Update documentation with Diffie-Hellman best practices.

- Do not advise generation of DH parameters with dsaparam to save
computation time.
- Promote use of custom parameters more, and explicitly forbid use of
built-in parameters weaker than 2048 bits.
- Advise the callback to ignore <keylength> - it is currently called
with 1024 bits, but this value can and should be safely ignored by
servers.

Reviewed-by: Rich Salz <rsalz@openssl.org>

diff --git a/doc/ssl/SSL_CTX_set_tmp_dh_callback.pod b/doc/ssl/SSL_CTX_set_tmp_dh_callback.pod
index 7a27eef50b173dbeba26a732a501530ffcb621e9..b754c16a86e66eaeab66df876000ffd760dd7c70 100644 (file)
--- a/doc/ssl/SSL_CTX_set_tmp_dh_callback.pod
+++ b/doc/ssl/SSL_CTX_set_tmp_dh_callback.pod
@@ -61,12 +61,12 @@ 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 (see especially
-the section about DSA parameters below), 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 to always enable
-this option.
+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).
 
 As generating DH parameters is extremely time consuming, an application
 should not generate the parameters on the fly but supply the parameters.
@@ -74,82 +74,62 @@ DH parameters can be reused, as the actual key is newly generated during
 the negotiation. The risk in reusing DH parameters is that an attacker
 may specialize on a very often used DH group. Applications should therefore
 generate their own DH parameters during the installation process using the
-openssl L<dhparam(1)|dhparam(1)> application. In order to reduce the computer
-time needed for this generation, it is possible to use DSA parameters
-instead (see L<dhparam(1)|dhparam(1)>), but in this case SSL_OP_SINGLE_DH_USE
-is mandatory.
+openssl L<dhparam(1)|dhparam(1)> application. This application
+guarantees that "strong" primes are used.
 
-Application authors may compile in DH parameters. Files dh512.pem,
-dh1024.pem, dh2048.pem, and dh4096.pem in the 'apps' directory of current
+Files dh2048.pem, and dh4096.pem in the 'apps' directory of the current
 version of the OpenSSL distribution contain the 'SKIP' DH parameters,
 which use safe primes and were generated verifiably pseudo-randomly.
 These files can be converted into C code using the B<-C> option of the
-L<dhparam(1)|dhparam(1)> application.
-Authors may also generate their own set of parameters using
-L<dhparam(1)|dhparam(1)>, but a user may not be sure how the parameters were
-generated. The generation of DH parameters during installation is therefore
-recommended.
+L<dhparam(1)|dhparam(1)> application. Generation of custom DH
+parameters during installation should still be preferred to stop an
+attacker from specializing on a commonly used group. Files dh1024.pem
+and dh512.pem contain old parameters that must not be used by
+applications.
 
 An application may either directly specify the DH parameters or
-can supply the DH parameters via a callback function. The callback approach
-has the advantage, that the callback may supply DH parameters for different
-key lengths.
+can supply the DH parameters via a callback function.
 
-The B<tmp_dh_callback> is called with the B<keylength> needed and
-the B<is_export> information. The B<is_export> flag is set, when the
-ephemeral DH key exchange is performed with an export cipher.
+Previous versions of the callback used B<is_export> and B<keylength>
+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<keylength> and B<is_export> and simply supply at least 2048-bit
+parameters in the callback.
 
 =head1 EXAMPLES
 
-Handle DH parameters for key lengths of 512 and 1024 bits. (Error handling
+Setup DH parameters with a key length of 2048 bits. (Error handling
 partly left out.)
 
- ...
- /* Set up ephemeral DH stuff */
- DH *dh_512 = NULL;
- DH *dh_1024 = NULL;
- FILE *paramfile;
+ Command-line parameter generation:
+ $ openssl dhparam -out dh_param_2048.pem 2048
+
+ Code for setting up parameters during server initialization:
 
  ...
- /* "openssl dhparam -out dh_param_512.pem -2 512" */
- paramfile = fopen("dh_param_512.pem", "r");
+ SSL_CTX ctx = SSL_CTX_new();
+ ...
+
+ /* Set up ephemeral DH parameters. */
+ DH *dh_2048 = NULL;
+ FILE *paramfile;
+ paramfile = fopen("dh_param_2048.pem", "r");
  if (paramfile) {
-   dh_512 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
+   dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
    fclose(paramfile);
+ } else {
+   /* Error. */
  }
- /* "openssl dhparam -out dh_param_1024.pem -2 1024" */
- paramfile = fopen("dh_param_1024.pem", "r");
- if (paramfile) {
-   dh_1024 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
-   fclose(paramfile);
+ if (dh_2048 == NULL) {
+  /* Error. */
  }
- ...
-
- /* "openssl dhparam -C -2 512" etc... */
- DH *get_dh512() { ... }
- DH *get_dh1024() { ... }
-
- DH *tmp_dh_callback(SSL *s, int is_export, int keylength)
- {
-    DH *dh_tmp=NULL;
-
-    switch (keylength) {
-    case 512:
-      if (!dh_512)
-        dh_512 = get_dh512();
-      dh_tmp = dh_512;
-      break;
-    case 1024:
-      if (!dh_1024)
-        dh_1024 = get_dh1024();
-      dh_tmp = dh_1024;
-      break;
-    default:
-      /* Generating a key on the fly is very costly, so use what is there */
-      setup_dh_parameters_like_above();
-    }
-    return(dh_tmp);
+ if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1) {
+   /* Error. */
  }
+ SSL_CTX_set_options(ctx, SSL_OP_SINGLE_DH_USE);
+ ...
 
 =head1 RETURN VALUES