From c5b831f21d0d29d1e517d139d9d101763f60c9a2 Mon Sep 17 00:00:00 2001 From: Matt Caswell Date: Thu, 17 Dec 2015 02:57:20 +0000 Subject: [PATCH] Always generate DH keys for ephemeral DH cipher suites Modified version of the commit ffaef3f15 in the master branch by Stephen Henson. This makes the SSL_OP_SINGLE_DH_USE option a no-op and always generates a new DH key for every handshake regardless. CVE-2016-0701 (fix part 2 or 2) Issue reported by Antonio Sanso Reviewed-by: Viktor Dukhovni --- doc/ssl/SSL_CTX_set_tmp_dh_callback.pod | 29 +++++-------------------- ssl/s3_lib.c | 14 ------------ ssl/s3_srvr.c | 17 +++------------ ssl/ssl.h | 2 +- 4 files changed, 9 insertions(+), 53 deletions(-) diff --git a/doc/ssl/SSL_CTX_set_tmp_dh_callback.pod b/doc/ssl/SSL_CTX_set_tmp_dh_callback.pod index b754c16a86..234fbc8450 100644 --- a/doc/ssl/SSL_CTX_set_tmp_dh_callback.pod +++ b/doc/ssl/SSL_CTX_set_tmp_dh_callback.pod @@ -48,25 +48,8 @@ even if he gets hold of the normal (certified) key, as this key was only used for signing. In order to perform a DH key exchange the server must use a DH group -(DH parameters) and generate a DH key. -The server will always generate a new DH key during the negotiation -if either the DH parameters are supplied via callback or the -SSL_OP_SINGLE_DH_USE option of SSL_CTX_set_options(3) is set (or both). -It will immediately create a DH key if DH parameters are supplied via -SSL_CTX_set_tmp_dh() and SSL_OP_SINGLE_DH_USE is not set. -In this case, -it may happen that a key is generated on initialization without later -being needed, while on the other hand the computer time during the -negotiation is being saved. - -If "strong" primes were used to generate the DH parameters, it is not strictly -necessary to generate a new key for each handshake but it does improve forward -secrecy. If it is not assured that "strong" primes were used, -SSL_OP_SINGLE_DH_USE must be used in order to prevent small subgroup -attacks. Always using SSL_OP_SINGLE_DH_USE has an impact on the -computer time needed during negotiation, but it is not very large, so -application authors/users should consider always enabling this option. -The option is required to implement perfect forward secrecy (PFS). +(DH parameters) and generate a DH key. The server will always generate +a new DH key during the negotiation. As generating DH parameters is extremely time consuming, an application should not generate the parameters on the fly but supply the parameters. @@ -93,10 +76,9 @@ can supply the DH parameters via a callback function. Previous versions of the callback used B and B parameters to control parameter generation for export and non-export cipher suites. Modern servers that do not support export ciphersuites -are advised to either use SSL_CTX_set_tmp_dh() in combination with -SSL_OP_SINGLE_DH_USE, or alternatively, use the callback but ignore -B and B and simply supply at least 2048-bit -parameters in the callback. +are advised to either use SSL_CTX_set_tmp_dh() or alternatively, use +the callback but ignore B and B and simply +supply at least 2048-bit parameters in the callback. =head1 EXAMPLES @@ -128,7 +110,6 @@ partly left out.) if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1) { /* Error. */ } - SSL_CTX_set_options(ctx, SSL_OP_SINGLE_DH_USE); ... =head1 RETURN VALUES diff --git a/ssl/s3_lib.c b/ssl/s3_lib.c index b7c5db3aa5..f846cb5b7b 100644 --- a/ssl/s3_lib.c +++ b/ssl/s3_lib.c @@ -3206,13 +3206,6 @@ long ssl3_ctrl(SSL *s, int cmd, long larg, void *parg) SSLerr(SSL_F_SSL3_CTRL, ERR_R_DH_LIB); return (ret); } - if (!(s->options & SSL_OP_SINGLE_DH_USE)) { - if (!DH_generate_key(dh)) { - DH_free(dh); - SSLerr(SSL_F_SSL3_CTRL, ERR_R_DH_LIB); - return (ret); - } - } if (s->cert->dh_tmp != NULL) DH_free(s->cert->dh_tmp); s->cert->dh_tmp = dh; @@ -3713,13 +3706,6 @@ long ssl3_ctx_ctrl(SSL_CTX *ctx, int cmd, long larg, void *parg) SSLerr(SSL_F_SSL3_CTX_CTRL, ERR_R_DH_LIB); return 0; } - if (!(ctx->options & SSL_OP_SINGLE_DH_USE)) { - if (!DH_generate_key(new)) { - SSLerr(SSL_F_SSL3_CTX_CTRL, ERR_R_DH_LIB); - DH_free(new); - return 0; - } - } if (cert->dh_tmp != NULL) DH_free(cert->dh_tmp); cert->dh_tmp = new; diff --git a/ssl/s3_srvr.c b/ssl/s3_srvr.c index 9b05f189d5..ab28702ee9 100644 --- a/ssl/s3_srvr.c +++ b/ssl/s3_srvr.c @@ -1687,20 +1687,9 @@ int ssl3_send_server_key_exchange(SSL *s) } s->s3->tmp.dh = dh; - if ((dhp->pub_key == NULL || - dhp->priv_key == NULL || - (s->options & SSL_OP_SINGLE_DH_USE))) { - if (!DH_generate_key(dh)) { - SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE, ERR_R_DH_LIB); - goto err; - } - } else { - dh->pub_key = BN_dup(dhp->pub_key); - dh->priv_key = BN_dup(dhp->priv_key); - if ((dh->pub_key == NULL) || (dh->priv_key == NULL)) { - SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE, ERR_R_DH_LIB); - goto err; - } + if (!DH_generate_key(dh)) { + SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE, ERR_R_DH_LIB); + goto err; } r[0] = dh->p; r[1] = dh->g; diff --git a/ssl/ssl.h b/ssl/ssl.h index a31c085711..ae8c92575e 100644 --- a/ssl/ssl.h +++ b/ssl/ssl.h @@ -625,7 +625,7 @@ struct ssl_session_st { # define SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION 0x00040000L /* If set, always create a new key when using tmp_ecdh parameters */ # define SSL_OP_SINGLE_ECDH_USE 0x00080000L -/* If set, always create a new key when using tmp_dh parameters */ +/* Does nothing: retained for compatibility */ # define SSL_OP_SINGLE_DH_USE 0x00100000L /* Does nothing: retained for compatibiity */ # define SSL_OP_EPHEMERAL_RSA 0x0