Only allow ephemeral RSA keys in export ciphersuites.

OpenSSL clients would tolerate temporary RSA keys in non-export ciphersuites. It also had an option SSL_OP_EPHEMERAL_RSA which enabled this server side. Remove both options as they are a protocol violation. Thanks to Karthikeyan Bhargavan for reporting this issue. (CVE-2015-0204) Reviewed-by: Matt Caswell <[email protected]>
openssl · Jan 6, 2015 · ce325c6 · ce325c6
1 parent b15f876
commit ce325c6
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Showing 7 changed files with 38 additions and 57 deletions.
diff --git a/CHANGES b/CHANGES
@@ -666,6 +666,14 @@
      (CVE-2014-3572)
      [Steve Henson]
 
+  *) Remove non-export ephemeral RSA code on client and server. This code
+     violated the TLS standard by allowing the use of temporary RSA keys in
+     non-export ciphersuites and could be used by a server to effectively
+     downgrade the RSA key length used to a value smaller than the server
+     certificate. Thanks for Karthikeyan Bhargavan for reporting this issue.
+     (CVE-2015-0204)
+     [Steve Henson]
+
   *) Ensure that the session ID context of an SSL is updated when its
      SSL_CTX is updated via SSL_set_SSL_CTX.
 

diff --git a/doc/ssl/SSL_CTX_set_options.pod b/doc/ssl/SSL_CTX_set_options.pod
@@ -151,15 +151,7 @@ temporary/ephemeral DH parameters are used.
 
 =item SSL_OP_EPHEMERAL_RSA
 
-Always use ephemeral (temporary) RSA key when doing RSA operations
-(see L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>).
-According to the specifications this is only done, when a RSA key
-can only be used for signature operations (namely under export ciphers
-with restricted RSA keylength). By setting this option, ephemeral
-RSA keys are always used. This option breaks compatibility with the
-SSL/TLS specifications and may lead to interoperability problems with
-clients and should therefore never be used. Ciphers with DHE (ephemeral
-Diffie-Hellman) key exchange should be used instead.
+This option is no longer implemented and is treated as no op.
 
 =item SSL_OP_CIPHER_SERVER_PREFERENCE
 

diff --git a/doc/ssl/SSL_CTX_set_tmp_rsa_callback.pod b/doc/ssl/SSL_CTX_set_tmp_rsa_callback.pod
@@ -74,21 +74,14 @@ exchange and use DHE (Ephemeral Diffie-Hellman) key exchange instead
 in order to achieve forward secrecy (see
 L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>).
 
-On OpenSSL servers ephemeral RSA key exchange is therefore disabled by default
-and must be explicitly enabled  using the SSL_OP_EPHEMERAL_RSA option of
-L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)>, violating the TLS/SSL
-standard. When ephemeral RSA key exchange is required for export ciphers,
-it will automatically be used without this option!
-
-An application may either directly specify the key or can supply the key via
-a callback function. The callback approach has the advantage, that the
-callback may generate the key only in case it is actually needed. As the
-generation of a RSA key is however costly, it will lead to a significant
-delay in the handshake procedure.  Another advantage of the callback function
-is that it can supply keys of different size (e.g. for SSL_OP_EPHEMERAL_RSA
-usage) while the explicit setting of the key is only useful for key size of
-512 bits to satisfy the export restricted ciphers and does give away key length
-if a longer key would be allowed.
+An application may either directly specify the key or can supply the key via a
+callback function. The callback approach has the advantage, that the callback
+may generate the key only in case it is actually needed. As the generation of a
+RSA key is however costly, it will lead to a significant delay in the handshake
+procedure.  Another advantage of the callback function is that it can supply
+keys of different size while the explicit setting of the key is only useful for
+key size of 512 bits to satisfy the export restricted ciphers and does give
+away key length if a longer key would be allowed.
 
 The B<tmp_rsa_callback> is called with the B<keylength> needed and
 the B<is_export> information. The B<is_export> flag is set, when the

diff --git a/ssl/d1_srvr.c b/ssl/d1_srvr.c
@@ -472,24 +472,15 @@ int dtls1_accept(SSL *s)
 		case SSL3_ST_SW_KEY_EXCH_B:
 			alg_k = s->s3->tmp.new_cipher->algorithm_mkey;
 
-			/* clear this, it may get reset by
-			 * send_server_key_exchange */
-			if ((s->options & SSL_OP_EPHEMERAL_RSA)
-#ifndef OPENSSL_NO_KRB5
-				&& !(alg_k & SSL_kKRB5)
-#endif /* OPENSSL_NO_KRB5 */
-				)
-				/* option SSL_OP_EPHEMERAL_RSA sends temporary RSA key
-				 * even when forbidden by protocol specs
-				 * (handshake may fail as clients are not required to
-				 * be able to handle this) */
-				s->s3->tmp.use_rsa_tmp=1;
-			else
-				s->s3->tmp.use_rsa_tmp=0;
+			/*
+			 * clear this, it may get reset by
+			 * send_server_key_exchange
+			 */
+			s->s3->tmp.use_rsa_tmp=0;
 
 			/* only send if a DH key exchange or
 			 * RSA but we have a sign only certificate */
-			if (s->s3->tmp.use_rsa_tmp
+			if (
 			/* PSK: send ServerKeyExchange if PSK identity
 			 * hint if provided */
 #ifndef OPENSSL_NO_PSK

diff --git a/ssl/s3_clnt.c b/ssl/s3_clnt.c
@@ -1636,6 +1636,13 @@ int ssl3_get_key_exchange(SSL *s)
 #ifndef OPENSSL_NO_RSA
 	if (alg_k & SSL_kRSA)
 		{
+		/* Temporary RSA keys only allowed in export ciphersuites */
+		if (!SSL_C_IS_EXPORT(s->s3->tmp.new_cipher))
+			{
+			al=SSL_AD_UNEXPECTED_MESSAGE;
+			SSLerr(SSL_F_SSL3_GET_SERVER_CERTIFICATE,SSL_R_UNEXPECTED_MESSAGE);
+			goto f_err;
+			}
 		if ((rsa=RSA_new()) == NULL)
 			{
 			SSLerr(SSL_F_SSL3_GET_KEY_EXCHANGE,ERR_R_MALLOC_FAILURE);

diff --git a/ssl/s3_srvr.c b/ssl/s3_srvr.c
@@ -453,20 +453,11 @@ int ssl3_accept(SSL *s)
 		case SSL3_ST_SW_KEY_EXCH_B:
 			alg_k = s->s3->tmp.new_cipher->algorithm_mkey;
 
-			/* clear this, it may get reset by
-			 * send_server_key_exchange */
-			if ((s->options & SSL_OP_EPHEMERAL_RSA)
-#ifndef OPENSSL_NO_KRB5
-				&& !(alg_k & SSL_kKRB5)
-#endif /* OPENSSL_NO_KRB5 */
-				)
-				/* option SSL_OP_EPHEMERAL_RSA sends temporary RSA key
-				 * even when forbidden by protocol specs
-				 * (handshake may fail as clients are not required to
-				 * be able to handle this) */
-				s->s3->tmp.use_rsa_tmp=1;
-			else
-				s->s3->tmp.use_rsa_tmp=0;
+			/*
+			 * clear this, it may get reset by
+			 * send_server_key_exchange
+			 */
+			s->s3->tmp.use_rsa_tmp=0;
 
 
 			/* only send if a DH key exchange, fortezza or
@@ -480,7 +471,7 @@ int ssl3_accept(SSL *s)
 			 * server certificate contains the server's
 			 * public key for key exchange.
 			 */
-			if (s->s3->tmp.use_rsa_tmp
+			if (
 			/* PSK: send ServerKeyExchange if PSK identity
 			 * hint if provided */
 #ifndef OPENSSL_NO_PSK

diff --git a/ssl/ssl.h b/ssl/ssl.h
@@ -606,9 +606,8 @@ struct ssl_session_st
 #define SSL_OP_SINGLE_ECDH_USE				0x00080000L
 /* If set, always create a new key when using tmp_dh parameters */
 #define SSL_OP_SINGLE_DH_USE				0x00100000L
-/* Set to always use the tmp_rsa key when doing RSA operations,
- * even when this violates protocol specs */
-#define SSL_OP_EPHEMERAL_RSA				0x00200000L
+/* Does nothing: retained for compatibiity */
+#define SSL_OP_EPHEMERAL_RSA				0x0
 /* Set on servers to choose the cipher according to the server's
  * preferences */
 #define SSL_OP_CIPHER_SERVER_PREFERENCE			0x00400000L