This change alters the processing of invalid, RSA pre-master secrets so
authorAdam Langley <agl@chromium.org>
Wed, 24 Apr 2013 18:45:44 +0000 (14:45 -0400)
committerEmilia Kasper <emilia@openssl.org>
Wed, 24 Sep 2014 13:46:26 +0000 (15:46 +0200)
that bad encryptions are treated like random session keys in constant
time.

(cherry picked from commit adb46dbc6dd7347750df2468c93e8c34bcb93a4b)

Reviewed-by: Rich Salz <rsalz@openssl.org>
ssl/s3_srvr.c

index c907f2b..127185a 100644 (file)
@@ -2167,6 +2167,10 @@ int ssl3_get_client_key_exchange(SSL *s)
 #ifndef OPENSSL_NO_RSA
        if (alg_k & SSL_kRSA)
                {
+               unsigned char rand_premaster_secret[SSL_MAX_MASTER_KEY_LENGTH];
+               int decrypt_len, decrypt_good_mask;
+               unsigned char version_good;
+
                /* FIX THIS UP EAY EAY EAY EAY */
                if (s->s3->tmp.use_rsa_tmp)
                        {
@@ -2214,54 +2218,94 @@ int ssl3_get_client_key_exchange(SSL *s)
                                n=i;
                        }
 
-               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); */
-                       }
-
-               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
-                        * protocol does not offer such protection for DH ciphersuites).
-                        * However, buggy clients exist that send the negotiated protocol
-                        * version instead if the server does not support the requested
-                        * protocol version.
-                        * If SSL_OP_TLS_ROLLBACK_BUG is set, tolerate such clients. */
-                       if (!((s->options & SSL_OP_TLS_ROLLBACK_BUG) &&
-                               (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); */
+               /* We must not leak whether a decryption failure occurs because
+                * of Bleichenbacher's attack on PKCS #1 v1.5 RSA padding (see
+                * RFC 2246, section 7.4.7.1). The code follows that advice of
+                * the TLS RFC and generates a random premaster secret for the
+                * case that the decrypt fails. See
+                * https://tools.ietf.org/html/rfc5246#section-7.4.7.1 */
 
-                               /* 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. */
-                               }
+               /* should be RAND_bytes, but we cannot work around a failure. */
+               if (RAND_pseudo_bytes(rand_premaster_secret,
+                                     sizeof(rand_premaster_secret)) <= 0)
+                       goto err;
+               decrypt_len = RSA_private_decrypt((int)n,p,p,rsa,RSA_PKCS1_PADDING);
+               ERR_clear_error();
+
+               /* decrypt_len should be SSL_MAX_MASTER_KEY_LENGTH.
+                * decrypt_good_mask will be zero if so and non-zero otherwise. */
+               decrypt_good_mask = decrypt_len ^ SSL_MAX_MASTER_KEY_LENGTH;
+
+               /* If the version in the decrypted pre-master secret is correct
+                * then version_good will be zero. 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". Thus version checks
+                * are done in constant time and are treated like any other
+                * decryption error. */
+               version_good = p[0] ^ (s->client_version>>8);
+               version_good |= 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 protocol does not offer such protection for
+                * DH ciphersuites). However, buggy clients exist that send the
+                * negotiated protocol version instead if the server does not
+                * support the requested protocol version. If
+                * SSL_OP_TLS_ROLLBACK_BUG is set, tolerate such clients. */
+               if (s->options & SSL_OP_TLS_ROLLBACK_BUG)
+                       {
+                       unsigned char workaround_mask = version_good;
+                       unsigned char workaround;
+
+                       /* workaround_mask will be 0xff if version_good is
+                        * non-zero (i.e. the version match failed). Otherwise
+                        * it'll be 0x00. */
+                       workaround_mask |= workaround_mask >> 4;
+                       workaround_mask |= workaround_mask >> 2;
+                       workaround_mask |= workaround_mask >> 1;
+                       workaround_mask = ~((workaround_mask & 1) - 1);
+
+                       workaround = p[0] ^ (s->version>>8);
+                       workaround |= p[1] ^ (s->version&0xff);
+
+                       /* If workaround_mask is 0xff (i.e. there was a version
+                        * mismatch) then we copy the value of workaround over
+                        * version_good. */
+                       version_good = (workaround & workaround_mask) |
+                                      (version_good & ~workaround_mask);
+                       }
+
+               /* If any bits in version_good are set then they'll poision
+                * decrypt_good_mask and cause rand_premaster_secret to be
+                * used. */
+               decrypt_good_mask |= version_good;
+
+               /* decrypt_good_mask will be zero iff decrypt_len ==
+                * SSL_MAX_MASTER_KEY_LENGTH and the version check passed. We
+                * fold the bottom 32 bits of it with an OR so that the LSB
+                * will be zero iff everything is good. This assumes that we'll
+                * never decrypt a value > 2**31 bytes, which seems safe. */
+               decrypt_good_mask |= decrypt_good_mask >> 16;
+               decrypt_good_mask |= decrypt_good_mask >> 8;
+               decrypt_good_mask |= decrypt_good_mask >> 4;
+               decrypt_good_mask |= decrypt_good_mask >> 2;
+               decrypt_good_mask |= decrypt_good_mask >> 1;
+               /* Now select only the LSB and subtract one. If decrypt_len ==
+                * SSL_MAX_MASTER_KEY_LENGTH and the version check passed then
+                * decrypt_good_mask will be all ones. Otherwise it'll be all
+                * zeros. */
+               decrypt_good_mask &= 1;
+               decrypt_good_mask--;
+
+               /* Now copy rand_premaster_secret over p using
+                * decrypt_good_mask. */
+               for (i = 0; i < (int) sizeof(rand_premaster_secret); i++)
+                       {
+                       p[i] = (p[i] & decrypt_good_mask) |
+                              (rand_premaster_secret[i] & ~decrypt_good_mask);
                        }
 
-               if (al != -1)
-                       {
-                       /* 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). */
-                       ERR_clear_error();
-                       i = SSL_MAX_MASTER_KEY_LENGTH;
-                       p[0] = s->client_version >> 8;
-                       p[1] = s->client_version & 0xff;
-                       if (RAND_pseudo_bytes(p+2, i-2) <= 0) /* should be RAND_bytes, but we cannot work around a failure */
-                               goto err;
-                       }
-       
                s->session->master_key_length=
                        s->method->ssl3_enc->generate_master_secret(s,
                                s->session->master_key,