Fix Bleichenbacher PKCS #1 1.5 countermeasure.

(The attack against SSL 3.1 and TLS 1.0 is impractical anyway,
otherwise this would be a security relevant patch.)

3 files changed, 47 insertions, 3 deletions

diff --git a/CHANGES b/CHANGES
index 03f9d02abe..6cea57be1d 100644
--- a/CHANGES
+++ b/CHANGES
@@ -11,6 +11,27 @@
          *) applies to 0.9.6a (/0.9.6b) and 0.9.7
          +) applies to 0.9.7 only
 
+  *) The countermeasure against Bleichbacher's attack on PKCS #1 v1.5
+     RSA encryption was accidentily removed in s3_srvr.c in OpenSSL 0.9.5
+     when fixing the server behaviour for backwards-compatible 'client
+     hello' messages.  (Note that the attack is impractical against
+     SSL 3.0 and TLS 1.0 anyway because length and version checking
+     means that the probability of guessing a valid ciphertext is
+     around 2^-40; see section 5 in Bleichenbacher's CRYPTO '98
+     paper.)
+
+     Before 0.9.5, the countermeasure (hide the error by generating a
+     random 'decryption result') did not work properly because
+     ERR_clear_error() was missing, meaning that SSL_get_error() would
+     detect the supposedly ignored error.
+
+     Both problems are now fixed.
+     [Bodo Moeller]
+
+  *) In crypto/bio/bf_buff.c, increase DEFAULT_BUFFER_SIZE to 4096
+     (previously it was 1024).
+     [Bodo Moeller]
+
   +) Fix a memory leak in 'sk_dup()' in the case reallocation fails. (Also
      tidy up some unecessarily weird code in 'sk_new()').
      [Geoff, reported by Diego Tartara <dtartara@novamens.com>]
diff --git a/ssl/s2_srvr.c b/ssl/s2_srvr.c
index 10de5ec3db..cc9f591427 100644
--- a/ssl/s2_srvr.c
+++ b/ssl/s2_srvr.c
@@ -405,12 +405,13 @@ static int get_client_master_key(SSL *s)
 	/* bad decrypt */
 #if 1
 	/* If a bad decrypt, continue with protocol but with a
-	 * dud master secret */
+	 * random master secret (Bleichenbacher attack) */
 	if ((i < 0) ||
 		((!is_export && (i != EVP_CIPHER_key_length(c)))
 		|| (is_export && ((i != ek) || (s->s2->tmp.clear+i !=
 			EVP_CIPHER_key_length(c))))))
 		{
+		ERR_clear_error();
 		if (is_export)
 			i=ek;
 		else
diff --git a/ssl/s3_srvr.c b/ssl/s3_srvr.c
index dce89bf2bd..0b0eceedb0 100644
--- a/ssl/s3_srvr.c
+++ b/ssl/s3_srvr.c
@@ -1333,14 +1333,15 @@ static int ssl3_get_client_key_exchange(SSL *s)
 
 		i=RSA_private_decrypt((int)n,p,p,rsa,RSA_PKCS1_PADDING);
 
+		al = -1;
+		
 		if (i != SSL_MAX_MASTER_KEY_LENGTH)
 			{
 			al=SSL_AD_DECODE_ERROR;
 			SSLerr(SSL_F_SSL3_GET_CLIENT_KEY_EXCHANGE,SSL_R_BAD_RSA_DECRYPT);
-			goto f_err;
 			}
 
-		if (!((p[0] == (s->client_version>>8)) && (p[1] == (s->client_version & 0xff))))
+		if ((al == -1) && !((p[0] == (s->client_version>>8)) && (p[1] == (s->client_version & 0xff))))
 			{
 			/* The premaster secret must contain the same version number as the
 			 * ClientHello to detect version rollback attacks (strangely, the
@@ -1358,6 +1359,27 @@ static int ssl3_get_client_key_exchange(SSL *s)
 				}
 			}
 
+		if (al != -1)
+			{
+#if 0
+			goto f_err;
+#else
+			/* Some decryption failure -- use random value instead as countermeasure
+			 * against Bleichenbacher's attack on PKCS #1 v1.5 RSA padding
+			 * (see RFC 2246, section 7.4.7.1).
+			 * But note that due to length and protocol version checking, the
+			 * attack is impractical anyway (see section 5 in D. Bleichenbacher:
+			 * "Chosen Ciphertext Attacks Against Protocols Based on the RSA
+			 * Encryption Standard PKCS #1", CRYPTO '98, LNCS 1462, pp. 1-12).
+			 */
+			ERR_clear_error();
+			i = SSL_MAX_MASTER_KEY_LENGTH;
+			p[0] = s->client_version >> 8;
+			p[1] = s->client_version & 0xff;
+			RAND_pseudo_bytes(p+2, i-2); /* should be RAND_bytes, but we cannot work around a failure */
+#endif
+			}
+	
 		s->session->master_key_length=
 			s->method->ssl3_enc->generate_master_secret(s,
 				s->session->master_key,