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 adb46dbc6d)
Reviewed-by: Rich Salz <rsalz@openssl.org>
This commit is contained in:
Adam Langley
Emilia Kasper
parent
9bed73adaa
commit
941af48fec
1 changed files with 85 additions and 41 deletions
132
ssl/s3_srvr.c
132
ssl/s3_srvr.c
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@ -2184,6 +2184,10 @@ int ssl3_get_client_key_exchange(SSL *s)
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#ifndef OPENSSL_NO_RSA
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#ifndef OPENSSL_NO_RSA
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if (alg_k & SSL_kRSA)
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if (alg_k & SSL_kRSA)
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{
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{
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unsigned char rand_premaster_secret[SSL_MAX_MASTER_KEY_LENGTH];
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int decrypt_len, decrypt_good_mask;
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unsigned char version_good;
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/* FIX THIS UP EAY EAY EAY EAY */
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/* FIX THIS UP EAY EAY EAY EAY */
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if (s->s3->tmp.use_rsa_tmp)
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if (s->s3->tmp.use_rsa_tmp)
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{
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{
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@ -2231,52 +2235,92 @@ int ssl3_get_client_key_exchange(SSL *s)
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n=i;
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n=i;
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}
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}
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i=RSA_private_decrypt((int)n,p,p,rsa,RSA_PKCS1_PADDING);
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/* We must not leak whether a decryption failure occurs because
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* of Bleichenbacher's attack on PKCS #1 v1.5 RSA padding (see
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* RFC 2246, section 7.4.7.1). The code follows that advice of
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* the TLS RFC and generates a random premaster secret for the
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* case that the decrypt fails. See
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* https://tools.ietf.org/html/rfc5246#section-7.4.7.1 */
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al = -1;
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/* should be RAND_bytes, but we cannot work around a failure. */
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if (RAND_pseudo_bytes(rand_premaster_secret,
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if (i != SSL_MAX_MASTER_KEY_LENGTH)
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sizeof(rand_premaster_secret)) <= 0)
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{
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al=SSL_AD_DECODE_ERROR;
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/* SSLerr(SSL_F_SSL3_GET_CLIENT_KEY_EXCHANGE,SSL_R_BAD_RSA_DECRYPT); */
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}
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if ((al == -1) && !((p[0] == (s->client_version>>8)) && (p[1] == (s->client_version & 0xff))))
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{
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/* The premaster secret must contain the same version number as the
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* ClientHello to detect version rollback attacks (strangely, the
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* protocol does not offer such protection for DH ciphersuites).
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* However, buggy clients exist that send the negotiated protocol
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* version instead if the server does not support the requested
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* protocol version.
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* If SSL_OP_TLS_ROLLBACK_BUG is set, tolerate such clients. */
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if (!((s->options & SSL_OP_TLS_ROLLBACK_BUG) &&
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(p[0] == (s->version>>8)) && (p[1] == (s->version & 0xff))))
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{
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al=SSL_AD_DECODE_ERROR;
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/* SSLerr(SSL_F_SSL3_GET_CLIENT_KEY_EXCHANGE,SSL_R_BAD_PROTOCOL_VERSION_NUMBER); */
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/* The Klima-Pokorny-Rosa extension of Bleichenbacher's attack
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* (http://eprint.iacr.org/2003/052/) exploits the version
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* number check as a "bad version oracle" -- an alert would
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* reveal that the plaintext corresponding to some ciphertext
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* made up by the adversary is properly formatted except
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* that the version number is wrong. To avoid such attacks,
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* we should treat this just like any other decryption error. */
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}
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}
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if (al != -1)
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{
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/* Some decryption failure -- use random value instead as countermeasure
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* against Bleichenbacher's attack on PKCS #1 v1.5 RSA padding
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* (see RFC 2246, section 7.4.7.1). */
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ERR_clear_error();
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i = SSL_MAX_MASTER_KEY_LENGTH;
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p[0] = s->client_version >> 8;
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p[1] = s->client_version & 0xff;
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if (RAND_pseudo_bytes(p+2, i-2) <= 0) /* should be RAND_bytes, but we cannot work around a failure */
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goto err;
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goto err;
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decrypt_len = RSA_private_decrypt((int)n,p,p,rsa,RSA_PKCS1_PADDING);
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ERR_clear_error();
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/* decrypt_len should be SSL_MAX_MASTER_KEY_LENGTH.
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* decrypt_good_mask will be zero if so and non-zero otherwise. */
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decrypt_good_mask = decrypt_len ^ SSL_MAX_MASTER_KEY_LENGTH;
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/* If the version in the decrypted pre-master secret is correct
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* then version_good will be zero. The Klima-Pokorny-Rosa
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* extension of Bleichenbacher's attack
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* (http://eprint.iacr.org/2003/052/) exploits the version
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* number check as a "bad version oracle". Thus version checks
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* are done in constant time and are treated like any other
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* decryption error. */
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version_good = p[0] ^ (s->client_version>>8);
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version_good |= p[1] ^ (s->client_version&0xff);
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/* The premaster secret must contain the same version number as
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* the ClientHello to detect version rollback attacks
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* (strangely, the protocol does not offer such protection for
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* DH ciphersuites). However, buggy clients exist that send the
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* negotiated protocol version instead if the server does not
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* support the requested protocol version. If
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* SSL_OP_TLS_ROLLBACK_BUG is set, tolerate such clients. */
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if (s->options & SSL_OP_TLS_ROLLBACK_BUG)
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{
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unsigned char workaround_mask = version_good;
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unsigned char workaround;
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/* workaround_mask will be 0xff if version_good is
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* non-zero (i.e. the version match failed). Otherwise
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* it'll be 0x00. */
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workaround_mask |= workaround_mask >> 4;
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workaround_mask |= workaround_mask >> 2;
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workaround_mask |= workaround_mask >> 1;
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workaround_mask = ~((workaround_mask & 1) - 1);
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workaround = p[0] ^ (s->version>>8);
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workaround |= p[1] ^ (s->version&0xff);
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/* If workaround_mask is 0xff (i.e. there was a version
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* mismatch) then we copy the value of workaround over
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* version_good. */
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version_good = (workaround & workaround_mask) |
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(version_good & ~workaround_mask);
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}
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/* If any bits in version_good are set then they'll poision
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* decrypt_good_mask and cause rand_premaster_secret to be
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* used. */
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decrypt_good_mask |= version_good;
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/* decrypt_good_mask will be zero iff decrypt_len ==
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* SSL_MAX_MASTER_KEY_LENGTH and the version check passed. We
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* fold the bottom 32 bits of it with an OR so that the LSB
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* will be zero iff everything is good. This assumes that we'll
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* never decrypt a value > 2**31 bytes, which seems safe. */
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decrypt_good_mask |= decrypt_good_mask >> 16;
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decrypt_good_mask |= decrypt_good_mask >> 8;
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decrypt_good_mask |= decrypt_good_mask >> 4;
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decrypt_good_mask |= decrypt_good_mask >> 2;
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decrypt_good_mask |= decrypt_good_mask >> 1;
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/* Now select only the LSB and subtract one. If decrypt_len ==
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* SSL_MAX_MASTER_KEY_LENGTH and the version check passed then
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* decrypt_good_mask will be all ones. Otherwise it'll be all
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* zeros. */
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decrypt_good_mask &= 1;
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decrypt_good_mask--;
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/* Now copy rand_premaster_secret over p using
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* decrypt_good_mask. */
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for (i = 0; i < (int) sizeof(rand_premaster_secret); i++)
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{
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p[i] = (p[i] & decrypt_good_mask) |
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(rand_premaster_secret[i] & ~decrypt_good_mask);
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}
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}
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s->session->master_key_length=
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s->session->master_key_length=
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