
+					},
+				},
+			},
+			iss:                 "some-issuer",
+			backendSetKeyID:     "key-id-1",
+			backendSetAlgorithm: "RS256",
+			supportedKeys: map[string]supportedKeyT{
+				"key-id-1": {
+					key:             &rsaKey1.PublicKey,
+					excludeFromOidc: true,
+				},
+			},
+
+			wantErr: fmt.Errorf("while validating header: key used for signing JWT (kid: key-id-1) is excluded from OIDC discovery docs"),
+		},
+		{
+			desc: "token signed with key that's excluded from OIDC but validation is disabled",
+			publicClaims: jwt.Claims{
+				Subject: "some-subject",
+				Audience: jwt.Audience{
+					"some-audience-1",
+					"some-audience-2",
+				},
+				ID: "key-id-1",
+			},
+			privateClaims: privateClaimsT{
+				Kubernetes: kubernetesT{
+					Namespace: "foo",
+					Svcacct: refT{
+						Name: "default",
+						UID:  "abcdef",
+					},
+				},
+			},
+			iss:                 "some-issuer",
+			backendSetKeyID:     "key-id-1",
+			backendSetAlgorithm: "RS256",
+			supportedKeys: map[string]supportedKeyT{
+				"key-id-1": {
+					key:             &rsaKey1.PublicKey,
+					excludeFromOidc: true,
+				},
+			},
+			allowSigningWithNonOIDCKeys: true,
+
+			wantClaims: unifiedClaimsT{
+				Issuer:  "some-issuer",
+				Subject: "some-subject",
+				Audience: jwt.Audience{
+					"some-audience-1",
+					"some-audience-2",
+				},
+				ID: "key-id-1",
+				Kubernetes: kubernetesT{
+					Namespace: "foo",
+					Svcacct: refT{
+						Name: "default",
+						UID:  "abcdef",
+					},
+				},
+			},
+		},
+		{
+			desc:                "empty key ID returned from signer",
+			iss:                 "some-issuer",
+			backendSetKeyID:     "",
+			backendSetAlgorithm: "RS256",
+			supportedKeys: map[string]supportedKeyT{
+				"key-id-1": {
+					key:             &rsaKey1.PublicKey,
+					excludeFromOidc: true,
+				},
+			},
+			wantErr: fmt.Errorf("while validating header: key id missing"),
+		},
+		{
+			desc:                "key id longer than 1024 bytes returned from signer",
+			iss:                 "some-issuer",
+			backendSetKeyID:     string(make([]byte, 1025)),
+			backendSetAlgorithm: "RS256",
+			supportedKeys: map[string]supportedKeyT{
+				"key-id-1": {
+					key:             &rsaKey1.PublicKey,
+					excludeFromOidc: true,
+				},
+			},
+			wantErr: fmt.Errorf("while validating header: key id longer than 1 kb"),
+		},
+		{
+			desc:                "unsupported alg returned from signer",
+			iss:                 "some-issuer",
+			backendSetKeyID:     "key-id-1",
+			backendSetAlgorithm: "something-unsupported",
+			supportedKeys: map[string]supportedKeyT{
+				"key-id-1": {
+					key:             &rsaKey1.PublicKey,
+					excludeFromOidc: true,
+				},
+			},
+			wantErr: fmt.Errorf("while validating header: bad signing algorithm \"something-unsupported\""),
+		},
+		{
+			desc:                "empty alg returned from signer",
+			iss:                 "some-issuer",
+			backendSetKeyID:     "key-id-1",
+			backendSetAlgorithm: "",
+			supportedKeys: map[string]supportedKeyT{
+				"key-id-1": {
+					key:             &rsaKey1.PublicKey,
+					excludeFromOidc: true,
+				},
+			},
+			wantErr: fmt.Errorf("while validating header: bad signing algorithm \"\""),
+		},
+	}
+
+	for i, tc := range testCases {
+		t.Run(tc.desc, func(t *testing.T) {
+			ctx := context.Background()
+
+			sockname := fmt.Sprintf("@test-external-token-generator-%d.sock", i)
+			t.Cleanup(func() { _ = os.Remove(sockname) })
+
+			addr := &net.UnixAddr{Name: sockname, Net: "unix"}
+			listener, err := net.ListenUnix(addr.Network(), addr)
+			if err != nil {
+				t.Fatalf("Failed to start fake backend: %v", err)
+			}
+
+			grpcServer := grpc.NewServer()
+
+			backend := &dummyExtrnalSigner{
+				keyID:              tc.backendSetKeyID,
+				signingAlgorithm:   tc.backendSetAlgorithm,
+				signature:          "abcdef",
+				supportedKeys:      tc.supportedKeys,
+				refreshHintSeconds: 10,
+				DataTimeStamp:      timestamppb.New(time.Time{}),
+			}
+			externaljwtv1alpha1.RegisterExternalJWTSignerServer(grpcServer, backend)
+
+			go func() {
+				if err := grpcServer.Serve(listener); err != nil {
+					panic(fmt.Errorf("error returned from grpcServer: %w", err))
+				}
+			}()
+			defer grpcServer.Stop()
+
+			clientConn, err := grpc.DialContext(
+				ctx,
+				sockname,
+				grpc.WithContextDialer(func(ctx context.Context, path string) (net.Conn, error) {
+					return (&net.Dialer{}).DialContext(ctx, "unix", path)
+				}),
+				grpc.WithTransportCredentials(insecure.NewCredentials()),
+			)
+			if err != nil {
+				t.Fatalf("Failed to dial buffconn client: %v", err)
+			}
+			defer func() { _ = clientConn.Close() }()
+
+			plugin := newPlugin(tc.iss, clientConn, tc.allowSigningWithNonOIDCKeys)
+			err = plugin.keyCache.initialFill(ctx)
+			if err != nil {
+				t.Fatalf("initial fill failed: %v", err)
+			}
+
+			gotToken, err := plugin.GenerateToken(ctx, &tc.publicClaims, tc.privateClaims)
+			if err != nil && tc.wantErr != nil {
+				if err.Error() != tc.wantErr.Error() {
+					t.Fatalf("want error: %v, got error: %v", tc.wantErr, err)
+				}
+				return
+			} else if err != nil && tc.wantErr == nil {
+				t.Fatalf("Unexpected error generating token: %v", err)
+			} else if err == nil && tc.wantErr != nil {
+				t.Fatalf("Wanted error %q, but got nil", tc.wantErr)
+			}
+
+			tokenPieces := strings.Split(gotToken, ".")
+			payloadBase64 := tokenPieces[1]
+
+			gotClaimBytes, err := base64.RawURLEncoding.DecodeString(payloadBase64)
+			if err != nil {
+				t.Fatalf("error converting received tokens to bytes: %v", err)
+			}
+
+			gotClaims := unifiedClaimsT{}
+			if err := json.Unmarshal(gotClaimBytes, &gotClaims); err != nil {
+				t.Fatalf("Error while unmarshaling claims from backend: %v", err)
+			}
+
+			if diff := cmp.Diff(gotClaims, tc.wantClaims); diff != "" {
+				t.Fatalf("Bad claims; diff (-got +want):\n%s", diff)
+			}
+
+			// Don't check header or signature values since we're not testing
+			// our (fake) backends.
+		})
+	}
+
+}
+
+func sortPublicKeySlice(a, b serviceaccount.PublicKey) bool {
+	return a.KeyID < b.KeyID
+}
+
+type headerT struct {
+	Algorithm string `json:"alg"`
+	KeyID     string `json:"kid,omitempty"`
+	Type      string `json:"typ"`
+}
+
+type unifiedClaimsT struct {
+	Issuer     string           `json:"iss,omitempty"`
+	Subject    string           `json:"sub,omitempty"`
+	Audience   jwt.Audience     `json:"aud,omitempty"`
+	Expiry     *jwt.NumericDate `json:"exp,omitempty"`
+	NotBefore  *jwt.NumericDate `json:"nbf,omitempty"`
+	IssuedAt   *jwt.NumericDate `json:"iat,omitempty"`
+	ID         string           `json:"jti,omitempty"`
+	Kubernetes kubernetesT      `json:"kubernetes.io,omitempty"`
+}
+
+type privateClaimsT struct {
+	Kubernetes kubernetesT `json:"kubernetes.io,omitempty"`
+}
+
+type kubernetesT struct {
+	Namespace string           `json:"namespace,omitempty"`
+	Svcacct   refT             `json:"serviceaccount,omitempty"`
+	Pod       *refT            `json:"pod,omitempty"`
+	Secret    *refT            `json:"secret,omitempty"`
+	Node      *refT            `json:"node,omitempty"`
+	WarnAfter *jwt.NumericDate `json:"warnafter,omitempty"`
+}
+
+type refT struct {
+	Name string `json:"name,omitempty"`
+	UID  string `json:"uid,omitempty"`
+}
+
+type supportedKeyT struct {
+	key             *rsa.PublicKey
+	excludeFromOidc bool
+}
+
+type dummyExtrnalSigner struct {
+	externaljwtv1alpha1.UnimplementedExternalJWTSignerServer
+
+	// required for Sign()
+	keyID            string
+	signingAlgorithm string
+	signature        string
+
+	// required for FetchKeys()
+	keyLock               sync.Mutex
+	supportedKeys         map[string]supportedKeyT
+	refreshHintSeconds    int
+	DataTimeStamp         *timestamppb.Timestamp
+	SupportedKeysOverride []*externaljwtv1alpha1.Key
+}
+
+func (des *dummyExtrnalSigner) Sign(ctx context.Context, r *externaljwtv1alpha1.SignJWTRequest) (*externaljwtv1alpha1.SignJWTResponse, error) {
+	header := &headerT{
+		Type:      "JWT",
+		Algorithm: des.signingAlgorithm,
+		KeyID:     des.keyID,
+	}
+
+	headerJSON, err := json.Marshal(header)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create header for JWT response")
+	}
+
+	resp := &externaljwtv1alpha1.SignJWTResponse{
+		Header:    base64.RawURLEncoding.EncodeToString(headerJSON),
+		Signature: des.signature,
+	}
+	return resp, nil
+}
+
+func (des *dummyExtrnalSigner) FetchKeys(ctx context.Context, r *externaljwtv1alpha1.FetchKeysRequest) (*externaljwtv1alpha1.FetchKeysResponse, error) {
+	des.keyLock.Lock()
+	defer des.keyLock.Unlock()
+
+	pbKeys := []*externaljwtv1alpha1.Key{}
+	if des.SupportedKeysOverride != nil {
+		pbKeys = des.SupportedKeysOverride
+	} else {
+		for kid, k := range des.supportedKeys {
+			keyBytes, err := x509.MarshalPKIXPublicKey(k.key)
+			if err != nil {
+				return nil, fmt.Errorf("while marshaling key: %w", err)
+			}
+			pbKey := &externaljwtv1alpha1.Key{
+				KeyId:                    kid,
+				Key:                      keyBytes,
+				ExcludeFromOidcDiscovery: k.excludeFromOidc,
+			}
+			pbKeys = append(pbKeys, pbKey)
+		}
+	}
+
+	return &externaljwtv1alpha1.FetchKeysResponse{
+		Keys:               pbKeys,
+		DataTimestamp:      des.DataTimeStamp,
+		RefreshHintSeconds: int64(des.refreshHintSeconds),
+	}, nil
+}

pkg/serviceaccount/externaljwt/plugin/testing/v1alpha1/externalsigner_mock.go

@@ -0,0 +1,279 @@
+/*
+Copyright 2024 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package v1alpha1
+
+import (
+	"context"
+	"crypto"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/base64"
+	"encoding/json"
+	"fmt"
+	"net"
+	"os"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"google.golang.org/grpc"
+	"google.golang.org/protobuf/types/known/timestamppb"
+
+	"k8s.io/externaljwt/apis/v1alpha1"
+	"k8s.io/klog/v2"
+)
+
+type MockSigner struct {
+	socketPath string
+	server     *grpc.Server
+	listener   net.Listener
+
+	SigningKey                *rsa.PrivateKey
+	SigningKeyID              string
+	SigningAlg                string
+	TokenType                 string
+	SupportedKeys             atomic.Pointer[map[string]KeyT]
+	AckKeyFetch               chan bool
+	MaxTokenExpirationSeconds int64
+
+	FetchError    error
+	MetadataError error
+	errorLock     sync.RWMutex
+}
+
+type KeyT struct {
+	Key                      []byte
+	ExcludeFromOidcDiscovery bool
+}
+
+// NewMockSigner starts and returns a new MockSigner
+// It servers on the provided socket.
+func NewMockSigner(t *testing.T, socketPath string) *MockSigner {
+	server := grpc.NewServer()
+
+	m := &MockSigner{
+		socketPath:                socketPath,
+		server:                    server,
+		AckKeyFetch:               make(chan bool),
+		MaxTokenExpirationSeconds: 10 * 60, // 10m
+	}
+
+	if err := m.Reset(); err != nil {
+		t.Fatalf("failed to load keys for mock signer: %v", err)
+	}
+
+	v1alpha1.RegisterExternalJWTSignerServer(server, m)
+	if err := m.start(t); err != nil {
+		t.Fatalf("failed to start Mock Signer with error: %v", err)
+	}
+
+	t.Cleanup(m.CleanUp)
+	if err := m.waitForMockServerToStart(); err != nil {
+		t.Fatalf("failed to start Mock Signer with error %v", err)
+	}
+
+	return m
+}
+
+func (m *MockSigner) Sign(ctx context.Context, req *v1alpha1.SignJWTRequest) (*v1alpha1.SignJWTResponse, error) {
+
+	header := &struct {
+		Algorithm string `json:"alg,omitempty"`
+		KeyID     string `json:"kid,omitempty"`
+		Type      string `json:"typ,omitempty"`
+	}{
+		Type:      m.TokenType,
+		Algorithm: m.SigningAlg,
+		KeyID:     m.SigningKeyID,
+	}
+
+	headerJSON, err := json.Marshal(header)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create header for JWT response")
+	}
+
+	base64Header := base64.RawURLEncoding.EncodeToString(headerJSON)
+
+	toBeSignedHash := hashBytes([]byte(base64Header + "." + req.Claims))
+
+	signature, err := rsa.SignPKCS1v15(rand.Reader, m.SigningKey, crypto.SHA256, toBeSignedHash)
+	if err != nil {
+		return nil, fmt.Errorf("unable to sign payload: %w", err)
+	}
+
+	return &v1alpha1.SignJWTResponse{
+		Header:    base64Header,
+		Signature: base64.RawURLEncoding.EncodeToString(signature),
+	}, nil
+}
+
+func (m *MockSigner) FetchKeys(ctx context.Context, req *v1alpha1.FetchKeysRequest) (*v1alpha1.FetchKeysResponse, error) {
+	m.errorLock.RLocker().Lock()
+	defer m.errorLock.RLocker().Unlock()
+	if m.FetchError != nil {
+		return nil, m.FetchError
+	}
+
+	keys := []*v1alpha1.Key{}
+
+	for id, k := range *m.SupportedKeys.Load() {
+		keys = append(keys, &v1alpha1.Key{
+			KeyId:                    id,
+			Key:                      k.Key,
+			ExcludeFromOidcDiscovery: k.ExcludeFromOidcDiscovery,
+		})
+	}
+
+	select {
+	case <-m.AckKeyFetch:
+	default:
+	}
+
+	return &v1alpha1.FetchKeysResponse{
+		RefreshHintSeconds: 5,
+		DataTimestamp:      &timestamppb.Timestamp{Seconds: time.Now().Unix()},
+		Keys:               keys,
+	}, nil
+}
+
+func (m *MockSigner) Metadata(ctx context.Context, req *v1alpha1.MetadataRequest) (*v1alpha1.MetadataResponse, error) {
+	m.errorLock.RLocker().Lock()
+	defer m.errorLock.RLocker().Unlock()
+	if m.MetadataError != nil {
+		return nil, m.MetadataError
+	}
+	return &v1alpha1.MetadataResponse{
+		MaxTokenExpirationSeconds: m.MaxTokenExpirationSeconds,
+	}, nil
+}
+
+// Reset genrate and adds signing/supported keys to MockSigner instance.
+func (m *MockSigner) Reset() error {
+
+	priv1, pub1, err := generateKeyPair()
+	if err != nil {
+		return err
+	}
+
+	_, pub2, err := generateKeyPair()
+	if err != nil {
+		return err
+	}
+
+	_, pub3, err := generateKeyPair()
+	if err != nil {
+		return err
+	}
+
+	m.SigningKey = priv1
+	m.SigningKeyID = "kid-1"
+	m.SigningAlg = "RS256"
+	m.TokenType = "JWT"
+	m.SupportedKeys.Store(&map[string]KeyT{
+		"kid-1": {Key: pub1},
+		"kid-2": {Key: pub2},
+		"kid-3": {Key: pub3},
+	})
+	m.errorLock.Lock()
+	defer m.errorLock.Unlock()
+	m.FetchError = nil
+	m.MetadataError = nil
+	m.MaxTokenExpirationSeconds = 10 * 60 // 10m
+
+	return nil
+}
+
+// start makes the gRpc MockServer listen on unix socket.
+func (m *MockSigner) start(t *testing.T) error {
+	var err error
+
+	m.listener, err = net.Listen("unix", m.socketPath)
+	if err != nil {
+		return fmt.Errorf("failed to listen on the unix socket, error: %w", err)
+	}
+
+	klog.Infof("Starting Mock Signer at socketPath %s", m.socketPath)
+	go func() {
+		if err := m.server.Serve(m.listener); err != nil {
+			t.Error(err)
+		}
+	}()
+	klog.Infof("Mock Signer listening at socketPath %s", m.socketPath)
+
+	return nil
+}
+
+// waitForMockServerToStart waits until Mock signer is ready to server.
+// waits for a max of 30s before failing.
+func (m *MockSigner) waitForMockServerToStart() error {
+	var gRPCErr error
+
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second*30)
+	defer cancel()
+	doneCh := ctx.Done()
+
+	for range 30 {
+		select {
+		case <-doneCh:
+			return fmt.Errorf("failed to start Mock signer: %w", ctx.Err())
+		default:
+		}
+		if _, gRPCErr = m.FetchKeys(context.Background(), &v1alpha1.FetchKeysRequest{}); gRPCErr == nil {
+			break
+		}
+		time.Sleep(time.Second)
+	}
+
+	if gRPCErr != nil {
+		return fmt.Errorf("failed to start Mock signer, gRPC error: %w", gRPCErr)
+	}
+
+	return nil
+}
+
+// CleanUp stops gRPC server and the underlying listener.
+func (m *MockSigner) CleanUp() {
+	m.server.Stop()
+	_ = m.listener.Close()
+	_ = os.Remove(m.socketPath)
+}
+
+func generateKeyPair() (*rsa.PrivateKey, []byte, error) {
+
+	// Generate a new private key
+	privateKey, err := rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		klog.Errorf("Error generating private key: %v", err)
+		return nil, nil, err
+	}
+
+	publicKeyBytes, err := x509.MarshalPKIXPublicKey(&privateKey.PublicKey)
+	if err != nil {
+		klog.Errorf("Error marshaling public key: %v", err)
+		return nil, nil, err
+	}
+
+	return privateKey, publicKeyBytes, nil
+}
+
+func hashBytes(bytes []byte) []byte {
+	hasher := crypto.SHA256.New()
+	hasher.Write(bytes)
+	return hasher.Sum(nil)
+}

pkg/serviceaccount/jwt.go

@@ -53,7 +53,7 @@ type TokenGenerator interface {
 	// the payload object. Public claims take precedent over private
 	// claims i.e. if both claims and privateClaims have an "exp" field,
 	// the value in claims will be used.
-	GenerateToken(claims *jwt.Claims, privateClaims interface{}) (string, error)
+	GenerateToken(ctx context.Context, claims *jwt.Claims, privateClaims interface{}) (string, error)
 }
 
 // JWTTokenGenerator returns a TokenGenerator that generates signed JWT tokens, using the given privateKey.
@@ -118,8 +118,9 @@ func signerFromRSAPrivateKey(keyPair *rsa.PrivateKey) (jose.Signer, error) {
 	}
 
 	// IMPORTANT: If this function is updated to support additional key sizes,
-	// algorithmForPublicKey in serviceaccount/openidmetadata.go must also be
-	// updated to support the same key sizes. Today we only support RS256.
+	// algorithmForPublicKey in serviceaccount/openidmetadata.go and
+	// validateJWTHeader in externaljwt/pkg/plugin/plugin.go must also
+	// be updated to support the same key sizes. Today we only support RS256.
 
 	// Wrap the RSA keypair in a JOSE JWK with the designated key ID.
 	privateJWK := &jose.JSONWebKey{
@@ -146,6 +147,11 @@ func signerFromRSAPrivateKey(keyPair *rsa.PrivateKey) (jose.Signer, error) {
 
 func signerFromECDSAPrivateKey(keyPair *ecdsa.PrivateKey) (jose.Signer, error) {
 	var alg jose.SignatureAlgorithm
+
+	// IMPORTANT: If this function is updated to support additional algorithms,
+	// validateJWTHeader in externaljwt/pkg/plugin/plugin.go must also be updated
+	// to support the same Algorithms. Today we only support "ES256", "ES384", "ES512".
+
 	switch keyPair.Curve {
 	case elliptic.P256():
 		alg = jose.ES256
@@ -211,15 +217,8 @@ type jwtTokenGenerator struct {
 	signer jose.Signer
 }
 
-func (j *jwtTokenGenerator) GenerateToken(claims *jwt.Claims, privateClaims interface{}) (string, error) {
-	// claims are applied in reverse precedence
-	return jwt.Signed(j.signer).
-		Claims(privateClaims).
-		Claims(claims).
-		Claims(&jwt.Claims{
-			Issuer: j.iss,
-		}).
-		CompactSerialize()
+func (j *jwtTokenGenerator) GenerateToken(ctx context.Context, claims *jwt.Claims, privateClaims interface{}) (string, error) {
+	return GenerateToken(j.signer, j.iss, claims, privateClaims)
 }
 
 // JWTTokenAuthenticator authenticates tokens as JWT tokens produced by JWTTokenGenerator
@@ -257,12 +256,13 @@ type PublicKeysGetter interface {
 
 	// GetPublicKeys returns public keys to use for verifying a token with the given key id.
 	// keyIDHint may be empty if the token did not have a kid header, or if all public keys are desired.
-	GetPublicKeys(keyIDHint string) []PublicKey
+	GetPublicKeys(ctx context.Context, keyIDHint string) []PublicKey
 }
 
 type PublicKey struct {
 	KeyID                    string
 	PublicKey                interface{}
+	ExcludeFromOIDCDiscovery bool
 }
 
 type staticPublicKeysGetter struct {
@@ -306,7 +306,7 @@ func (s staticPublicKeysGetter) GetCacheAgeMaxSeconds() int {
 	return 3600
 }
 
-func (s staticPublicKeysGetter) GetPublicKeys(keyID string) []PublicKey {
+func (s staticPublicKeysGetter) GetPublicKeys(ctx context.Context, keyID string) []PublicKey {
 	if len(keyID) == 0 {
 		return s.allPublicKeys
 	}
@@ -357,7 +357,7 @@ func (j *jwtTokenAuthenticator[PrivateClaims]) AuthenticateToken(ctx context.Con
 		found   bool
 		errlist []error
 	)
-	keys := j.keysGetter.GetPublicKeys(kid)
+	keys := j.keysGetter.GetPublicKeys(ctx, kid)
 	if len(keys) == 0 {
 		return nil, false, fmt.Errorf("invalid signature, no keys found")
 	}
@@ -438,3 +438,15 @@ func (j *jwtTokenAuthenticator[PrivateClaims]) hasCorrectIssuer(tokenData string
 	}
 	return j.issuers[claims.Issuer]
 }
+
+// GenerateToken is shared between internal and external signer code to ensure that claim merging logic remains consistent between them.
+func GenerateToken(signer jose.Signer, iss string, claims *jwt.Claims, privateClaims interface{}) (string, error) {
+	// claims are applied in reverse precedence
+	return jwt.Signed(signer).
+		Claims(privateClaims).
+		Claims(claims).
+		Claims(&jwt.Claims{
+			Issuer: iss,
+		}).
+		CompactSerialize()
+}

pkg/serviceaccount/jwt_test.go

@@ -174,7 +174,8 @@ func TestTokenGenerateAndValidate(t *testing.T) {
 	if err != nil {
 		t.Fatalf("error making generator: %v", err)
 	}
-	rsaToken, err := rsaGenerator.GenerateToken(serviceaccount.LegacyClaims(*serviceAccount, *rsaSecret))
+	c, pc := serviceaccount.LegacyClaims(*serviceAccount, *rsaSecret)
+	rsaToken, err := rsaGenerator.GenerateToken(context.TODO(), c, pc)
 	if err != nil {
 		t.Fatalf("error generating token: %v", err)
 	}
@@ -188,7 +189,8 @@ func TestTokenGenerateAndValidate(t *testing.T) {
 	checkJSONWebSignatureHasKeyID(t, rsaToken, rsaKeyID)
 
 	// Generate RSA token with invalidAutoSecret
-	invalidAutoSecretToken, err := rsaGenerator.GenerateToken(serviceaccount.LegacyClaims(*serviceAccount, *invalidAutoSecret))
+	c, pc = serviceaccount.LegacyClaims(*serviceAccount, *invalidAutoSecret)
+	invalidAutoSecretToken, err := rsaGenerator.GenerateToken(context.TODO(), c, pc)
 	if err != nil {
 		t.Fatalf("error generating token: %v", err)
 	}
@@ -217,7 +219,8 @@ func TestTokenGenerateAndValidate(t *testing.T) {
 	if err != nil {
 		t.Fatalf("error making generator: %v", err)
 	}
-	badIssuerToken, err := badIssuerGenerator.GenerateToken(serviceaccount.LegacyClaims(*serviceAccount, *rsaSecret))
+	c, pc = serviceaccount.LegacyClaims(*serviceAccount, *rsaSecret)
+	badIssuerToken, err := badIssuerGenerator.GenerateToken(context.TODO(), c, pc)
 	if err != nil {
 		t.Fatalf("error generating token: %v", err)
 	}
@@ -227,7 +230,8 @@ func TestTokenGenerateAndValidate(t *testing.T) {
 	if err != nil {
 		t.Fatalf("error making generator: %v", err)
 	}
-	differentIssuerToken, err := differentIssuerGenerator.GenerateToken(serviceaccount.LegacyClaims(*serviceAccount, *rsaSecret))
+	c, pc = serviceaccount.LegacyClaims(*serviceAccount, *rsaSecret)
+	differentIssuerToken, err := differentIssuerGenerator.GenerateToken(context.TODO(), c, pc)
 	if err != nil {
 		t.Fatalf("error generating token: %v", err)
 	}
@@ -394,7 +398,7 @@ func TestTokenGenerateAndValidate(t *testing.T) {
 		authn := serviceaccount.JWTTokenAuthenticator([]string{serviceaccount.LegacyIssuer, "bar"}, keysGetter, auds, validator)
 
 		// An invalid, non-JWT token should always fail
-		ctx := authenticator.WithAudiences(context.Background(), auds)
+		ctx := authenticator.WithAudiences(context.TODO(), auds)
 		if _, ok, err := authn.AuthenticateToken(ctx, "invalid token"); err != nil || ok {
 			t.Errorf("%s: Expected err=nil, ok=false for non-JWT token", k)
 			continue
@@ -445,15 +449,15 @@ type keyIDPrefixer struct {
 	keyIDPrefix string
 }
 
-func (k *keyIDPrefixer) GetPublicKeys(keyIDHint string) []serviceaccount.PublicKey {
+func (k *keyIDPrefixer) GetPublicKeys(ctx context.Context, keyIDHint string) []serviceaccount.PublicKey {
 	if k.keyIDPrefix == "" {
-		return k.PublicKeysGetter.GetPublicKeys(keyIDHint)
+		return k.PublicKeysGetter.GetPublicKeys(context.TODO(), keyIDHint)
 	}
 	if keyIDHint != "" {
 		keyIDHint = k.keyIDPrefix + keyIDHint
 	}
 	var retval []serviceaccount.PublicKey
-	for _, key := range k.PublicKeysGetter.GetPublicKeys(keyIDHint) {
+	for _, key := range k.PublicKeysGetter.GetPublicKeys(context.TODO(), keyIDHint) {
 		key.KeyID = k.keyIDPrefix + key.KeyID
 		retval = append(retval, key)
 	}
@@ -503,7 +507,8 @@ func generateECDSAToken(t *testing.T, iss string, serviceAccount *v1.ServiceAcco
 	if err != nil {
 		t.Fatalf("error making generator: %v", err)
 	}
-	ecdsaToken, err := ecdsaGenerator.GenerateToken(serviceaccount.LegacyClaims(*serviceAccount, *ecdsaSecret))
+	c, pc := serviceaccount.LegacyClaims(*serviceAccount, *ecdsaSecret)
+	ecdsaToken, err := ecdsaGenerator.GenerateToken(context.TODO(), c, pc)
 	if err != nil {
 		t.Fatalf("error generating token: %v", err)
 	}
@@ -590,17 +595,17 @@ func TestStaticPublicKeysGetter(t *testing.T) {
 				t.Fatalf("unexpected construction error: %v", err)
 			}
 
-			bogusKeys := getter.GetPublicKeys("bogus")
+			bogusKeys := getter.GetPublicKeys(context.TODO(), "bogus")
 			if len(bogusKeys) != 0 {
 				t.Fatalf("unexpected bogus keys: %#v", bogusKeys)
 			}
 
-			allKeys := getter.GetPublicKeys("")
+			allKeys := getter.GetPublicKeys(context.TODO(), "")
 			if !reflect.DeepEqual(tc.ExpectKeys, allKeys) {
 				t.Fatalf("unexpected keys: %#v", allKeys)
 			}
 			for _, key := range allKeys {
-				keysByID := getter.GetPublicKeys(key.KeyID)
+				keysByID := getter.GetPublicKeys(context.TODO(), key.KeyID)
 				if len(keysByID) != 1 {
 					t.Fatalf("expected 1 key for id %s, got %d", key.KeyID, len(keysByID))
 				}

pkg/serviceaccount/openidmetadata.go

@@ -17,6 +17,7 @@ limitations under the License.
 package serviceaccount
 
 import (
+	"context"
 	"crypto"
 	"crypto/ecdsa"
 	"crypto/elliptic"
@@ -75,7 +76,17 @@ func (p *openidConfigProvider) Enqueue() {
 	}
 }
 func (p *openidConfigProvider) Update() error {
-	pubKeys := p.pubKeyGetter.GetPublicKeys("")
+	pubKeys := []PublicKey{}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+	unfilteredPubKeys := p.pubKeyGetter.GetPublicKeys(ctx, "")
+	for _, key := range unfilteredPubKeys {
+		if !key.ExcludeFromOIDCDiscovery {
+			pubKeys = append(pubKeys, key)
+		}
+	}
+
 	if len(pubKeys) == 0 {
 		return fmt.Errorf("no keys provided for validating keyset")
 	}

pkg/serviceaccount/openidmetadata_test.go

@@ -17,6 +17,7 @@ limitations under the License.
 package serviceaccount_test
 
 import (
+	"context"
 	"crypto/ecdsa"
 	"crypto/rsa"
 	"crypto/x509"
@@ -161,10 +162,18 @@ func expectConfiguration(t *testing.T, reqURL string, want Configuration) {
 func TestServeKeys(t *testing.T) {
 	wantPubRSA := getPublicKey(rsaPublicKey).(*rsa.PublicKey)
 	wantPubECDSA := getPublicKey(ecdsaPublicKey).(*ecdsa.PublicKey)
+
+	alternateGetter, err := serviceaccount.StaticPublicKeysGetter([]interface{}{wantPubRSA})
+	if err != nil {
+		t.Fatal(err)
+	}
+
 	var serveKeysTests = []struct {
 		Name               string
 		Keys               []interface{}
 		WantKeys           []jose.JSONWebKey
+		updatedKeysGetter  serviceaccount.PublicKeysGetter
+		WantKeysPostUpdate []jose.JSONWebKey
 	}{
 		{
 			Name: "configured public keys",
@@ -220,6 +229,97 @@ func TestServeKeys(t *testing.T) {
 				},
 			},
 		},
+		{
+			Name: "configured public keys reacting to update",
+			Keys: []interface{}{
+				getPublicKey(rsaPublicKey),
+				getPublicKey(ecdsaPublicKey),
+			},
+			WantKeys: []jose.JSONWebKey{
+				{
+					Algorithm:                   "RS256",
+					Key:                         wantPubRSA,
+					KeyID:                       rsaKeyID,
+					Use:                         "sig",
+					Certificates:                []*x509.Certificate{},
+					CertificateThumbprintSHA1:   []uint8{},
+					CertificateThumbprintSHA256: []uint8{},
+				},
+				{
+					Algorithm:                   "ES256",
+					Key:                         wantPubECDSA,
+					KeyID:                       ecdsaKeyID,
+					Use:                         "sig",
+					Certificates:                []*x509.Certificate{},
+					CertificateThumbprintSHA1:   []uint8{},
+					CertificateThumbprintSHA256: []uint8{},
+				},
+			},
+			updatedKeysGetter: alternateGetter,
+			WantKeysPostUpdate: []jose.JSONWebKey{
+				{
+					Algorithm:                   "RS256",
+					Key:                         wantPubRSA,
+					KeyID:                       rsaKeyID,
+					Use:                         "sig",
+					Certificates:                []*x509.Certificate{},
+					CertificateThumbprintSHA1:   []uint8{},
+					CertificateThumbprintSHA256: []uint8{},
+				},
+			},
+		},
+		{
+			Name: "configured public keys reacting to update while excluding keys",
+			Keys: []interface{}{
+				getPublicKey(rsaPublicKey),
+				getPublicKey(ecdsaPublicKey),
+			},
+			WantKeys: []jose.JSONWebKey{
+				{
+					Algorithm:                   "RS256",
+					Key:                         wantPubRSA,
+					KeyID:                       rsaKeyID,
+					Use:                         "sig",
+					Certificates:                []*x509.Certificate{},
+					CertificateThumbprintSHA1:   []uint8{},
+					CertificateThumbprintSHA256: []uint8{},
+				},
+				{
+					Algorithm:                   "ES256",
+					Key:                         wantPubECDSA,
+					KeyID:                       ecdsaKeyID,
+					Use:                         "sig",
+					Certificates:                []*x509.Certificate{},
+					CertificateThumbprintSHA1:   []uint8{},
+					CertificateThumbprintSHA256: []uint8{},
+				},
+			},
+			updatedKeysGetter: dummyPublicKeyGetter{
+				keys: []serviceaccount.PublicKey{
+					{
+						KeyID:                    rsaKeyID,
+						PublicKey:                wantPubRSA,
+						ExcludeFromOIDCDiscovery: true,
+					},
+					{
+						KeyID:                    ecdsaKeyID,
+						PublicKey:                wantPubECDSA,
+						ExcludeFromOIDCDiscovery: false,
+					},
+				},
+			},
+			WantKeysPostUpdate: []jose.JSONWebKey{
+				{
+					Algorithm:                   "ES256",
+					Key:                         wantPubECDSA,
+					KeyID:                       ecdsaKeyID,
+					Use:                         "sig",
+					Certificates:                []*x509.Certificate{},
+					CertificateThumbprintSHA1:   []uint8{},
+					CertificateThumbprintSHA256: []uint8{},
+				},
+			},
+		},
 	}
 
 	for _, tt := range serveKeysTests {
@@ -228,10 +328,6 @@ func TestServeKeys(t *testing.T) {
 			if err != nil {
 				t.Fatal(err)
 			}
-			updatedKeysGetter, err := serviceaccount.StaticPublicKeysGetter([]interface{}{wantPubRSA})
-			if err != nil {
-				t.Fatal(err)
-			}
 			keysGetter := &proxyKeyGetter{PublicKeysGetter: initialKeysGetter}
 			s, _ := setupServer(t, exampleIssuer, keysGetter)
 			defer s.Close()
@@ -239,23 +335,17 @@ func TestServeKeys(t *testing.T) {
 			reqURL := s.URL + "/openid/v1/jwks"
 			expectKeys(t, reqURL, tt.WantKeys)
 
+			if tt.updatedKeysGetter != nil {
 				// modify the underlying keys, expect the same response
-			keysGetter.PublicKeysGetter = updatedKeysGetter
+				keysGetter.PublicKeysGetter = tt.updatedKeysGetter
 				expectKeys(t, reqURL, tt.WantKeys)
 
 				// notify the metadata the keys changed, expected a modified response
 				for _, listener := range keysGetter.listeners {
 					listener.Enqueue()
 				}
-			expectKeys(t, reqURL, []jose.JSONWebKey{{
-				Algorithm:                   "RS256",
-				Key:                         wantPubRSA,
-				KeyID:                       rsaKeyID,
-				Use:                         "sig",
-				Certificates:                []*x509.Certificate{},
-				CertificateThumbprintSHA1:   []uint8{},
-				CertificateThumbprintSHA256: []uint8{},
-			}})
+				expectKeys(t, reqURL, tt.WantKeysPostUpdate)
+			}
 		})
 	}
 }
@@ -479,3 +569,19 @@ func TestNewOpenIDMetadata(t *testing.T) {
 		})
 	}
 }
+
+type dummyPublicKeyGetter struct {
+	keys []serviceaccount.PublicKey
+}
+
+func (d dummyPublicKeyGetter) AddListener(listener serviceaccount.Listener) {
+	// no-op
+}
+
+func (d dummyPublicKeyGetter) GetCacheAgeMaxSeconds() int {
+	return 3600
+}
+
+func (d dummyPublicKeyGetter) GetPublicKeys(ctx context.Context, keyIDHint string) []serviceaccount.PublicKey {
+	return d.keys
+}

staging/publishing/import-restrictions.yaml

@@ -317,3 +317,7 @@
   - k8s.io/cri-client
   - k8s.io/klog/v2
   - k8s.io/utils
+
+- baseImportPath: "./staging/src/k8s.io/externaljwt"
+  allowedImports:
+  - k8s.io/externaljwt

staging/publishing/rules.yaml

@@ -2441,6 +2441,13 @@ rules:
       branch: release-1.31
       dirs:
       - staging/src/k8s.io/endpointslice
+- destination: externaljwt
+  branches:
+  - name: master
+    source:
+      branch: master
+      dirs:
+      - staging/src/k8s.io/externaljwt
 recursive-delete-patterns:
 - '*/.gitattributes'
 default-go-version: 1.23.2

staging/src/k8s.io/externaljwt/.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,2 @@
+Sorry, we do not accept changes directly against this repository. Please see
+CONTRIBUTING.md for information on where and how to contribute instead.

staging/src/k8s.io/externaljwt/CONTRIBUTING.md

@@ -0,0 +1,7 @@
+# Contributing guidelines
+
+Do not open pull requests directly against this repository, they will be ignored. Instead, please open pull requests against [kubernetes/kubernetes](https://git.k8s.io/kubernetes/). Please follow the same [contributing guide](https://git.k8s.io/kubernetes/CONTRIBUTING.md) you would follow for any other pull request made to kubernetes/kubernetes.
+
+This repository is published from [kubernetes/kubernetes/staging/src/k8s.io/externaljwt](https://git.k8s.io/kubernetes/staging/src/k8s.io/externaljwt) by the [kubernetes publishing-bot](https://git.k8s.io/publishing-bot).
+
+Please see [Staging Directory and Publishing](https://git.k8s.io/community/contributors/devel/sig-architecture/staging.md) for more information

staging/src/k8s.io/externaljwt/LICENSE

@@ -0,0 +1,201 @@
+                                 Apache License
+                           Version 2.0, January 2004
+                        http://www.apache.org/licenses/
+
+   TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION
+
+   1. Definitions.
+
+      "License" shall mean the terms and conditions for use, reproduction,
+      and distribution as defined by Sections 1 through 9 of this document.
+
+      "Licensor" shall mean the copyright owner or entity authorized by
+      the copyright owner that is granting the License.
+
+      "Legal Entity" shall mean the union of the acting entity and all
+      other entities that control, are controlled by, or are under common
+      control with that entity. For the purposes of this definition,
+      "control" means (i) the power, direct or indirect, to cause the
+      direction or management of such entity, whether by contract or
+      otherwise, or (ii) ownership of fifty percent (50%) or more of the
+      outstanding shares, or (iii) beneficial ownership of such entity.
+
+      "You" (or "Your") shall mean an individual or Legal Entity
+      exercising permissions granted by this License.
+
+      "Source" form shall mean the preferred form for making modifications,
+      including but not limited to software source code, documentation
+      source, and configuration files.
+
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+      transformation or translation of a Source form, including but
+      not limited to compiled object code, generated documentation,
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+
+      "Work" shall mean the work of authorship, whether in Source or
+      Object form, made available under the License, as indicated by a
+      copyright notice that is included in or attached to the work
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+
+      "Derivative Works" shall mean any work, whether in Source or Object
+      form, that is based on (or derived from) the Work and for which the
+      editorial revisions, annotations, elaborations, or other modifications
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+      of this License, Derivative Works shall not include works that remain
+      separable from, or merely link (or bind by name) to the interfaces of,
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+
+      "Contribution" shall mean any work of authorship, including
+      the original version of the Work and any modifications or additions
+      to that Work or Derivative Works thereof, that is intentionally
+      submitted to Licensor for inclusion in the Work by the copyright owner
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+      to the Licensor or its representatives, including but not limited to
+      communication on electronic mailing lists, source code control systems,
+      and issue tracking systems that are managed by, or on behalf of, the
+      Licensor for the purpose of discussing and improving the Work, but
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+      designated in writing by the copyright owner as "Not a Contribution."
+
+      "Contributor" shall mean Licensor and any individual or Legal Entity
+      on behalf of whom a Contribution has been received by Licensor and
+      subsequently incorporated within the Work.
+
+   2. Grant of Copyright License. Subject to the terms and conditions of
+      this License, each Contributor hereby grants to You a perpetual,
+      worldwide, non-exclusive, no-charge, royalty-free, irrevocable
+      copyright license to reproduce, prepare Derivative Works of,
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+      Work and such Derivative Works in Source or Object form.
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+   3. Grant of Patent License. Subject to the terms and conditions of
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+      worldwide, non-exclusive, no-charge, royalty-free, irrevocable
+      (except as stated in this section) patent license to make, have made,
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+   4. Redistribution. You may reproduce and distribute copies of the
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+      meet the following conditions:
+
+      (a) You must give any other recipients of the Work or
+          Derivative Works a copy of this License; and
+
+      (b) You must cause any modified files to carry prominent notices
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+
+      (c) You must retain, in the Source form of any Derivative Works
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+          the Derivative Works; and
+
+      (d) If the Work includes a "NOTICE" text file as part of its
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+          of the NOTICE file are for informational purposes only and
+          do not modify the License. You may add Your own attribution
+          notices within Derivative Works that You distribute, alongside
+          or as an addendum to the NOTICE text from the Work, provided
+          that such additional attribution notices cannot be construed
+          as modifying the License.
+
+      You may add Your own copyright statement to Your modifications and
+      may provide additional or different license terms and conditions
+      for use, reproduction, or distribution of Your modifications, or
+      for any such Derivative Works as a whole, provided Your use,
+      reproduction, and distribution of the Work otherwise complies with
+      the conditions stated in this License.
+
+   5. Submission of Contributions. Unless You explicitly state otherwise,
+      any Contribution intentionally submitted for inclusion in the Work
+      by You to the Licensor shall be under the terms and conditions of
+      this License, without any additional terms or conditions.
+      Notwithstanding the above, nothing herein shall supersede or modify
+      the terms of any separate license agreement you may have executed
+      with Licensor regarding such Contributions.
+
+   6. Trademarks. This License does not grant permission to use the trade
+      names, trademarks, service marks, or product names of the Licensor,
+      except as required for reasonable and customary use in describing the
+      origin of the Work and reproducing the content of the NOTICE file.
+
+   7. Disclaimer of Warranty. Unless required by applicable law or
+      agreed to in writing, Licensor provides the Work (and each
+      Contributor provides its Contributions) on an "AS IS" BASIS,
+      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+      implied, including, without limitation, any warranties or conditions
+      of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A
+      PARTICULAR PURPOSE. You are solely responsible for determining the
+      appropriateness of using or redistributing the Work and assume any
+      risks associated with Your exercise of permissions under this License.
+
+   8. Limitation of Liability. In no event and under no legal theory,
+      whether in tort (including negligence), contract, or otherwise,
+      unless required by applicable law (such as deliberate and grossly
+      negligent acts) or agreed to in writing, shall any Contributor be
+      liable to You for damages, including any direct, indirect, special,
+      incidental, or consequential damages of any character arising as a
+      result of this License or out of the use or inability to use the
+      Work (including but not limited to damages for loss of goodwill,
+      work stoppage, computer failure or malfunction, or any and all
+      other commercial damages or losses), even if such Contributor
+      has been advised of the possibility of such damages.
+
+   9. Accepting Warranty or Additional Liability. While redistributing
+      the Work or Derivative Works thereof, You may choose to offer,
+      and charge a fee for, acceptance of support, warranty, indemnity,
+      or other liability obligations and/or rights consistent with this
+      License. However, in accepting such obligations, You may act only
+      on Your own behalf and on Your sole responsibility, not on behalf
+      of any other Contributor, and only if You agree to indemnify,
+      defend, and hold each Contributor harmless for any liability
+      incurred by, or claims asserted against, such Contributor by reason
+      of your accepting any such warranty or additional liability.
+
+   END OF TERMS AND CONDITIONS
+
+   APPENDIX: How to apply the Apache License to your work.
+
+      To apply the Apache License to your work, attach the following
+      boilerplate notice, with the fields enclosed by brackets "{}"
+      replaced with your own identifying information. (Don't include
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+      file or class name and description of purpose be included on the
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+
+   Copyright {yyyy} {name of copyright owner}
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.

staging/src/k8s.io/externaljwt/OWNERS

@@ -0,0 +1,8 @@
+# See the OWNERS docs at https://go.k8s.io/owners
+
+approvers:
+  - sig-auth-serviceaccounts-approvers
+reviewers:
+  - sig-auth-serviceaccounts-reviewers
+labels:
+  - sig/auth

staging/src/k8s.io/externaljwt/README.md

@@ -0,0 +1,20 @@
+# ExternalJWT
+
+This repository contains proto APIs which enable plugging external JWT signing and key management.
+
+See [KEP 740](https://github.com/kubernetes/enhancements/tree/master/keps/sig-auth/740-service-account-external-signing) for more details.
+
+## Community, discussion, contribution, and support
+
+ExternalJWT a sub-project of [SIG-Auth](https://github.com/kubernetes/community/tree/master/sig-auth).
+
+You can reach the maintainers of this project at:
+
+- Slack: [#sig-auth](https://kubernetes.slack.com/messages/sig-auth)
+- Mailing List: [kubernetes-sig-auth](https://groups.google.com/forum/#!forum/kubernetes-sig-auth)
+
+Learn how to engage with the Kubernetes community on the [community page](http://kubernetes.io/community/).
+
+### Code of conduct
+
+Participation in the Kubernetes community is governed by the [Kubernetes Code of Conduct](code-of-conduct.md).

staging/src/k8s.io/externaljwt/SECURITY_CONTACTS

@@ -0,0 +1,15 @@
+# Defined below are the security contacts for this repo.
+#
+# They are the contact point for the Product Security Committee to reach out
+# to for triaging and handling of incoming issues.
+#
+# The below names agree to abide by the
+# [Embargo Policy](https://git.k8s.io/security/private-distributors-list.md#embargo-policy)
+# and will be removed and replaced if they violate that agreement.
+#
+# DO NOT REPORT SECURITY VULNERABILITIES DIRECTLY TO THESE NAMES, FOLLOW THE
+# INSTRUCTIONS AT https://kubernetes.io/security/
+
+liggitt
+enj
+taahm

staging/src/k8s.io/externaljwt/apis/OWNERS

@@ -0,0 +1,9 @@
+# See the OWNERS docs at https://go.k8s.io/owners
+
+# Disable inheritance as this is an api owners file
+options:
+  no_parent_owners: true
+approvers:
+  - api-approvers
+reviewers:
+  - sig-auth-api-reviewers

staging/src/k8s.io/externaljwt/apis/v1alpha1/api.pb.go

@@ -0,0 +1,591 @@
+/*
+Copyright The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// Code generated by protoc-gen-gogo. DO NOT EDIT.
+// source: api.proto
+
+package v1alpha1
+
+import (
+	context "context"
+	fmt "fmt"
+	proto "github.com/gogo/protobuf/proto"
+	grpc "google.golang.org/grpc"
+	codes "google.golang.org/grpc/codes"
+	status "google.golang.org/grpc/status"
+	timestamppb "google.golang.org/protobuf/types/known/timestamppb"
+	math "math"
+)
+
+// Reference imports to suppress errors if they are not otherwise used.
+var _ = proto.Marshal
+var _ = fmt.Errorf
+var _ = math.Inf
+
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the proto package it is being compiled against.
+// A compilation error at this line likely means your copy of the
+// proto package needs to be updated.
+const _ = proto.GoGoProtoPackageIsVersion3 // please upgrade the proto package
+
+type SignJWTRequest struct {
+	// URL-safe base64 wrapped payload to be signed.
+	// Exactly as it appears in the second segment of the JWT
+	Claims               string   `protobuf:"bytes,1,opt,name=claims,proto3" json:"claims,omitempty"`
+	XXX_NoUnkeyedLiteral struct{} `json:"-"`
+	XXX_unrecognized     []byte   `json:"-"`
+	XXX_sizecache        int32    `json:"-"`
+}
+
+func (m *SignJWTRequest) Reset()         { *m = SignJWTRequest{} }
+func (m *SignJWTRequest) String() string { return proto.CompactTextString(m) }
+func (*SignJWTRequest) ProtoMessage()    {}
+func (*SignJWTRequest) Descriptor() ([]byte, []int) {
+	return fileDescriptor_00212fb1f9d3bf1c, []int{0}
+}
+func (m *SignJWTRequest) XXX_Unmarshal(b []byte) error {
+	return xxx_messageInfo_SignJWTRequest.Unmarshal(m, b)
+}
+func (m *SignJWTRequest) XXX_Marshal(b []byte, deterministic bool) ([]byte, error) {
+	return xxx_messageInfo_SignJWTRequest.Marshal(b, m, deterministic)
+}
+func (m *SignJWTRequest) XXX_Merge(src proto.Message) {
+	xxx_messageInfo_SignJWTRequest.Merge(m, src)
+}
+func (m *SignJWTRequest) XXX_Size() int {
+	return xxx_messageInfo_SignJWTRequest.Size(m)
+}
+func (m *SignJWTRequest) XXX_DiscardUnknown() {
+	xxx_messageInfo_SignJWTRequest.DiscardUnknown(m)
+}
+
+var xxx_messageInfo_SignJWTRequest proto.InternalMessageInfo
+
+func (m *SignJWTRequest) GetClaims() string {
+	if m != nil {
+		return m.Claims
+	}
+	return ""
+}
+
+type SignJWTResponse struct {
+	// header must contain only alg, kid, typ claims.
+	// typ must be “JWT”.
+	// kid must be non-empty, <=1024 characters, and its corresponding public key should not be excluded from OIDC discovery.
+	// alg must be one of the algorithms supported by kube-apiserver (currently RS256, ES256, ES384, ES512).
+	// header cannot have any additional data that kube-apiserver does not recognize.
+	// Already wrapped in URL-safe base64, exactly as it appears in the first segment of the JWT.
+	Header string `protobuf:"bytes,1,opt,name=header,proto3" json:"header,omitempty"`
+	// The signature for the JWT.
+	// Already wrapped in URL-safe base64, exactly as it appears in the final segment of the JWT.
+	Signature            string   `protobuf:"bytes,2,opt,name=signature,proto3" json:"signature,omitempty"`
+	XXX_NoUnkeyedLiteral struct{} `json:"-"`
+	XXX_unrecognized     []byte   `json:"-"`
+	XXX_sizecache        int32    `json:"-"`
+}
+
+func (m *SignJWTResponse) Reset()         { *m = SignJWTResponse{} }
+func (m *SignJWTResponse) String() string { return proto.CompactTextString(m) }
+func (*SignJWTResponse) ProtoMessage()    {}
+func (*SignJWTResponse) Descriptor() ([]byte, []int) {
+	return fileDescriptor_00212fb1f9d3bf1c, []int{1}
+}
+func (m *SignJWTResponse) XXX_Unmarshal(b []byte) error {
+	return xxx_messageInfo_SignJWTResponse.Unmarshal(m, b)
+}
+func (m *SignJWTResponse) XXX_Marshal(b []byte, deterministic bool) ([]byte, error) {
+	return xxx_messageInfo_SignJWTResponse.Marshal(b, m, deterministic)
+}
+func (m *SignJWTResponse) XXX_Merge(src proto.Message) {
+	xxx_messageInfo_SignJWTResponse.Merge(m, src)
+}
+func (m *SignJWTResponse) XXX_Size() int {
+	return xxx_messageInfo_SignJWTResponse.Size(m)
+}
+func (m *SignJWTResponse) XXX_DiscardUnknown() {
+	xxx_messageInfo_SignJWTResponse.DiscardUnknown(m)
+}
+
+var xxx_messageInfo_SignJWTResponse proto.InternalMessageInfo
+
+func (m *SignJWTResponse) GetHeader() string {
+	if m != nil {
+		return m.Header
+	}
+	return ""
+}
+
+func (m *SignJWTResponse) GetSignature() string {
+	if m != nil {
+		return m.Signature
+	}
+	return ""
+}
+
+type FetchKeysRequest struct {
+	XXX_NoUnkeyedLiteral struct{} `json:"-"`
+	XXX_unrecognized     []byte   `json:"-"`
+	XXX_sizecache        int32    `json:"-"`
+}
+
+func (m *FetchKeysRequest) Reset()         { *m = FetchKeysRequest{} }
+func (m *FetchKeysRequest) String() string { return proto.CompactTextString(m) }
+func (*FetchKeysRequest) ProtoMessage()    {}
+func (*FetchKeysRequest) Descriptor() ([]byte, []int) {
+	return fileDescriptor_00212fb1f9d3bf1c, []int{2}
+}
+func (m *FetchKeysRequest) XXX_Unmarshal(b []byte) error {
+	return xxx_messageInfo_FetchKeysRequest.Unmarshal(m, b)
+}
+func (m *FetchKeysRequest) XXX_Marshal(b []byte, deterministic bool) ([]byte, error) {
+	return xxx_messageInfo_FetchKeysRequest.Marshal(b, m, deterministic)
+}
+func (m *FetchKeysRequest) XXX_Merge(src proto.Message) {
+	xxx_messageInfo_FetchKeysRequest.Merge(m, src)
+}
+func (m *FetchKeysRequest) XXX_Size() int {
+	return xxx_messageInfo_FetchKeysRequest.Size(m)
+}
+func (m *FetchKeysRequest) XXX_DiscardUnknown() {
+	xxx_messageInfo_FetchKeysRequest.DiscardUnknown(m)
+}
+
+var xxx_messageInfo_FetchKeysRequest proto.InternalMessageInfo
+
+type FetchKeysResponse struct {
+	Keys []*Key `protobuf:"bytes,1,rep,name=keys,proto3" json:"keys,omitempty"`
+	// The timestamp when this data was pulled from the authoritative source of
+	// truth for verification keys.
+	// kube-apiserver can export this from metrics, to enable end-to-end SLOs.
+	DataTimestamp *timestamppb.Timestamp `protobuf:"bytes,2,opt,name=data_timestamp,json=dataTimestamp,proto3" json:"data_timestamp,omitempty"`
+	// refresh interval for verification keys to pick changes if any.
+	// any value <= 0 is considered a misconfiguration.
+	RefreshHintSeconds   int64    `protobuf:"varint,3,opt,name=refresh_hint_seconds,json=refreshHintSeconds,proto3" json:"refresh_hint_seconds,omitempty"`
+	XXX_NoUnkeyedLiteral struct{} `json:"-"`
+	XXX_unrecognized     []byte   `json:"-"`
+	XXX_sizecache        int32    `json:"-"`
+}
+
+func (m *FetchKeysResponse) Reset()         { *m = FetchKeysResponse{} }
+func (m *FetchKeysResponse) String() string { return proto.CompactTextString(m) }
+func (*FetchKeysResponse) ProtoMessage()    {}
+func (*FetchKeysResponse) Descriptor() ([]byte, []int) {
+	return fileDescriptor_00212fb1f9d3bf1c, []int{3}
+}
+func (m *FetchKeysResponse) XXX_Unmarshal(b []byte) error {
+	return xxx_messageInfo_FetchKeysResponse.Unmarshal(m, b)
+}
+func (m *FetchKeysResponse) XXX_Marshal(b []byte, deterministic bool) ([]byte, error) {
+	return xxx_messageInfo_FetchKeysResponse.Marshal(b, m, deterministic)
+}
+func (m *FetchKeysResponse) XXX_Merge(src proto.Message) {
+	xxx_messageInfo_FetchKeysResponse.Merge(m, src)
+}
+func (m *FetchKeysResponse) XXX_Size() int {
+	return xxx_messageInfo_FetchKeysResponse.Size(m)
+}
+func (m *FetchKeysResponse) XXX_DiscardUnknown() {
+	xxx_messageInfo_FetchKeysResponse.DiscardUnknown(m)
+}
+
+var xxx_messageInfo_FetchKeysResponse proto.InternalMessageInfo
+
+func (m *FetchKeysResponse) GetKeys() []*Key {
+	if m != nil {
+		return m.Keys
+	}
+	return nil
+}
+
+func (m *FetchKeysResponse) GetDataTimestamp() *timestamppb.Timestamp {
+	if m != nil {
+		return m.DataTimestamp
+	}
+	return nil
+}
+
+func (m *FetchKeysResponse) GetRefreshHintSeconds() int64 {
+	if m != nil {
+		return m.RefreshHintSeconds
+	}
+	return 0
+}
+
+type Key struct {
+	// A unique identifier for this key.
+	// Length must be <=1024.
+	KeyId string `protobuf:"bytes,1,opt,name=key_id,json=keyId,proto3" json:"key_id,omitempty"`
+	// The public key, PKIX-serialized.
+	// must be a public key supported by kube-apiserver (currently RSA 256 or ECDSA 256/384/521)
+	Key []byte `protobuf:"bytes,2,opt,name=key,proto3" json:"key,omitempty"`
+	// Set only for keys that are not used to sign bound tokens.
+	// eg: supported keys for legacy tokens.
+	// If set, key is used for verification but excluded from OIDC discovery docs.
+	// if set, external signer should not use this key to sign a JWT.
+	ExcludeFromOidcDiscovery bool     `protobuf:"varint,3,opt,name=exclude_from_oidc_discovery,json=excludeFromOidcDiscovery,proto3" json:"exclude_from_oidc_discovery,omitempty"`
+	XXX_NoUnkeyedLiteral     struct{} `json:"-"`
+	XXX_unrecognized         []byte   `json:"-"`
+	XXX_sizecache            int32    `json:"-"`
+}
+
+func (m *Key) Reset()         { *m = Key{} }
+func (m *Key) String() string { return proto.CompactTextString(m) }
+func (*Key) ProtoMessage()    {}
+func (*Key) Descriptor() ([]byte, []int) {
+	return fileDescriptor_00212fb1f9d3bf1c, []int{4}
+}
+func (m *Key) XXX_Unmarshal(b []byte) error {
+	return xxx_messageInfo_Key.Unmarshal(m, b)
+}
+func (m *Key) XXX_Marshal(b []byte, deterministic bool) ([]byte, error) {
+	return xxx_messageInfo_Key.Marshal(b, m, deterministic)
+}
+func (m *Key) XXX_Merge(src proto.Message) {
+	xxx_messageInfo_Key.Merge(m, src)
+}
+func (m *Key) XXX_Size() int {
+	return xxx_messageInfo_Key.Size(m)
+}
+func (m *Key) XXX_DiscardUnknown() {
+	xxx_messageInfo_Key.DiscardUnknown(m)
+}
+
+var xxx_messageInfo_Key proto.InternalMessageInfo
+
+func (m *Key) GetKeyId() string {
+	if m != nil {
+		return m.KeyId
+	}
+	return ""
+}
+
+func (m *Key) GetKey() []byte {
+	if m != nil {
+		return m.Key
+	}
+	return nil
+}
+
+func (m *Key) GetExcludeFromOidcDiscovery() bool {
+	if m != nil {
+		return m.ExcludeFromOidcDiscovery
+	}
+	return false
+}
+
+type MetadataRequest struct {
+	XXX_NoUnkeyedLiteral struct{} `json:"-"`
+	XXX_unrecognized     []byte   `json:"-"`
+	XXX_sizecache        int32    `json:"-"`
+}
+
+func (m *MetadataRequest) Reset()         { *m = MetadataRequest{} }
+func (m *MetadataRequest) String() string { return proto.CompactTextString(m) }
+func (*MetadataRequest) ProtoMessage()    {}
+func (*MetadataRequest) Descriptor() ([]byte, []int) {
+	return fileDescriptor_00212fb1f9d3bf1c, []int{5}
+}
+func (m *MetadataRequest) XXX_Unmarshal(b []byte) error {
+	return xxx_messageInfo_MetadataRequest.Unmarshal(m, b)
+}
+func (m *MetadataRequest) XXX_Marshal(b []byte, deterministic bool) ([]byte, error) {
+	return xxx_messageInfo_MetadataRequest.Marshal(b, m, deterministic)
+}
+func (m *MetadataRequest) XXX_Merge(src proto.Message) {
+	xxx_messageInfo_MetadataRequest.Merge(m, src)
+}
+func (m *MetadataRequest) XXX_Size() int {
+	return xxx_messageInfo_MetadataRequest.Size(m)
+}
+func (m *MetadataRequest) XXX_DiscardUnknown() {
+	xxx_messageInfo_MetadataRequest.DiscardUnknown(m)
+}
+
+var xxx_messageInfo_MetadataRequest proto.InternalMessageInfo
+
+type MetadataResponse struct {
+	// used by kube-apiserver for defaulting/validation of JWT lifetime while accounting for configuration flag values:
+	// 1. `--service-account-max-token-expiration`
+	// 2. `--service-account-extend-token-expiration`
+	//
+	// * If `--service-account-max-token-expiration` is greater than `max_token_expiration_seconds`, kube-apiserver treats that as misconfiguration and exits.
+	// * If `--service-account-max-token-expiration` is not explicitly set, kube-apiserver defaults to `max_token_expiration_seconds`.
+	// * If `--service-account-extend-token-expiration` is true, the extended expiration is `min(1 year, max_token_expiration_seconds)`.
+	//
+	// `max_token_expiration_seconds` must be at least 600s.
+	MaxTokenExpirationSeconds int64    `protobuf:"varint,1,opt,name=max_token_expiration_seconds,json=maxTokenExpirationSeconds,proto3" json:"max_token_expiration_seconds,omitempty"`
+	XXX_NoUnkeyedLiteral      struct{} `json:"-"`
+	XXX_unrecognized          []byte   `json:"-"`
+	XXX_sizecache             int32    `json:"-"`
+}
+
+func (m *MetadataResponse) Reset()         { *m = MetadataResponse{} }
+func (m *MetadataResponse) String() string { return proto.CompactTextString(m) }
+func (*MetadataResponse) ProtoMessage()    {}
+func (*MetadataResponse) Descriptor() ([]byte, []int) {
+	return fileDescriptor_00212fb1f9d3bf1c, []int{6}
+}
+func (m *MetadataResponse) XXX_Unmarshal(b []byte) error {
+	return xxx_messageInfo_MetadataResponse.Unmarshal(m, b)
+}
+func (m *MetadataResponse) XXX_Marshal(b []byte, deterministic bool) ([]byte, error) {
+	return xxx_messageInfo_MetadataResponse.Marshal(b, m, deterministic)
+}
+func (m *MetadataResponse) XXX_Merge(src proto.Message) {
+	xxx_messageInfo_MetadataResponse.Merge(m, src)
+}
+func (m *MetadataResponse) XXX_Size() int {
+	return xxx_messageInfo_MetadataResponse.Size(m)
+}
+func (m *MetadataResponse) XXX_DiscardUnknown() {
+	xxx_messageInfo_MetadataResponse.DiscardUnknown(m)
+}
+
+var xxx_messageInfo_MetadataResponse proto.InternalMessageInfo
+
+func (m *MetadataResponse) GetMaxTokenExpirationSeconds() int64 {
+	if m != nil {
+		return m.MaxTokenExpirationSeconds
+	}
+	return 0
+}
+
+func init() {
+	proto.RegisterType((*SignJWTRequest)(nil), "v1alpha1.SignJWTRequest")
+	proto.RegisterType((*SignJWTResponse)(nil), "v1alpha1.SignJWTResponse")
+	proto.RegisterType((*FetchKeysRequest)(nil), "v1alpha1.FetchKeysRequest")
+	proto.RegisterType((*FetchKeysResponse)(nil), "v1alpha1.FetchKeysResponse")
+	proto.RegisterType((*Key)(nil), "v1alpha1.Key")
+	proto.RegisterType((*MetadataRequest)(nil), "v1alpha1.MetadataRequest")
+	proto.RegisterType((*MetadataResponse)(nil), "v1alpha1.MetadataResponse")
+}
+
+func init() { proto.RegisterFile("api.proto", fileDescriptor_00212fb1f9d3bf1c) }
+
+var fileDescriptor_00212fb1f9d3bf1c = []byte{
+	// 483 bytes of a gzipped FileDescriptorProto
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+	0x62, 0x72, 0xaf, 0xe0, 0x83, 0xc1, 0x58, 0xd6, 0x3e, 0x49, 0x27, 0xf9, 0x60, 0xff, 0x98, 0xc4,
+	0x6f, 0xa5, 0xc3, 0xb8, 0x8b, 0x2d, 0xe7, 0xcf, 0xdf, 0xca, 0xa5, 0x39, 0x93, 0xae, 0x7e, 0x76,
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+	0x03, 0x00, 0x00,
+}
+
+// Reference imports to suppress errors if they are not otherwise used.
+var _ context.Context
+var _ grpc.ClientConn
+
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the grpc package it is being compiled against.
+const _ = grpc.SupportPackageIsVersion4
+
+// ExternalJWTSignerClient is the client API for ExternalJWTSigner service.
+//
+// For semantics around ctx use and closing/ending streaming RPCs, please refer to https://godoc.org/google.golang.org/grpc#ClientConn.NewStream.
+type ExternalJWTSignerClient interface {
+	// Sign takes a serialized JWT payload, and returns the serialized header and
+	// signature.  The caller can then assemble the JWT from the header, payload,
+	// and signature.
+	//
+	// The plugin MUST set a key id in the returned JWT header.
+	Sign(ctx context.Context, in *SignJWTRequest, opts ...grpc.CallOption) (*SignJWTResponse, error)
+	// FetchKeys returns the set of public keys that are trusted to sign
+	// Kubernetes service account tokens. Kube-apiserver will call this RPC:
+	//
+	// * Every time it tries to validate a JWT from the service account issuer with an unknown key ID, and
+	//
+	//   - Periodically, so it can serve reasonably-up-to-date keys from the OIDC
+	//     JWKs endpoint.
+	FetchKeys(ctx context.Context, in *FetchKeysRequest, opts ...grpc.CallOption) (*FetchKeysResponse, error)
+	// Metadata is meant to be called once on startup.
+	// Enables sharing metadata with kube-apiserver (eg: the max token lifetime that signer supports)
+	Metadata(ctx context.Context, in *MetadataRequest, opts ...grpc.CallOption) (*MetadataResponse, error)
+}
+
+type externalJWTSignerClient struct {
+	cc *grpc.ClientConn
+}
+
+func NewExternalJWTSignerClient(cc *grpc.ClientConn) ExternalJWTSignerClient {
+	return &externalJWTSignerClient{cc}
+}
+
+func (c *externalJWTSignerClient) Sign(ctx context.Context, in *SignJWTRequest, opts ...grpc.CallOption) (*SignJWTResponse, error) {
+	out := new(SignJWTResponse)
+	err := c.cc.Invoke(ctx, "/v1alpha1.ExternalJWTSigner/Sign", in, out, opts...)
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+func (c *externalJWTSignerClient) FetchKeys(ctx context.Context, in *FetchKeysRequest, opts ...grpc.CallOption) (*FetchKeysResponse, error) {
+	out := new(FetchKeysResponse)
+	err := c.cc.Invoke(ctx, "/v1alpha1.ExternalJWTSigner/FetchKeys", in, out, opts...)
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+func (c *externalJWTSignerClient) Metadata(ctx context.Context, in *MetadataRequest, opts ...grpc.CallOption) (*MetadataResponse, error) {
+	out := new(MetadataResponse)
+	err := c.cc.Invoke(ctx, "/v1alpha1.ExternalJWTSigner/Metadata", in, out, opts...)
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+// ExternalJWTSignerServer is the server API for ExternalJWTSigner service.
+type ExternalJWTSignerServer interface {
+	// Sign takes a serialized JWT payload, and returns the serialized header and
+	// signature.  The caller can then assemble the JWT from the header, payload,
+	// and signature.
+	//
+	// The plugin MUST set a key id in the returned JWT header.
+	Sign(context.Context, *SignJWTRequest) (*SignJWTResponse, error)
+	// FetchKeys returns the set of public keys that are trusted to sign
+	// Kubernetes service account tokens. Kube-apiserver will call this RPC:
+	//
+	// * Every time it tries to validate a JWT from the service account issuer with an unknown key ID, and
+	//
+	//   - Periodically, so it can serve reasonably-up-to-date keys from the OIDC
+	//     JWKs endpoint.
+	FetchKeys(context.Context, *FetchKeysRequest) (*FetchKeysResponse, error)
+	// Metadata is meant to be called once on startup.
+	// Enables sharing metadata with kube-apiserver (eg: the max token lifetime that signer supports)
+	Metadata(context.Context, *MetadataRequest) (*MetadataResponse, error)
+}
+
+// UnimplementedExternalJWTSignerServer can be embedded to have forward compatible implementations.
+type UnimplementedExternalJWTSignerServer struct {
+}
+
+func (*UnimplementedExternalJWTSignerServer) Sign(ctx context.Context, req *SignJWTRequest) (*SignJWTResponse, error) {
+	return nil, status.Errorf(codes.Unimplemented, "method Sign not implemented")
+}
+func (*UnimplementedExternalJWTSignerServer) FetchKeys(ctx context.Context, req *FetchKeysRequest) (*FetchKeysResponse, error) {
+	return nil, status.Errorf(codes.Unimplemented, "method FetchKeys not implemented")
+}
+func (*UnimplementedExternalJWTSignerServer) Metadata(ctx context.Context, req *MetadataRequest) (*MetadataResponse, error) {
+	return nil, status.Errorf(codes.Unimplemented, "method Metadata not implemented")
+}
+
+func RegisterExternalJWTSignerServer(s *grpc.Server, srv ExternalJWTSignerServer) {
+	s.RegisterService(&_ExternalJWTSigner_serviceDesc, srv)
+}
+
+func _ExternalJWTSigner_Sign_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
+	in := new(SignJWTRequest)
+	if err := dec(in); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(ExternalJWTSignerServer).Sign(ctx, in)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: "/v1alpha1.ExternalJWTSigner/Sign",
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(ExternalJWTSignerServer).Sign(ctx, req.(*SignJWTRequest))
+	}
+	return interceptor(ctx, in, info, handler)
+}
+
+func _ExternalJWTSigner_FetchKeys_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
+	in := new(FetchKeysRequest)
+	if err := dec(in); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(ExternalJWTSignerServer).FetchKeys(ctx, in)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: "/v1alpha1.ExternalJWTSigner/FetchKeys",
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(ExternalJWTSignerServer).FetchKeys(ctx, req.(*FetchKeysRequest))
+	}
+	return interceptor(ctx, in, info, handler)
+}
+
+func _ExternalJWTSigner_Metadata_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
+	in := new(MetadataRequest)
+	if err := dec(in); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(ExternalJWTSignerServer).Metadata(ctx, in)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: "/v1alpha1.ExternalJWTSigner/Metadata",
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(ExternalJWTSignerServer).Metadata(ctx, req.(*MetadataRequest))
+	}
+	return interceptor(ctx, in, info, handler)
+}
+
+var _ExternalJWTSigner_serviceDesc = grpc.ServiceDesc{
+	ServiceName: "v1alpha1.ExternalJWTSigner",
+	HandlerType: (*ExternalJWTSignerServer)(nil),
+	Methods: []grpc.MethodDesc{
+		{
+			MethodName: "Sign",
+			Handler:    _ExternalJWTSigner_Sign_Handler,
+		},
+		{
+			MethodName: "FetchKeys",
+			Handler:    _ExternalJWTSigner_FetchKeys_Handler,
+		},
+		{
+			MethodName: "Metadata",
+			Handler:    _ExternalJWTSigner_Metadata_Handler,
+		},
+	},
+	Streams:  []grpc.StreamDesc{},
+	Metadata: "api.proto",
+}

staging/src/k8s.io/externaljwt/apis/v1alpha1/api.proto

@@ -0,0 +1,113 @@
+/*
+Copyright 2024 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// To regenerate api.pb.go run `hack/update-codegen.sh protobindings`
+syntax = "proto3";
+
+package v1alpha1;
+
+option go_package = "k8s.io/externaljwt/apis/v1alpha1";
+
+import "google/protobuf/timestamp.proto";
+
+// This service is served by a process on a local Unix Domain Socket.
+service ExternalJWTSigner {
+    // Sign takes a serialized JWT payload, and returns the serialized header and
+    // signature.  The caller can then assemble the JWT from the header, payload,
+    // and signature.
+    //
+    // The plugin MUST set a key id in the returned JWT header.
+    rpc Sign(SignJWTRequest) returns (SignJWTResponse) {}
+  
+    // FetchKeys returns the set of public keys that are trusted to sign
+    // Kubernetes service account tokens. Kube-apiserver will call this RPC:
+    //
+    // * Every time it tries to validate a JWT from the service account issuer with an unknown key ID, and
+    //
+    // * Periodically, so it can serve reasonably-up-to-date keys from the OIDC
+    //   JWKs endpoint.
+    rpc FetchKeys(FetchKeysRequest) returns (FetchKeysResponse) {}
+    
+    // Metadata is meant to be called once on startup.
+    // Enables sharing metadata with kube-apiserver (eg: the max token lifetime that signer supports)
+    rpc Metadata(MetadataRequest) returns (MetadataResponse) {}
+  }
+  
+  message SignJWTRequest {
+    // URL-safe base64 wrapped payload to be signed.
+    // Exactly as it appears in the second segment of the JWT
+    string claims = 1;
+  }
+  
+  message SignJWTResponse {
+    // header must contain only alg, kid, typ claims.
+    // typ must be “JWT”.
+    // kid must be non-empty, <=1024 characters, and its corresponding public key should not be excluded from OIDC discovery.
+    // alg must be one of the algorithms supported by kube-apiserver (currently RS256, ES256, ES384, ES512).
+    // header cannot have any additional data that kube-apiserver does not recognize.
+    // Already wrapped in URL-safe base64, exactly as it appears in the first segment of the JWT.
+    string header = 1;
+  
+    // The signature for the JWT.
+    // Already wrapped in URL-safe base64, exactly as it appears in the final segment of the JWT.
+    string signature = 2;
+  }
+  
+  message FetchKeysRequest {}
+  
+  message FetchKeysResponse {
+    repeated Key keys = 1;
+  
+    // The timestamp when this data was pulled from the authoritative source of
+    // truth for verification keys.
+    // kube-apiserver can export this from metrics, to enable end-to-end SLOs.
+    google.protobuf.Timestamp data_timestamp = 2;
+  
+    // refresh interval for verification keys to pick changes if any.
+    // any value <= 0 is considered a misconfiguration.
+    int64 refresh_hint_seconds = 3;
+  }
+  
+  message Key {
+    // A unique identifier for this key.
+    // Length must be <=1024.
+    string key_id = 1;
+  
+    // The public key, PKIX-serialized.
+    // must be a public key supported by kube-apiserver (currently RSA 256 or ECDSA 256/384/521)
+    bytes key = 2;
+  
+    // Set only for keys that are not used to sign bound tokens.
+    // eg: supported keys for legacy tokens.
+    // If set, key is used for verification but excluded from OIDC discovery docs.
+    // if set, external signer should not use this key to sign a JWT.
+    bool exclude_from_oidc_discovery = 3;
+  }
+  
+  message MetadataRequest {}
+  
+  message MetadataResponse {
+    // used by kube-apiserver for defaulting/validation of JWT lifetime while accounting for configuration flag values:
+    // 1. `--service-account-max-token-expiration`
+    // 2. `--service-account-extend-token-expiration`
+    //
+    // * If `--service-account-max-token-expiration` is greater than `max_token_expiration_seconds`, kube-apiserver treats that as misconfiguration and exits.
+    // * If `--service-account-max-token-expiration` is not explicitly set, kube-apiserver defaults to `max_token_expiration_seconds`.
+    // * If `--service-account-extend-token-expiration` is true, the extended expiration is `min(1 year, max_token_expiration_seconds)`.
+    //
+    // `max_token_expiration_seconds` must be at least 600s.
+    int64 max_token_expiration_seconds = 1;
+  }

staging/src/k8s.io/externaljwt/code-of-conduct.md

@@ -0,0 +1,3 @@
+# Kubernetes Community Code of Conduct
+
+Please refer to our [Kubernetes Community Code of Conduct](https://git.k8s.io/community/code-of-conduct.md)

staging/src/k8s.io/externaljwt/docs.go

@@ -0,0 +1,18 @@
+/*
+Copyright 2024 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// Package externaljwt contains the proto definitions for the ExternalJWTSigner.
+package externaljwt // import "k8s.io/externaljwt"

staging/src/k8s.io/externaljwt/go.mod

@@ -0,0 +1,20 @@
+// This is a generated file. Do not edit directly.
+
+module k8s.io/externaljwt
+
+go 1.23.0
+
+godebug default=go1.23
+
+require (
+	github.com/gogo/protobuf v1.3.2
+	google.golang.org/grpc v1.65.0
+	google.golang.org/protobuf v1.35.1
+)
+
+require (
+	golang.org/x/net v0.30.0 // indirect
+	golang.org/x/sys v0.26.0 // indirect
+	golang.org/x/text v0.19.0 // indirect
+	google.golang.org/genproto/googleapis/rpc v0.0.0-20240826202546-f6391c0de4c7 // indirect
+)

staging/src/k8s.io/externaljwt/go.sum

@@ -0,0 +1,62 @@
+cel.dev/expr v0.15.0/go.mod h1:TRSuuV7DlVCE/uwv5QbAiW/v8l5O8C4eEPHeu7gf7Sg=
+cloud.google.com/go/compute/metadata v0.3.0/go.mod h1:zFmK7XCadkQkj6TtorcaGlCW1hT1fIilQDwofLpJ20k=
+github.com/census-instrumentation/opencensus-proto v0.4.1/go.mod h1:4T9NM4+4Vw91VeyqjLS6ao50K5bOcLKN6Q42XnYaRYw=
+github.com/cespare/xxhash/v2 v2.3.0/go.mod h1:VGX0DQ3Q6kWi7AoAeZDth3/j3BFtOZR5XLFGgcrjCOs=
+github.com/cncf/xds/go v0.0.0-20240423153145-555b57ec207b/go.mod h1:W+zGtBO5Y1IgJhy4+A9GOqVhqLpfZi+vwmdNXUehLA8=
+github.com/envoyproxy/go-control-plane v0.12.0/go.mod h1:ZBTaoJ23lqITozF0M6G4/IragXCQKCnYbmlmtHvwRG0=
+github.com/envoyproxy/protoc-gen-validate v1.0.4/go.mod h1:qys6tmnRsYrQqIhm2bvKZH4Blx/1gTIZ2UKVY1M+Yew=
+github.com/gogo/protobuf v1.3.2 h1:Ov1cvc58UF3b5XjBnZv7+opcTcQFZebYjWzi34vdm4Q=
+github.com/gogo/protobuf v1.3.2/go.mod h1:P1XiOD3dCwIKUDQYPy72D8LYyHL2YPYrpS2s69NZV8Q=
+github.com/golang/glog v1.2.1/go.mod h1:6AhwSGph0fcJtXVM/PEHPqZlFeoLxhs7/t5UDAwmO+w=
+github.com/golang/protobuf v1.5.4/go.mod h1:lnTiLA8Wa4RWRcIUkrtSVa5nRhsEGBg48fD6rSs7xps=
+github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
+github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
+github.com/google/uuid v1.6.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
+github.com/kisielk/errcheck v1.5.0/go.mod h1:pFxgyoBC7bSaBwPgfKdkLd5X25qrDl4LWUI2bnpBCr8=
+github.com/kisielk/gotool v1.0.0/go.mod h1:XhKaO+MFFWcvkIS/tQcRk01m1F5IRFswLeQ+oQHNcck=
+github.com/yuin/goldmark v1.1.27/go.mod h1:3hX8gzYuyVAZsxl0MRgGTJEmQBFcNTphYh9decYSb74=
+github.com/yuin/goldmark v1.2.1/go.mod h1:3hX8gzYuyVAZsxl0MRgGTJEmQBFcNTphYh9decYSb74=
+golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
+golang.org/x/crypto v0.0.0-20191011191535-87dc89f01550/go.mod h1:yigFU9vqHzYiE8UmvKecakEJjdnWj3jj499lnFckfCI=
+golang.org/x/crypto v0.0.0-20200622213623-75b288015ac9/go.mod h1:LzIPMQfyMNhhGPhUkYOs5KpL4U8rLKemX1yGLhDgUto=
+golang.org/x/crypto v0.28.0/go.mod h1:rmgy+3RHxRZMyY0jjAJShp2zgEdOqj2AO7U0pYmeQ7U=
+golang.org/x/mod v0.2.0/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
+golang.org/x/mod v0.3.0/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
+golang.org/x/mod v0.17.0/go.mod h1:hTbmBsO62+eylJbnUtE2MGJUyE7QWk4xUqPFrRgJ+7c=
+golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
+golang.org/x/net v0.0.0-20190620200207-3b0461eec859/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
+golang.org/x/net v0.0.0-20200226121028-0de0cce0169b/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
+golang.org/x/net v0.0.0-20201021035429-f5854403a974/go.mod h1:sp8m0HH+o8qH0wwXwYZr8TS3Oi6o0r6Gce1SSxlDquU=
+golang.org/x/net v0.30.0 h1:AcW1SDZMkb8IpzCdQUaIq2sP4sZ4zw+55h6ynffypl4=
+golang.org/x/net v0.30.0/go.mod h1:2wGyMJ5iFasEhkwi13ChkO/t1ECNC4X4eBKkVFyYFlU=
+golang.org/x/oauth2 v0.20.0/go.mod h1:XYTD2NtWslqkgxebSiOHnXEap4TF09sJSc7H1sXbhtI=
+golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sync v0.0.0-20190911185100-cd5d95a43a6e/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sync v0.0.0-20201020160332-67f06af15bc9/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sync v0.8.0/go.mod h1:Czt+wKu1gCyEFDUtn0jG5QVvpJ6rzVqr5aXyt9drQfk=
+golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
+golang.org/x/sys v0.0.0-20190412213103-97732733099d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20200930185726-fdedc70b468f/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.26.0 h1:KHjCJyddX0LoSTb3J+vWpupP9p0oznkqVk/IfjymZbo=
+golang.org/x/sys v0.26.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
+golang.org/x/term v0.25.0/go.mod h1:RPyXicDX+6vLxogjjRxjgD2TKtmAO6NZBsBRfrOLu7M=
+golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+golang.org/x/text v0.3.3/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
+golang.org/x/text v0.19.0 h1:kTxAhCbGbxhK0IwgSKiMO5awPoDQ0RpfiVYBfK860YM=
+golang.org/x/text v0.19.0/go.mod h1:BuEKDfySbSR4drPmRPG/7iBdf8hvFMuRexcpahXilzY=
+golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
+golang.org/x/tools v0.0.0-20191119224855-298f0cb1881e/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
+golang.org/x/tools v0.0.0-20200619180055-7c47624df98f/go.mod h1:EkVYQZoAsY45+roYkvgYkIh4xh/qjgUK9TdY2XT94GE=
+golang.org/x/tools v0.0.0-20210106214847-113979e3529a/go.mod h1:emZCQorbCU4vsT4fOWvOPXz4eW1wZW4PmDk9uLelYpA=
+golang.org/x/tools v0.21.1-0.20240508182429-e35e4ccd0d2d/go.mod h1:aiJjzUbINMkxbQROHiO6hDPo2LHcIPhhQsa9DLh0yGk=
+golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
+golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
+golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
+golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
+google.golang.org/genproto/googleapis/api v0.0.0-20240528184218-531527333157/go.mod h1:99sLkeliLXfdj2J75X3Ho+rrVCaJze0uwN7zDDkjPVU=
+google.golang.org/genproto/googleapis/rpc v0.0.0-20240826202546-f6391c0de4c7 h1:2035KHhUv+EpyB+hWgJnaWKJOdX1E95w2S8Rr4uWKTs=
+google.golang.org/genproto/googleapis/rpc v0.0.0-20240826202546-f6391c0de4c7/go.mod h1:UqMtugtsSgubUsoxbuAoiCXvqvErP7Gf0so0mK9tHxU=
+google.golang.org/grpc v1.65.0 h1:bs/cUb4lp1G5iImFFd3u5ixQzweKizoZJAwBNLR42lc=
+google.golang.org/grpc v1.65.0/go.mod h1:WgYC2ypjlB0EiQi6wdKixMqukr6lBc0Vo+oOgjrM5ZQ=
+google.golang.org/protobuf v1.35.1 h1:m3LfL6/Ca+fqnjnlqQXNpFPABW1UD7mjh8KO2mKFytA=
+google.golang.org/protobuf v1.35.1/go.mod h1:9fA7Ob0pmnwhb644+1+CVWFRbNajQ6iRojtC/QF5bRE=

test/e2e_node/services/apiserver.go

@@ -102,7 +102,7 @@ func (a *APIServer) Start(ctx context.Context) error {
 	go func() {
 		defer close(errCh)
 		defer cancel(errors.New("shutting down")) // Calling Stop is optional, but cancel always should be invoked.
-		completedOptions, err := o.Complete()
+		completedOptions, err := o.Complete(ctx)
 		if err != nil {
 			errCh <- fmt.Errorf("set apiserver default options error: %w", err)
 			return

test/featuregates_linter/test_data/versioned_feature_list.yaml

@@ -452,6 +452,12 @@
     lockToDefault: false
     preRelease: GA
     version: "1.20"
+- name: ExternalServiceAccountTokenSigner
+  versionedSpecs:
+  - default: false
+    lockToDefault: false
+    preRelease: Alpha
+    version: "1.32"
 - name: GracefulNodeShutdown
   versionedSpecs:
   - default: false

test/integration/auth/svcaccttoken_test.go

@@ -691,7 +691,7 @@ func TestServiceAccountTokenCreate(t *testing.T) {
 		if err != nil {
 			t.Fatalf("err calling Claims: %v", err)
 		}
-		tok, err := tokenGenerator.GenerateToken(sc, pc)
+		tok, err := tokenGenerator.GenerateToken(context.TODO(), sc, pc)
 		if err != nil {
 			t.Fatalf("err signing expired token: %v", err)
 		}

test/integration/etcd/server.go

@@ -106,7 +106,7 @@ func StartRealAPIServerOrDie(t *testing.T, configFuncs ...func(*options.ServerRu
 	for _, f := range configFuncs {
 		f(opts)
 	}
-	completedOptions, err := opts.Complete()
+	completedOptions, err := opts.Complete(tCtx)
 	if err != nil {
 		t.Fatal(err)
 	}

test/integration/framework/test_server.go

@@ -158,7 +158,7 @@ func StartTestServer(ctx context.Context, t testing.TB, setup TestServerSetup) (
 		setup.ModifyServerRunOptions(opts)
 	}
 
-	completedOptions, err := opts.Complete()
+	completedOptions, err := opts.Complete(ctx)
 	if err != nil {
 		t.Fatal(err)
 	}

test/integration/serviceaccount/external_jwt_signer_test.go

@@ -0,0 +1,305 @@
+/*
+Copyright 2024 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package serviceaccount
+
+import (
+	"context"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"fmt"
+	"os"
+	"strings"
+	"testing"
+	"time"
+
+	"k8s.io/kubernetes/cmd/kube-apiserver/app/options"
+	"k8s.io/kubernetes/test/integration/framework"
+	"k8s.io/kubernetes/test/utils/ktesting"
+
+	authv1 "k8s.io/api/authentication/v1"
+	corev1 "k8s.io/api/core/v1"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	utilfeature "k8s.io/apiserver/pkg/util/feature"
+	featuregatetesting "k8s.io/component-base/featuregate/testing"
+	"k8s.io/kubernetes/pkg/features"
+	v1alpha1testing "k8s.io/kubernetes/pkg/serviceaccount/externaljwt/plugin/testing/v1alpha1"
+)
+
+func TestExternalJWTSigningAndAuth(t *testing.T) {
+	// Enable feature gate for external JWT signer.
+	featuregatetesting.SetFeatureGateDuringTest(t, utilfeature.DefaultFeatureGate, features.ExternalServiceAccountTokenSigner, true)
+
+	// Prep some keys to use with test.
+	key1, err := rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		panic("Error while generating first RSA key")
+	}
+	pubKey1Bytes, err := x509.MarshalPKIXPublicKey(&key1.PublicKey)
+	if err != nil {
+		panic("Error while marshaling first public key")
+	}
+
+	tCtx := ktesting.Init(t)
+	ctx, cancel := context.WithCancel(tCtx)
+	defer cancel()
+
+	// create and start mock signer.
+	socketPath := "@mock-external-jwt-signer.sock"
+	t.Cleanup(func() { _ = os.Remove(socketPath) })
+	mockSigner := v1alpha1testing.NewMockSigner(t, socketPath)
+	defer mockSigner.CleanUp()
+
+	// Start Api server configured with external signer.
+	client, _, tearDownFn := framework.StartTestServer(ctx, t, framework.TestServerSetup{
+		ModifyServerRunOptions: func(opt *options.ServerRunOptions) {
+			opt.ServiceAccountSigningEndpoint = socketPath
+			opt.ServiceAccountSigningKeyFile = ""
+			opt.Authentication.ServiceAccounts.KeyFiles = []string{}
+		},
+	})
+	defer tearDownFn()
+
+	// Create Namesapce (ns-1) to work with.
+	if _, err := client.CoreV1().Namespaces().Create(ctx, &corev1.Namespace{
+		ObjectMeta: metav1.ObjectMeta{
+			Name: "ns-1",
+		},
+	}, metav1.CreateOptions{}); err != nil {
+		t.Fatalf("Error when creating namespace: %v", err)
+	}
+
+	// Create ServiceAccount (sa-1) to work with.
+	if _, err := client.CoreV1().ServiceAccounts("ns-1").Create(ctx, &corev1.ServiceAccount{
+		ObjectMeta: metav1.ObjectMeta{
+			Name: "sa-1",
+		},
+	}, metav1.CreateOptions{}); err != nil {
+		t.Fatalf("Error when creating service-account: %v", err)
+	}
+
+	testCases := []struct {
+		desc                      string
+		preTestSignerUpdate       func()
+		preValidationSignerUpdate func()
+		wantTokenReqErr           error
+		shouldPassAuth            bool
+	}{
+		{
+			desc:                      "signing key supported.",
+			preTestSignerUpdate:       func() { /*no-op*/ },
+			preValidationSignerUpdate: func() { /*no-op*/ },
+			shouldPassAuth:            true,
+		},
+		{
+			desc: "signing key not among supported set",
+			preTestSignerUpdate: func() {
+				mockSigner.SigningKey = key1
+				mockSigner.SigningKeyID = "updated-kid-1"
+			},
+			preValidationSignerUpdate: func() { /*no-op*/ },
+			shouldPassAuth:            false,
+		},
+		{
+			desc: "signing key corresponds to public key that is excluded from OIDC",
+			preTestSignerUpdate: func() {
+				mockSigner.SigningKey = key1
+				mockSigner.SigningKeyID = "updated-kid-1"
+
+				cpy := make(map[string]v1alpha1testing.KeyT)
+				for key, value := range *mockSigner.SupportedKeys.Load() {
+					cpy[key] = value
+				}
+				cpy["updated-kid-1"] = v1alpha1testing.KeyT{
+					Key:                      pubKey1Bytes,
+					ExcludeFromOidcDiscovery: true,
+				}
+				mockSigner.SupportedKeys.Store(&cpy)
+			},
+			preValidationSignerUpdate: func() { /*no-op*/ },
+			wantTokenReqErr:           fmt.Errorf("failed to generate token: while validating header: key used for signing JWT (kid: updated-kid-1) is excluded from OIDC discovery docs"),
+		},
+		{
+			desc: "different signing and supported keys with same id",
+			preTestSignerUpdate: func() {
+				mockSigner.SigningKey = key1
+			},
+			preValidationSignerUpdate: func() { /*no-op*/ },
+			shouldPassAuth:            false,
+		},
+		{
+			desc: "token gen failure with un-supported Alg type",
+			preTestSignerUpdate: func() {
+				mockSigner.SigningAlg = "ABC"
+			},
+			preValidationSignerUpdate: func() { /*no-op*/ },
+			wantTokenReqErr:           fmt.Errorf("failed to generate token: while validating header: bad signing algorithm \"ABC\""),
+		},
+		{
+			desc: "token gen failure with un-supported token type",
+			preTestSignerUpdate: func() {
+				mockSigner.TokenType = "ABC"
+			},
+			preValidationSignerUpdate: func() { /*no-op*/ },
+			wantTokenReqErr:           fmt.Errorf("failed to generate token: while validating header: bad type"),
+		},
+		{
+			desc: "change of supported keys not picked immediately",
+			preTestSignerUpdate: func() {
+				mockSigner.SigningKey = key1
+			},
+			preValidationSignerUpdate: func() {
+				mockSigner.SupportedKeys.Store(&map[string]v1alpha1testing.KeyT{})
+			},
+			shouldPassAuth: false,
+		},
+		{
+			desc: "change of supported keys picked up after periodic sync",
+			preTestSignerUpdate: func() {
+				mockSigner.SigningKey = key1
+			},
+			preValidationSignerUpdate: func() {
+				cpy := make(map[string]v1alpha1testing.KeyT)
+				for key, value := range *mockSigner.SupportedKeys.Load() {
+					cpy[key] = value
+				}
+				cpy["kid-1"] = v1alpha1testing.KeyT{Key: pubKey1Bytes}
+				mockSigner.SupportedKeys.Store(&cpy)
+				mockSigner.AckKeyFetch <- true
+			},
+			shouldPassAuth: true,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.desc, func(t *testing.T) {
+			// Start fresh
+			err := mockSigner.Reset()
+			if err != nil {
+				t.Fatalf("failed to reset signer for the test %q: %v", tc.desc, err)
+			}
+			mockSigner.AckKeyFetch <- true
+
+			// Adjust parameters on mock signer for the test.
+			tc.preTestSignerUpdate()
+
+			// Request a token for ns-1:sa-1.
+			tokenExpirationSec := int64(2 * 60 * 60) // 2h
+			tokenRequest, err := client.CoreV1().ServiceAccounts("ns-1").CreateToken(ctx, "sa-1", &authv1.TokenRequest{
+				Spec: authv1.TokenRequestSpec{
+					ExpirationSeconds: &tokenExpirationSec,
+				},
+			}, metav1.CreateOptions{})
+			if tc.wantTokenReqErr != nil {
+				if err == nil || !strings.Contains(err.Error(), tc.wantTokenReqErr.Error()) {
+					t.Fatalf("wanted error: %v, got error: %v", tc.wantTokenReqErr, err)
+				}
+				return
+			} else if err != nil {
+				t.Fatalf("Error when creating token: %v", err)
+			}
+
+			// Adjust parameters on mock signer for the test.
+			tc.preValidationSignerUpdate()
+
+			// Try Validating the token.
+			tokenReviewResult, err := client.AuthenticationV1().TokenReviews().Create(ctx, &authv1.TokenReview{
+				Spec: authv1.TokenReviewSpec{
+					Token: tokenRequest.Status.Token,
+				},
+			}, metav1.CreateOptions{})
+			if err != nil {
+				t.Fatalf("Error when validating token: %v", err)
+			}
+
+			if !tokenReviewResult.Status.Authenticated && tc.shouldPassAuth {
+				t.Fatal("Expected Authentication to succeed")
+			} else if tokenReviewResult.Status.Authenticated && !tc.shouldPassAuth {
+				t.Fatal("Expected Authentication to fail")
+			}
+		})
+	}
+}
+
+func TestDelayedStartForSigner(t *testing.T) {
+	// Enable feature gate for external JWT signer.
+	featuregatetesting.SetFeatureGateDuringTest(t, utilfeature.DefaultFeatureGate, features.ExternalServiceAccountTokenSigner, true)
+
+	tCtx := ktesting.Init(t)
+	ctx, cancel := context.WithCancel(tCtx)
+	defer cancel()
+
+	// Schedule signer to start on socket after 20 sec
+	socketPath := "@mock-external-jwt-signer.sock"
+	t.Cleanup(func() { _ = os.Remove(socketPath) })
+	go func() {
+		time.Sleep(20 * time.Second)
+		v1alpha1testing.NewMockSigner(t, socketPath)
+	}()
+
+	// Start Api server configured with external signer.
+	client, _, tearDownFn := framework.StartTestServer(ctx, t, framework.TestServerSetup{
+		ModifyServerRunOptions: func(opt *options.ServerRunOptions) {
+			opt.ServiceAccountSigningEndpoint = socketPath
+			opt.ServiceAccountSigningKeyFile = ""
+			opt.Authentication.ServiceAccounts.KeyFiles = []string{}
+		},
+	})
+	defer tearDownFn()
+
+	// Create Namesapce (ns-1) to work with.
+	if _, err := client.CoreV1().Namespaces().Create(ctx, &corev1.Namespace{
+		ObjectMeta: metav1.ObjectMeta{
+			Name: "ns-1",
+		},
+	}, metav1.CreateOptions{}); err != nil {
+		t.Fatalf("Error when creating namespace: %v", err)
+	}
+
+	// Create ServiceAccount (sa-1) to work with.
+	if _, err := client.CoreV1().ServiceAccounts("ns-1").Create(ctx, &corev1.ServiceAccount{
+		ObjectMeta: metav1.ObjectMeta{
+			Name: "sa-1",
+		},
+	}, metav1.CreateOptions{}); err != nil {
+		t.Fatalf("Error when creating service-account: %v", err)
+	}
+
+	// Request a token for ns-1:sa-1.
+	tokenExpirationSec := int64(2 * 60 * 60) // 2h
+	tokenRequest, err := client.CoreV1().ServiceAccounts("ns-1").CreateToken(ctx, "sa-1", &authv1.TokenRequest{
+		Spec: authv1.TokenRequestSpec{
+			ExpirationSeconds: &tokenExpirationSec,
+		},
+	}, metav1.CreateOptions{})
+	if err != nil {
+		t.Fatalf("Error when creating token: %v", err)
+	}
+
+	// Try Validating the token.
+	tokenReviewResult, err := client.AuthenticationV1().TokenReviews().Create(ctx, &authv1.TokenReview{
+		Spec: authv1.TokenReviewSpec{
+			Token: tokenRequest.Status.Token,
+		},
+	}, metav1.CreateOptions{})
+	if err != nil {
+		t.Fatalf("Error when validating token: %v", err)
+	}
+	if !tokenReviewResult.Status.Authenticated {
+		t.Fatal("Expected Authentication to succeed")
+	}
+}

vendor/modules.txt

@@ -1055,6 +1055,8 @@ gopkg.in/yaml.v3
 ## explicit; go 1.23.0
 # k8s.io/endpointslice v0.0.0 => ./staging/src/k8s.io/endpointslice
 ## explicit; go 1.23.0
+# k8s.io/externaljwt v0.0.0 => ./staging/src/k8s.io/externaljwt
+## explicit; go 1.23.0
 # k8s.io/gengo/v2 v2.0.0-20240911193312-2b36238f13e9
 ## explicit; go 1.20
 k8s.io/gengo/v2