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Fix Bleichenbacher PKCS #1 1.5 countermeasure.

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(The attack against SSL 3.1 and TLS 1.0 is impractical anyway,
otherwise this would be a security relevant patch.)
This commit is contained in:
Bodo Möller
2001-06-01 09:43:23 +00:00
parent 41d208db79
commit 83583e9479
3 changed files with 47 additions and 3 deletions
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21
CHANGES
View File

@@ -4,6 +4,27 @@
Changes between 0.9.6a and 0.9.6b [XX xxx XXXX]
*) 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 for compatibility mode trust settings: ignore trust settings
unless some valid trust or reject settings are present.
[Steve Henson]

3
ssl/s2_srvr.c
View File

@@ -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

26
ssl/s3_srvr.c
View File

@@ -1322,14 +1322,15 @@ 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
@@ -1347,6 +1348,27 @@ 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,