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Disallow Ed448 signature malleability

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Check that s is less than the order before attempting to verify the
signature as per RFC8032 5.2.7

Fixes #7706

Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/7748)

(cherry picked from commit 08afd2f37a)
This commit is contained in:
Matt Caswell 2018-12-03 17:01:07 +00:00
parent 488521d77f
commit f807ad17f3
1 changed files with 28 additions and 2 deletions
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30
crypto/ec/curve448/eddsa.c
View file

@ -246,10 +246,36 @@ c448_error_t c448_ed448_verify(
uint8_t context_len)
{
curve448_point_t pk_point, r_point;
c448_error_t error =
curve448_point_decode_like_eddsa_and_mul_by_ratio(pk_point, pubkey);
c448_error_t error;
curve448_scalar_t challenge_scalar;
curve448_scalar_t response_scalar;
/* Order in little endian format */
static const uint8_t order[] = {
0xF3, 0x44, 0x58, 0xAB, 0x92, 0xC2, 0x78, 0x23, 0x55, 0x8F, 0xC5, 0x8D,
0x72, 0xC2, 0x6C, 0x21, 0x90, 0x36, 0xD6, 0xAE, 0x49, 0xDB, 0x4E, 0xC4,
0xE9, 0x23, 0xCA, 0x7C, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x3F, 0x00
};
int i;
/*
* Check that s (second 57 bytes of the sig) is less than the order. Both
* s and the order are in little-endian format. This can be done in
* variable time, since if this is not the case the signature if publicly
* invalid.
*/
for (i = EDDSA_448_PUBLIC_BYTES - 1; i >= 0; i--) {
if (signature[i + EDDSA_448_PUBLIC_BYTES] > order[i])
return C448_FAILURE;
if (signature[i + EDDSA_448_PUBLIC_BYTES] < order[i])
break;
}
if (i < 0)
return C448_FAILURE;
error =
curve448_point_decode_like_eddsa_and_mul_by_ratio(pk_point, pubkey);
if (C448_SUCCESS != error)
return error;