OpenSSL Security Advisory [19 March 2003]

Klima-Pokorny-Rosa attack on RSA in SSL/TLS

Czech cryptologists Vlastimil Klima, Ondrej Pokorny, and Tomas Rosa
have come up with an extension of the "Bleichenbacher attack" on RSA
with PKCS #1 v1.5 padding as used in SSL 3.0 and TLS 1.0. Their
attack requires the attacker to open millions of SSL/TLS connections
to the server under attack; the server's behaviour when faced with
specially made-up RSA ciphertexts can reveal information that in
effect allows the attacker to perform a single RSA private key
operation on a ciphertext of its choice using the server's RSA key.
Note that the server's RSA key is not compromised in this attack.

This problem affects all applications using the OpenSSL SSL/TLS library.
OpenSSL releases up to 0.9.6i and 0.9.7a are vulnerable. The enclosed
patch modifies SSL/TLS server behaviour to avoid the vulnerability.

Security Patch

The following patch can be applied to OpenSSL releases 0.9.6b up to 0.9.6i,
0.9.7, and 0.9.7a.

— s3_srvr.c 29 Nov 2002 11:31:51 -0000 1.85.2.14
+++ s3_srvr.c 19 Mar 2003 18:00:00 -0000
@@ -1447,7 +1447,7 @@
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))))
@@ -1463,30 +1463,29 @@
(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. */
  

•                           p[0] = (char)(int) "CAN-2003-0131 patch 2003-03-19";
                            }
                    }
  
            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=

References

Report "Attacking RSA-based Sessions in SSL/TLS" by V. Klima, O. Pokorny,
and T. Rosa:
http://eprint.iacr.org/2003/052/

The Common Vulnerabilities and Exposures project (cve.mitre.org) has
assigned the name CAN-2003-0131 to this issue.
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2003-0131

URL for this Security Advisory:
http://www.openssl.org/news/secadv_20030319.txt