Remove context-as-nonce, add docs, and properly support datakey

CHANGELOG.md

@@ -29,9 +29,10 @@ FEATURES:
    deprecates App-ID. [GH-1426]
  * **Convergent Encryption in `Transit`**: The `transit` backend now supports a
    convergent encryption mode where the same plaintext will produce the same
-   ciphertext. Although very useful in some situations, this has security
-   implications, which are mostly mitigated by requiring the use of key
-   derivation when convergent encryption is enabled. See [the `transit`
+   ciphertext. Although very useful in some situations, this has potential
+   security implications, which are mostly mitigated by requiring the use of
+   key derivation when convergent encryption is enabled. See [the `transit`
+   backend
    documentation](https://www.vaultproject.io/docs/secrets/transit/index.html)
    for more details. [GH-1537]
  * **Improved LDAP Group Filters**: The `ldap` auth backend now uses templates

builtin/logical/transit/backend_test.go

@@ -230,7 +230,6 @@ func testAccStepReadPolicy(t *testing.T, name string, expectNone, derived bool)
 				KDFMode              string           `mapstructure:"kdf_mode"`
 				DeletionAllowed      bool             `mapstructure:"deletion_allowed"`
 				ConvergentEncryption bool             `mapstructure:"convergent_encryption"`
-				ContextAsNonce       bool             `mapstructure:"context_as_nonce"`
 			}
 			if err := mapstructure.Decode(resp.Data, &d); err != nil {
 				return err
@@ -608,6 +607,16 @@ func TestConvergentEncryption(t *testing.T) {
 		t.Fatalf("expected error response, got %#v", *resp)
 	}
 
+	// Ensure we fail if we do not provide a nonce
+	req.Data = map[string]interface{}{
+		"plaintext": "emlwIHphcA==", // "zip zap"
+		"context":   "pWZ6t/im3AORd0lVYE0zBdKpX6Bl3/SvFtoVTPWbdkzjG788XmMAnOlxandSdd7S",
+	}
+	resp, err = b.HandleRequest(req)
+	if err == nil && (resp == nil || !resp.IsError()) {
+		t.Fatal("expected error response")
+	}
+
 	// Now test encrypting the same value twice
 	req.Data = map[string]interface{}{
 		"plaintext": "emlwIHphcA==",     // "zip zap"
@@ -702,126 +711,6 @@ func TestConvergentEncryption(t *testing.T) {
 	}
 }
 
-func TestConvergentEncryptionContextAsNonce(t *testing.T) {
-	var b *backend
-	sysView := logical.TestSystemView()
-	storage := &logical.InmemStorage{}
-
-	b = Backend(&logical.BackendConfig{
-		StorageView: storage,
-		System:      sysView,
-	})
-
-	req := &logical.Request{
-		Storage:   storage,
-		Operation: logical.UpdateOperation,
-		Path:      "keys/testkeynonderived",
-		Data: map[string]interface{}{
-			"derived":               false,
-			"convergent_encryption": true,
-			"context_as_nonce":      true,
-		},
-	}
-
-	resp, err := b.HandleRequest(req)
-	if err != nil {
-		t.Fatal(err)
-	}
-	if resp == nil {
-		t.Fatal("expected non-nil response")
-	}
-	if !resp.IsError() {
-		t.Fatalf("bad: expected error response, got %#v", *resp)
-	}
-
-	req.Path = "keys/testkey"
-	req.Data = map[string]interface{}{
-		"derived":               true,
-		"convergent_encryption": true,
-		"context_as_nonce":      true,
-	}
-
-	resp, err = b.HandleRequest(req)
-	if err != nil {
-		t.Fatal(err)
-	}
-	if resp != nil {
-		t.Fatalf("bad: got resp %#v", *resp)
-	}
-
-	// First, test using an invalid length of nonce
-	req.Path = "encrypt/testkey"
-	req.Data = map[string]interface{}{
-		"plaintext": "emlwIHphcA==", // "zip zap"
-		"context":   "Zm9vIGJhcg==", // "foo bar"
-	}
-	resp, err = b.HandleRequest(req)
-	if resp == nil {
-		t.Fatal("expected non-nil response")
-	}
-	if !resp.IsError() {
-		t.Fatalf("expected error response, got %#v", *resp)
-	}
-
-	// Now test encrypting the same value twice
-	req.Data = map[string]interface{}{
-		"plaintext": "emlwIHphcA==",     // "zip zap"
-		"context":   "b25ldHdvdGhyZWVl", // "onetwothreee"
-	}
-	resp, err = b.HandleRequest(req)
-	if resp == nil {
-		t.Fatal("expected non-nil response")
-	}
-	if resp.IsError() {
-		t.Fatalf("got error response: %#v", *resp)
-	}
-	ciphertext1 := resp.Data["ciphertext"].(string)
-
-	resp, err = b.HandleRequest(req)
-	if resp == nil {
-		t.Fatal("expected non-nil response")
-	}
-	if resp.IsError() {
-		t.Fatalf("got error response: %#v", *resp)
-	}
-	ciphertext2 := resp.Data["ciphertext"].(string)
-
-	if ciphertext1 != ciphertext2 {
-		t.Fatalf("expected the same ciphertext but got %s and %s", ciphertext1, ciphertext2)
-	}
-
-	// For sanity, also check a different value
-	req.Data = map[string]interface{}{
-		"plaintext": "emlwIHphcA==",     // "zip zap"
-		"context":   "dHdvdGhyZWVmb3Vy", // "twothreefour"
-	}
-	resp, err = b.HandleRequest(req)
-	if resp == nil {
-		t.Fatal("expected non-nil response")
-	}
-	if resp.IsError() {
-		t.Fatalf("got error response: %#v", *resp)
-	}
-	ciphertext3 := resp.Data["ciphertext"].(string)
-
-	resp, err = b.HandleRequest(req)
-	if resp == nil {
-		t.Fatal("expected non-nil response")
-	}
-	if resp.IsError() {
-		t.Fatalf("got error response: %#v", *resp)
-	}
-	ciphertext4 := resp.Data["ciphertext"].(string)
-
-	if ciphertext3 != ciphertext4 {
-		t.Fatalf("expected the same ciphertext but got %s and %s", ciphertext3, ciphertext4)
-	}
-	if ciphertext1 == ciphertext3 {
-		t.Fatalf("expected different ciphertexts")
-	}
-
-}
-
 func TestPolicyFuzzing(t *testing.T) {
 	var be *backend
 	sysView := logical.TestSystemView()

builtin/logical/transit/lock_manager.go

@@ -105,42 +105,42 @@ func (lm *lockManager) UnlockPolicy(lock *sync.RWMutex, lockType bool) {
 // is needed (for instance, for an upgrade/migration), give up the read lock,
 // call again with an exclusive lock, then swap back out for a read lock.
 func (lm *lockManager) GetPolicyShared(storage logical.Storage, name string) (*Policy, *sync.RWMutex, error) {
-	p, lock, _, err := lm.getPolicyCommon(storage, name, false, false, false, false, shared)
+	p, lock, _, err := lm.getPolicyCommon(storage, name, false, false, false, shared)
 	if err == nil ||
 		(err != nil && err != errNeedExclusiveLock) {
 		return p, lock, err
 	}
 
 	// Try again while asking for an exlusive lock
-	p, lock, _, err = lm.getPolicyCommon(storage, name, false, false, false, false, exclusive)
+	p, lock, _, err = lm.getPolicyCommon(storage, name, false, false, false, exclusive)
 	if err != nil || p == nil || lock == nil {
 		return p, lock, err
 	}
 
 	lock.Unlock()
 
-	p, lock, _, err = lm.getPolicyCommon(storage, name, false, false, false, false, shared)
+	p, lock, _, err = lm.getPolicyCommon(storage, name, false, false, false, shared)
 	return p, lock, err
 }
 
 // Get the policy with an exclusive lock
 func (lm *lockManager) GetPolicyExclusive(storage logical.Storage, name string) (*Policy, *sync.RWMutex, error) {
-	p, lock, _, err := lm.getPolicyCommon(storage, name, false, false, false, false, exclusive)
+	p, lock, _, err := lm.getPolicyCommon(storage, name, false, false, false, exclusive)
 	return p, lock, err
 }
 
 // Get the policy with a read lock; if it returns that an exclusive lock is
 // needed, retry. If successful, call one more time to get a read lock and
 // return the value.
-func (lm *lockManager) GetPolicyUpsert(storage logical.Storage, name string, derived, convergent, contextAsNonce bool) (*Policy, *sync.RWMutex, bool, error) {
-	p, lock, _, err := lm.getPolicyCommon(storage, name, true, derived, convergent, contextAsNonce, shared)
+func (lm *lockManager) GetPolicyUpsert(storage logical.Storage, name string, derived, convergent bool) (*Policy, *sync.RWMutex, bool, error) {
+	p, lock, _, err := lm.getPolicyCommon(storage, name, true, derived, convergent, shared)
 	if err == nil ||
 		(err != nil && err != errNeedExclusiveLock) {
 		return p, lock, false, err
 	}
 
 	// Try again while asking for an exlusive lock
-	p, lock, upserted, err := lm.getPolicyCommon(storage, name, true, derived, convergent, contextAsNonce, exclusive)
+	p, lock, upserted, err := lm.getPolicyCommon(storage, name, true, derived, convergent, exclusive)
 	if err != nil || p == nil || lock == nil {
 		return p, lock, upserted, err
 	}
@@ -148,14 +148,14 @@ func (lm *lockManager) GetPolicyUpsert(storage logical.Storage, name string, der
 	lock.Unlock()
 
 	// Now get a shared lock for the return, but preserve the value of upsert
-	p, lock, _, err = lm.getPolicyCommon(storage, name, true, derived, convergent, contextAsNonce, shared)
+	p, lock, _, err = lm.getPolicyCommon(storage, name, true, derived, convergent, shared)
 
 	return p, lock, upserted, err
 }
 
 // When the function returns, a lock will be held on the policy if err == nil.
 // It is the caller's responsibility to unlock.
-func (lm *lockManager) getPolicyCommon(storage logical.Storage, name string, upsert, derived, convergent, contextAsNonce, lockType bool) (*Policy, *sync.RWMutex, bool, error) {
+func (lm *lockManager) getPolicyCommon(storage logical.Storage, name string, upsert, derived, convergent, lockType bool) (*Policy, *sync.RWMutex, bool, error) {
 	lock := lm.policyLock(name, lockType)
 
 	var p *Policy
@@ -204,8 +204,6 @@ func (lm *lockManager) getPolicyCommon(storage logical.Storage, name string, ups
 		if derived {
 			p.KDFMode = kdfMode
 			p.ConvergentEncryption = convergent
-			p.ContextAsNonce = new(bool)
-			*p.ContextAsNonce = contextAsNonce
 		}
 
 		err = p.rotate(storage)

builtin/logical/transit/path_datakey.go

@@ -30,6 +30,11 @@ ciphertext; "wrapped" will return the ciphertext only.`,
 				Description: "Context for key derivation. Required for derived keys.",
 			},
 
+			"nonce": &framework.FieldSchema{
+				Type:        framework.TypeString,
+				Description: "Nonce for when convergent encryption is used",
+			},
+
 			"bits": &framework.FieldSchema{
 				Type: framework.TypeInt,
 				Description: `Number of bits for the key; currently 128, 256,
@@ -61,17 +66,28 @@ func (b *backend) pathDatakeyWrite(
 		return logical.ErrorResponse("Invalid path, must be 'plaintext' or 'wrapped'"), logical.ErrInvalidRequest
 	}
 
+	var err error
+
 	// Decode the context if any
 	contextRaw := d.Get("context").(string)
 	var context []byte
 	if len(contextRaw) != 0 {
-		var err error
 		context, err = base64.StdEncoding.DecodeString(contextRaw)
 		if err != nil {
 			return logical.ErrorResponse("failed to decode context as base64"), logical.ErrInvalidRequest
 		}
 	}
 
+	// Decode the nonce if any
+	nonceRaw := d.Get("nonce").(string)
+	var nonce []byte
+	if len(nonceRaw) != 0 {
+		nonce, err = base64.StdEncoding.DecodeString(nonceRaw)
+		if err != nil {
+			return logical.ErrorResponse("failed to decode nonce as base64"), logical.ErrInvalidRequest
+		}
+	}
+
 	// Get the policy
 	p, lock, err := b.lm.GetPolicyShared(req.Storage, name)
 	if lock != nil {
@@ -100,7 +116,7 @@ func (b *backend) pathDatakeyWrite(
 		return nil, err
 	}
 
-	ciphertext, err := p.Encrypt(context, nil, base64.StdEncoding.EncodeToString(newKey))
+	ciphertext, err := p.Encrypt(context, nonce, base64.StdEncoding.EncodeToString(newKey))
 	if err != nil {
 		switch err.(type) {
 		case errutil.UserError:

builtin/logical/transit/path_decrypt.go

@@ -30,7 +30,7 @@ func (b *backend) pathDecrypt() *framework.Path {
 
 			"nonce": &framework.FieldSchema{
 				Type:        framework.TypeString,
-				Description: "Nonce for when convergent encryption is used and the context is not used as the nonce",
+				Description: "Nonce for when convergent encryption is used",
 			},
 		},
 

builtin/logical/transit/path_encrypt.go

@@ -31,7 +31,7 @@ func (b *backend) pathEncrypt() *framework.Path {
 
 			"nonce": &framework.FieldSchema{
 				Type:        framework.TypeString,
-				Description: "Nonce for when convergent encryption is used and the context is not used as the nonce",
+				Description: "Nonce for when convergent encryption is used",
 			},
 		},
 
@@ -95,7 +95,7 @@ func (b *backend) pathEncryptWrite(
 	var lock *sync.RWMutex
 	var upserted bool
 	if req.Operation == logical.CreateOperation {
-		p, lock, upserted, err = b.lm.GetPolicyUpsert(req.Storage, name, len(context) != 0, false, false)
+		p, lock, upserted, err = b.lm.GetPolicyUpsert(req.Storage, name, len(context) != 0, false)
 	} else {
 		p, lock, err = b.lm.GetPolicyShared(req.Storage, name)
 	}

builtin/logical/transit/path_keys.go

@@ -29,31 +29,14 @@ allows for per-transaction unique keys.`,
 This is only supported when using a key with
 key derivation enabled and will require all
 requests to carry both a context and 96-bit
-(12-byte) nonce, unless the "context_as_nonce"
-feature is also enabled. The given nonce will
-be used in place of a randomly generated nonce.
-As a result, when the same context and nonce
-(or context, if "context_as_nonce" is enabled)
-are supplied, the same ciphertext is emitted
-from the encryption function. It is *very
-important* when using this mode that you ensure
-that all nonces are unique for a given context,
-or, when using "context_as_nonce", that all
-contexts are unique for a given key. Failing to
-do so will severely impact the ciphertext's
-security.`,
-			},
-
-			"context_as_nonce": &framework.FieldSchema{
-				Type: framework.TypeBool,
-				Description: `Whether to use the context value as the
-nonce in the convergent encryption operation
-mode. If set true, the user will have to
-supply a 96-bit (12-byte) context value.
-It is *very important* when using this
-mode that you ensure that all contexts are
-*globally unique*. Failing to do so will
-severely impact the security of the key.`,
+(12-byte) nonce. The given nonce will be used
+in place of a randomly generated nonce. As a
+result, when the same context and nonce are
+supplied, the same ciphertext is generated. It
+is *very important* when using this mode that
+you ensure that all nonces are unique for a
+given context. Failing to do so will severely
+impact the ciphertext's security.`,
 			},
 		},
 
@@ -73,13 +56,12 @@ func (b *backend) pathPolicyWrite(
 	name := d.Get("name").(string)
 	derived := d.Get("derived").(bool)
 	convergent := d.Get("convergent_encryption").(bool)
-	contextAsNonce := d.Get("context_as_nonce").(bool)
 
 	if !derived && convergent {
 		return logical.ErrorResponse("convergent encryption requires derivation to be enabled"), nil
 	}
 
-	p, lock, upserted, err := b.lm.GetPolicyUpsert(req.Storage, name, derived, convergent, contextAsNonce)
+	p, lock, upserted, err := b.lm.GetPolicyUpsert(req.Storage, name, derived, convergent)
 	if lock != nil {
 		defer lock.RUnlock()
 	}
@@ -127,9 +109,6 @@ func (b *backend) pathPolicyRead(
 	if p.Derived {
 		resp.Data["kdf_mode"] = p.KDFMode
 		resp.Data["convergent_encryption"] = p.ConvergentEncryption
-		if p.ContextAsNonce != nil {
-			resp.Data["context_as_nonce"] = *p.ContextAsNonce
-		}
 	}
 
 	retKeys := map[string]int64{}

builtin/logical/transit/path_rewrap.go

@@ -30,7 +30,7 @@ func (b *backend) pathRewrap() *framework.Path {
 
 			"nonce": &framework.FieldSchema{
 				Type:        framework.TypeString,
-				Description: "Nonce for when convergent encryption is used and the context is not used as the nonce",
+				Description: "Nonce for when convergent encryption is used",
 			},
 		},
 

builtin/logical/transit/policy.go

@@ -72,7 +72,6 @@ type Policy struct {
 	Derived              bool   `json:"derived"`
 	KDFMode              string `json:"kdf_mode"`
 	ConvergentEncryption bool   `json:"convergent_encryption"`
-	ContextAsNonce       *bool  `json:"context_as_nonce"`
 
 	// The minimum version of the key allowed to be used
 	// for decryption
@@ -260,10 +259,6 @@ func (p *Policy) needsUpgrade() bool {
 		return true
 	}
 
-	if p.ConvergentEncryption && p.ContextAsNonce == nil {
-		return true
-	}
-
 	return false
 }
 
@@ -293,14 +288,6 @@ func (p *Policy) upgrade(storage logical.Storage) error {
 		persistNeeded = true
 	}
 
-	// Originally the context-as-nonce mode was the only mode, so keep that
-	// behavior if convergent encryption is already in use
-	if p.ConvergentEncryption && p.ContextAsNonce == nil {
-		p.ContextAsNonce = new(bool)
-		*p.ContextAsNonce = true
-		persistNeeded = true
-	}
-
 	if persistNeeded {
 		err := p.Persist(storage)
 		if err != nil {
@@ -377,17 +364,9 @@ func (p *Policy) Encrypt(context, nonce []byte, value string) (string, error) {
 	}
 
 	if p.ConvergentEncryption {
-
-		if *p.ContextAsNonce {
-			if len(context) != gcm.NonceSize() {
-				return "", errutil.UserError{Err: fmt.Sprintf("base64-decoded context must be %d bytes long when using convergent encryption with context-as-nonce with this key", gcm.NonceSize())}
-			}
-			nonce = context
-
-		} else if len(nonce) != gcm.NonceSize() {
+		if len(nonce) != gcm.NonceSize() {
 			return "", errutil.UserError{Err: fmt.Sprintf("base64-decoded nonce must be %d bytes long when using convergent encryption with this key", gcm.NonceSize())}
 		}
-
 	} else {
 		// Compute random nonce
 		nonce = make([]byte, gcm.NonceSize())
@@ -421,7 +400,7 @@ func (p *Policy) Decrypt(context, nonce []byte, value string) (string, error) {
 		return "", errutil.UserError{Err: "invalid ciphertext: no prefix"}
 	}
 
-	if p.ConvergentEncryption && !*p.ContextAsNonce && (nonce == nil || len(nonce) == 0) {
+	if p.ConvergentEncryption && (nonce == nil || len(nonce) == 0) {
 		return "", errutil.UserError{Err: "invalid convergent nonce supplied"}
 	}
 
@@ -483,9 +462,6 @@ func (p *Policy) Decrypt(context, nonce []byte, value string) (string, error) {
 	// Extract the nonce and ciphertext
 	var ciphertext []byte
 	if p.ConvergentEncryption {
-		if *p.ContextAsNonce {
-			nonce = context
-		}
 		ciphertext = decoded
 	} else {
 		nonce = decoded[:gcm.NonceSize()]

builtin/logical/transit/policy_test.go

@@ -22,7 +22,7 @@ func Test_KeyUpgrade(t *testing.T) {
 
 func testKeyUpgradeCommon(t *testing.T, lm *lockManager) {
 	storage := &logical.InmemStorage{}
-	p, lock, upserted, err := lm.GetPolicyUpsert(storage, "test", false, false, false)
+	p, lock, upserted, err := lm.GetPolicyUpsert(storage, "test", false, false)
 	if lock != nil {
 		defer lock.RUnlock()
 	}
@@ -68,7 +68,7 @@ func testArchivingUpgradeCommon(t *testing.T, lm *lockManager) {
 
 	storage := &logical.InmemStorage{}
 
-	p, lock, _, err := lm.GetPolicyUpsert(storage, "test", false, false, false)
+	p, lock, _, err := lm.GetPolicyUpsert(storage, "test", false, false)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -198,7 +198,7 @@ func testArchivingCommon(t *testing.T, lm *lockManager) {
 
 	storage := &logical.InmemStorage{}
 
-	p, lock, _, err := lm.GetPolicyUpsert(storage, "test", false, false, false)
+	p, lock, _, err := lm.GetPolicyUpsert(storage, "test", false, false)
 	if lock != nil {
 		defer lock.RUnlock()
 	}

website/source/docs/secrets/transit/index.html.md

@@ -33,7 +33,7 @@ data's attack surface.
 Key derivation is supported, which allows the same key to be used for multiple
 purposes by deriving a new key based on a user-supplied context value. In this
 mode, convergent encryption can optionally be supported, which allows the same
-context and plaintext to produce the same ciphertext.
+input values to produce the same ciphertext.
 
 The backend also supports key rotation, which allows a new version of the named
 key to be generated. All data encrypted with the key will use the newest
@@ -156,12 +156,16 @@ only encrypt or decrypt using the named keys they need access to.
         <span class="param-flags">optional</span>
         If set, the key will support convergent encryption, where the same
         plaintext creates the same ciphertext. This requires _derived_ to be
-        set to `true`. When enabled, the context value must be exactly 12 bytes
-        (96 bits) and will both be used to derive the key and as the nonce for
-        the encryption operation. Note that while this is useful for particular
-        situations, it also has security implications. In particular, you must
-        ensure that you do **not** use the same context value for more than one
-        plaintext value. Defaults to false.
+        set to `true`. When enabled, each
+        encryption(/decryption/rewrap/datakey) operation will require a `nonce`
+        value to be specified. Note that while this is useful for particular
+        situations, all nonce values used with a given context value **must be
+        unique** or it will compromise the security of your key. A common way
+        to use this will be to generate a unique identifier for the given data
+        (for instance, a SHA-512 sum), then separate the bytes so that twelve
+        bytes are used as the nonce and the remaining as the context, ensuring
+        that all bits of unique identity are used as a part of the encryption
+        operation. Defaults to false.
       </li>
     </ul>
   </dd>
@@ -347,6 +351,15 @@ only encrypt or decrypt using the named keys they need access to.
         The key derivation context, provided as base64 encoded.
         Must be provided if derivation is enabled.
       </li>
+      <li>
+        <span class="param">nonce</span>
+        <span class="param-flags">optional</span>
+        The nonce value, provided as base64 encoded. Must be provided if
+        convergent encryption is enabled for this key. The value must be
+        exactly 96 bits (12 bytes) long and the user must ensure that for any
+        given context (and thus, any given encryption key) this nonce value is
+        **never reused**.
+      </li>
     </ul>
   </dd>
 
@@ -393,6 +406,12 @@ only encrypt or decrypt using the named keys they need access to.
         The key derivation context, provided as base64 encoded.
         Must be provided if derivation is enabled.
       </li>
+      <li>
+        <span class="param">nonce</span>
+        <span class="param-flags">optional</span>
+        The nonce value used during encryption, provided as base64 encoded.
+        Must be provided if convergent encryption is enabled for this key.
+      </li>
     </ul>
   </dd>
 
@@ -441,6 +460,12 @@ only encrypt or decrypt using the named keys they need access to.
         The key derivation context, provided as base64 encoded.
         Must be provided if derivation is enabled.
       </li>
+      <li>
+        <span class="param">nonce</span>
+        <span class="param-flags">optional</span>
+        The nonce value used during encryption, provided as base64 encoded.
+        Must be provided if convergent encryption is enabled for this key.
+      </li>
     </ul>
   </dd>
 
@@ -493,6 +518,15 @@ only encrypt or decrypt using the named keys they need access to.
         The key derivation context, provided as base64 encoded.
         Must be provided if derivation is enabled.
       </li>
+      <li>
+        <span class="param">nonce</span>
+        <span class="param-flags">optional</span>
+        The nonce value, provided as base64 encoded. Must be provided if
+        convergent encryption is enabled for this key. The value must be
+        exactly 96 bits (12 bytes) long and the user must ensure that for any
+        given context (and thus, any given encryption key) this nonce value is
+        **never reused**.
+      </li>
       <li>
         <span class="param">bits</span>
         <span class="param-flags">optional</span>