Move constant time memcmp for signature verification
This commit replaces the ad-hoc code for constant-time double-checking the PKCS1 v1.5 RSA signature by an invocation of
`mbedtls_safer_memcmp`.
@@ -71,6 +71,20 @@ static void mbedtls_zeroize( void *v, size_t n ) {
volatile unsigned char *p = (unsigned char*)v; while( n-- ) *p++ = 0;
}
+/* constant-time buffer comparison */
+static inline int mbedtls_safer_memcmp( const void *a, const void *b, size_t n )
+{
+ size_t i;
+ const unsigned char *A = (const unsigned char *) a;
+ const unsigned char *B = (const unsigned char *) b;
+ unsigned char diff = 0;
+
+ for( i = 0; i < n; i++ )
+ diff |= A[i] ^ B[i];
+
+ return( diff );
+}
+
/*
* Initialize an RSA context
*/
@@ -1162,9 +1176,6 @@ int mbedtls_rsa_rsassa_pkcs1_v15_sign( mbedtls_rsa_context *ctx,
unsigned char *p = sig;
const char *oid = NULL;
unsigned char *sig_try = NULL, *verif = NULL;
- size_t i;
- unsigned char diff;
- volatile unsigned char diff_no_optimize;
int ret;
if( mode == MBEDTLS_RSA_PRIVATE && ctx->padding != MBEDTLS_RSA_PKCS_V15 )
@@ -1249,12 +1260,7 @@ int mbedtls_rsa_rsassa_pkcs1_v15_sign( mbedtls_rsa_context *ctx,
MBEDTLS_MPI_CHK( mbedtls_rsa_private( ctx, f_rng, p_rng, sig, sig_try ) );
MBEDTLS_MPI_CHK( mbedtls_rsa_public( ctx, sig_try, verif ) );
- /* Compare in constant time just in case */
- for( diff = 0, i = 0; i < ctx->len; i++ )
- diff |= verif[i] ^ sig[i];
- diff_no_optimize = diff;
-
- if( diff_no_optimize != 0 )
+ if( mbedtls_safer_memcmp( verif, sig, ctx->len ) != 0 )
{
ret = MBEDTLS_ERR_RSA_PRIVATE_FAILED;
goto cleanup;
