Disallow Ed448 signature malleability
Check that s is less than the order before attempting to verify the
signature as per RFC8032 5.2.7
Fixes
@@ -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;
