From ce60fbeb301b2aa2fdf50349d49a3f19cf324716 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Manuel=20P=C3=A9gouri=C3=A9-Gonnard?= <mpg@elzevir.fr>
Date: Wed, 15 Apr 2015 16:45:52 +0200
Subject: [PATCH] Fix potential timing difference with RSA PMS
---
library/ssl_srv.c | 16 +++++++++-------
1 file changed, 9 insertions(+), 7 deletions(-)
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 2ac0902c2..dad6872ea 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -2887,7 +2887,7 @@ static int ssl_parse_encrypted_pms( ssl_context *ssl,
unsigned char *pms = ssl->handshake->premaster + pms_offset;
unsigned char fake_pms[48], peer_pms[48];
unsigned char mask;
- size_t i;
+ size_t i, diff, peer_pmslen;
if( ! pk_can_do( ssl_own_key( ssl ), POLARSSL_PK_RSA ) )
{
@@ -2929,16 +2929,17 @@ static int ssl_parse_encrypted_pms( ssl_context *ssl,
return( ret );
ret = pk_decrypt( ssl_own_key( ssl ), p, len,
- peer_pms, &ssl->handshake->pmslen,
+ peer_pms, &peer_pmslen,
sizeof( peer_pms ),
ssl->f_rng, ssl->p_rng );
- ret |= ssl->handshake->pmslen - 48;
- ret |= peer_pms[0] - ssl->handshake->max_major_ver;
- ret |= peer_pms[1] - ssl->handshake->max_minor_ver;
+ diff = (size_t) ret;
+ diff |= peer_pmslen ^ 48;
+ diff |= peer_pms[0] ^ ssl->handshake->max_major_ver;
+ diff |= peer_pms[1] ^ ssl->handshake->max_minor_ver;
#if defined(POLARSSL_SSL_DEBUG_ALL)
- if( ret != 0 )
+ if( diff != 0 )
SSL_DEBUG_MSG( 1, ( "bad client key exchange message" ) );
#endif
@@ -2950,7 +2951,8 @@ static int ssl_parse_encrypted_pms( ssl_context *ssl,
}
ssl->handshake->pmslen = 48;
- mask = (unsigned char)( - ( ret != 0 ) ); /* ret ? 0xff : 0x00 */
+ mask = ( diff | - diff ) >> ( sizeof( size_t ) * 8 - 1 );
+ mask = (unsigned char)( - ( ret != 0 ) ); /* mask = diff ? 0xff : 0x00 */
for( i = 0; i < ssl->handshake->pmslen; i++ )
pms[i] = ( mask & fake_pms[i] ) | ( (~mask) & peer_pms[i] );