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Move constant time memcmp for signature verification

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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`.
This commit is contained in:
Hanno Becker 2017-09-06 12:32:16 +01:00
parent 558477d073
commit 171a8f1c95
1 changed files with 15 additions and 9 deletions
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24
library/rsa.c
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@ -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;