countermeasure against new Klima-Pokorny-Rosa atack

diff --git a/CHANGES b/CHANGES
index b3f3ffb4d05a1aa9ed3245575e43d6c729f370b7..46916103b98a27f3315704b31d7ef50e0df4978a 100644 (file)
--- a/CHANGES
+++ b/CHANGES
@@ -4,7 +4,11 @@
 
  Changes between 0.9.6i and 0.9.6j  [xx XXX 2003]
 
-  *) 
+  *) Countermeasure against the Klima-Pokorny-Rosa extension of
+     Bleichbacher's attack on PKCS #1 v1.5 padding: treat
+     a protocol version number mismatch like a decryption error
+     in ssl3_get_client_key_exchange (ssl/s3_srvr.c).
+     [Bodo Moeller]
 
  Changes between 0.9.6h and 0.9.6i  [19 Feb 2003]
 

diff --git a/ssl/s3_srvr.c b/ssl/s3_srvr.c
index 6b414cfa5c8bf53f1d86dcadc5dcc685246d971a..3f4818e888a72b2a383b2e070b896b934373802e 100644 (file)
--- a/ssl/s3_srvr.c
+++ b/ssl/s3_srvr.c
@@ -1425,7 +1425,7 @@ static int ssl3_get_client_key_exchange(SSL *s)
                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);
+                       /* SSLerr(SSL_F_SSL3_GET_CLIENT_KEY_EXCHANGE,SSL_R_BAD_RSA_DECRYPT); */
                        }
 
                if ((al == -1) && !((p[0] == (s->client_version>>8)) && (p[1] == (s->client_version & 0xff))))
@@ -1441,30 +1441,28 @@ static int ssl3_get_client_key_exchange(SSL *s)
                                (p[0] == (s->version>>8)) && (p[1] == (s->version & 0xff))))
                                {
                                al=SSL_AD_DECODE_ERROR;
-                               SSLerr(SSL_F_SSL3_GET_CLIENT_KEY_EXCHANGE,SSL_R_BAD_PROTOCOL_VERSION_NUMBER);
-                               goto f_err;
+                               /* SSLerr(SSL_F_SSL3_GET_CLIENT_KEY_EXCHANGE,SSL_R_BAD_PROTOCOL_VERSION_NUMBER); */
+
+                               /* The Klima-Pokorny-Rosa extension of Bleichenbacher's attack
+                                * (http://eprint.iacr.org/2003/052/) exploits the version
+                                * number check as a "bad version oracle" -- an alert would
+                                * reveal that the plaintext corresponding to some ciphertext
+                                * made up by the adversary is properly formatted except
+                                * that the version number is wrong.  To avoid such attacks,
+                                * we should treat this just like any other decryption error. */
                                }
                        }
 
                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).
-                        */
+                        * (see RFC 2246, section 7.4.7.1). */
                        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=