Add counter-measure against RSA-CRT attack
https://securityblog.redhat.com/2015/09/02/factoring-rsa-keys-with-tls-perfect-forward-secrecy/
diff --git a/library/rsa.c b/library/rsa.c
index f4ab6b2a..3883d09 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -51,6 +51,8 @@
#else
#include <stdio.h>
#define mbedtls_printf printf
+#define mbedtls_calloc calloc
+#define mbedtls_free free
#endif
/*
@@ -1005,6 +1007,11 @@
size_t nb_pad, olen, oid_size = 0;
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 )
return( MBEDTLS_ERR_RSA_BAD_INPUT_DATA );
@@ -1067,9 +1074,39 @@
memcpy( p, hash, hashlen );
}
- return( ( mode == MBEDTLS_RSA_PUBLIC )
- ? mbedtls_rsa_public( ctx, sig, sig )
- : mbedtls_rsa_private( ctx, f_rng, p_rng, sig, sig ) );
+ if( mode == MBEDTLS_RSA_PUBLIC )
+ return( mbedtls_rsa_public( ctx, sig, sig ) );
+
+ /*
+ * In order to prevent Lenstra's attack, make the signature in a
+ * temporary buffer and check it before returning it.
+ */
+ sig_try = mbedtls_calloc( 1, ctx->len );
+ verif = mbedtls_calloc( 1, ctx->len );
+ if( sig_try == NULL || verif == NULL )
+ return( MBEDTLS_ERR_MPI_ALLOC_FAILED );
+
+ 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 )
+ {
+ ret = MBEDTLS_ERR_RSA_PRIVATE_FAILED;
+ goto cleanup;
+ }
+
+ memcpy( sig, sig_try, ctx->len );
+
+cleanup:
+ mbedtls_free( sig_try );
+ mbedtls_free( verif );
+
+ return( ret );
}
#endif /* MBEDTLS_PKCS1_V15 */
