From ffaef3f1526ed87a46f82fa4924d5b08f2a2e631 Mon Sep 17 00:00:00 2001 From: "Dr. Stephen Henson" Date: Thu, 17 Dec 2015 02:57:20 +0000 Subject: Always generate DH keys for ephemeral DH cipher suites. Reviewed-by: Matt Caswell --- doc/ssl/SSL_CTX_set_tmp_dh_callback.pod | 29 +++++------------------------ 1 file changed, 5 insertions(+), 24 deletions(-) (limited to 'doc') diff --git a/doc/ssl/SSL_CTX_set_tmp_dh_callback.pod b/doc/ssl/SSL_CTX_set_tmp_dh_callback.pod index 3e08b88203..57bf211075 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. @@ -92,10 +75,9 @@ 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 @@ -127,7 +109,6 @@ 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 -- cgit v1.2.3