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common/rsa: Add support for exponent 3 RSA keys

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These keys are much quicker to verify (259ms to 51ms on a -M0 at
48 Mhz), so they can be used when timing is critical and
verification needs to be performed on the board.

BRANCH=none
BUG=chromium:663631
TEST=make buildall -j && make run-rsa
TEST=make run-rsa3 (next commit)

Change-Id: I0da4b3e21543bb6f7b18e8b6ddc5e153046a61b8
Reviewed-on: https://chromium-review.googlesource.com/408006
Commit-Ready: Nicolas Boichat <drinkcat@chromium.org>
Tested-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>
This commit is contained in:
Nicolas Boichat
2016-11-07 15:17:53 +08:00
committed by chrome-bot
parent 5edf3f5a8c
commit ef0f355e47
2 changed files with 50 additions and 2 deletions
Download Patch File Download Diff File Expand all files Collapse all files

49
common/rsa.c
View File

@@ -69,6 +69,42 @@ static void mont_mul_add(const struct rsa_public_key *key,
sub_mod(key, c);
}
#ifdef CONFIG_RSA_EXPONENT_3
/**
* Montgomery c[] += 0 * b[] / R % mod
*/
static void mont_mul_add_0(const struct rsa_public_key *key,
uint32_t *c,
const uint32_t *b)
{
uint32_t d0 = c[0] * key->n0inv;
uint64_t B = (uint64_t)d0 * key->n[0] + c[0];
uint32_t i;
for (i = 1; i < RSANUMWORDS; ++i) {
B = (B >> 32) + (uint64_t)d0 * key->n[i] + c[i];
c[i - 1] = (uint32_t)B;
}
c[i - 1] = B >> 32;
}
/* Montgomery c[] = a[] * 1 / R % key. */
static void mont_mul_1(const struct rsa_public_key *key,
uint32_t *c,
const uint32_t *a)
{
int i;
for (i = 0; i < RSANUMWORDS; ++i)
c[i] = 0;
mont_mul_add(key, c, 1, a);
for (i = 1; i < RSANUMWORDS; ++i)
mont_mul_add_0(key, c, a);
}
#endif
/**
* Montgomery c[] = a[] * b[] / R % mod
*/
@@ -87,13 +123,14 @@ static void mont_mul(const struct rsa_public_key *key,
/**
* In-place public exponentiation.
* Exponent depends on the configuration (65537 (default), or 3).
*
* @param key Key to use in signing
* @param inout Input and output big-endian byte array
* @param workbuf32 Work buffer; caller must verify this is
* 3 x RSANUMWORDS elements long.
*/
static void mod_pow_F4(const struct rsa_public_key *key, uint8_t *inout,
static void mod_pow(const struct rsa_public_key *key, uint8_t *inout,
uint32_t *workbuf32)
{
uint32_t *a = workbuf32;
@@ -112,12 +149,20 @@ static void mod_pow_F4(const struct rsa_public_key *key, uint8_t *inout,
a[i] = tmp;
}
/* TODO(drinkcat): This operation could be precomputed to save time. */
mont_mul(key, a_r, a, key->rr); /* a_r = a * RR / R mod M */
#ifdef CONFIG_RSA_EXPONENT_3
mont_mul(key, aa_r, a_r, a_r);
mont_mul(key, a, aa_r, a_r);
mont_mul_1(key, aaa, a);
#else
/* Exponent 65537 */
for (i = 0; i < 16; i += 2) {
mont_mul(key, aa_r, a_r, a_r); /* aa_r = a_r * a_r / R mod M */
mont_mul(key, a_r, aa_r, aa_r);/* a_r = aa_r * aa_r / R mod M */
}
mont_mul(key, aaa, a_r, a); /* aaa = a_r * a / R mod M */
#endif
/* Make sure aaa < mod; aaa is at most 1x mod too large. */
if (ge_mod(key, aaa))
@@ -200,7 +245,7 @@ int rsa_verify(const struct rsa_public_key *key, const uint8_t *signature,
/* Copy input to local workspace. */
memcpy(buf, signature, RSANUMBYTES);
mod_pow_F4(key, buf, workbuf32); /* In-place exponentiation. */
mod_pow(key, buf, workbuf32); /* In-place exponentiation. */
/* Check the PKCS#1 padding */
if (check_padding(buf) != 0)

3
include/config.h
View File

@@ -1664,6 +1664,9 @@
/* Define the RSA key size. */
#undef CONFIG_RSA_KEY_SIZE
/* Use RSA exponent 3 instead of F4 (65537) */
#undef CONFIG_RSA_EXPONENT_3
/*
* Verify the RW firmware using the RSA signature.
* (for accessories without software sync)