=pod

=head1 NAME

SSL_CTX_set_tmp_dh_callback, SSL_CTX_set_tmp_dh, SSL_set_tmp_dh_callback, SSL_set_tmp_dh - handle DH keys for ephemeral key exchange

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx,
            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,
            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
used when a DH parameters are required to B<tmp_dh_callback>.
The callback is inherited by all B<ssl> objects created from B<ctx>.

SSL_CTX_set_tmp_dh() sets DH parameters to be used to be B<dh>.
The key is inherited by all B<ssl> objects created from B<ctx>.

SSL_set_tmp_dh_callback() sets the callback only for B<ssl>.

SSL_set_tmp_dh() sets the paramters 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
ephemeral/temporary DH key and the key supplied and certified
by the certificate chain is only used for signing.

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
DH key inside the server application that is lost when the application
is left, it becomes impossible for an attacker to decrypt past sessions,
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 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.

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.

=head1 EXAMPLES

Handle DH parameters for key lengths of 512 and 1024 bits. (Error handling
partly left out.)

 ...
 /* Set up ephemeral DH stuff */
 DH *dh_512 = NULL;
 DH *dh_1024 = NULL;
 FILE *paramfile;

 ...
 /* "openssl dhparam -out dh_param_512.pem -2 512" */
 paramfile = fopen("dh_param_512.pem", "r");
 if (paramfile) {
   dh_512 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
   fclose(paramfile);
 }
 /* "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);
 }
 ...

 /* "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);
 }

=head1 RETURN VALUES

SSL_CTX_set_tmp_dh_callback() and SSL_set_tmp_dh_callback() do not return
diagnostic output.

SSL_CTX_set_tmp_dh() and SSL_set_tmp_dh() do return 1 on success and 0
on failure. Check the error queue to find out the reason of failure.

=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_options(3)|SSL_CTX_set_options(3)>,
L<ciphers(1)|ciphers(1)>, L<dhparam(1)|dhparam(1)>

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