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SUSE:SLE-12-SP2:GA
openssl.6981
openssl-RSA_premaster_secret_in_constant_time.p...
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File openssl-RSA_premaster_secret_in_constant_time.patch of Package openssl.6981
commit 133145686e054a28f78b94e8a996ef7ae13d30b7 Author: Adam Langley <agl@chromium.org> Date: Wed Apr 24 14:45:44 2013 -0400 This change alters the processing of invalid, RSA pre-master secrets so that bad encryptions are treated like random session keys in constant time. (cherry picked from commit adb46dbc6dd7347750df2468c93e8c34bcb93a4b) Reviewed-by: Rich Salz <rsalz@openssl.org> diff --git a/ssl/s3_srvr.c b/ssl/s3_srvr.c index c907f2b..127185a 100644 --- a/ssl/s3_srvr.c +++ b/ssl/s3_srvr.c @@ -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,
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