DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);
- void SSL_set_tmp_dh_callback(SSL_CTX *ctx,
+ void SSL_set_tmp_dh_callback(SSL *ctx,
DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
long SSL_set_tmp_dh(SSL *ssl, DH *dh)
- DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
-
=head1 DESCRIPTION
SSL_CTX_set_tmp_dh_callback() sets the callback function for B<ctx> to be
SSL_set_tmp_dh_callback() sets the callback only for B<ssl>.
-SSL_set_tmp_dh() sets the paramters only for B<ssl>.
+SSL_set_tmp_dh() sets the parameters only for B<ssl>.
These functions apply to SSL/TLS servers only.
=head1 NOTES
When using a cipher with RSA authentication, an ephemeral DH key exchange
-can take place. Ciphers with DSA keys always use ephemeral DH keys as well
-as anonymous ciphers. In this case the session data are negotiated using the
+can take place. Ciphers with DSA keys always use ephemeral DH keys as well.
+In these cases, the session data are negotiated using the
ephemeral/temporary DH key and the key supplied and certified
by the certificate chain is only used for signing.
+Anonymous ciphers (without a permanent server key) also use ephemeral DH keys.
Using ephemeral DH key exchange yields forward secrecy, as the connection
can only be decrypted, when the DH key is known. By generating a temporary
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 automatically
-generate the DH key when required, as it is computationally cheap
-(retrieve a random number). The server will reuse the DH key for further
-connections, unless the SSL_OP_SINGLE_DH_USE option of
-L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)> is set, in which case
-a new DH key for each negotiation will be generated.
+(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).
As generating DH parameters is extremely time consuming, an application
should not generate the parameters on the fly but supply the parameters.
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. Therefore application authors
-should not copy the DH parameters from other applications or the OpenSSL
-example application, if they compile in parameters, but generate their
-own set of parameters using e.g. the openssl L<dhparam(1)|dhparam(1)>
-application with the B<-C> option. An application may also generate
-its own set of DH parameters during the installation procedure on a specific
-host, so that each host uses different parameters.
-
-An application my 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.
-
-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.
+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. This application
+guarantees that "strong" primes are used.
+
+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. Generation of custom DH
+parameters during installation should still be preferred to stop an
+attacker from specializing on a commonly used group. File dh1024.pem
+contains 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.
+
+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:
+
+ ...
+ SSL_CTX ctx = SSL_CTX_new();
...
- /* "openssl dhparam -out dh_param_512.pem -2 512" */
- paramfile = fopen("dh_param_512.pem", "r");
+
+ /* 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
=head1 SEE ALSO
L<ssl(3)|ssl(3)>, L<SSL_CTX_set_cipher_list(3)|SSL_CTX_set_cipher_list(3)>,
+L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>,
L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)>,
L<ciphers(1)|ciphers(1)>, L<dhparam(1)|dhparam(1)>
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