=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 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.
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
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 always generate a new
DH key during the negotiation, when the DH parameters are supplied via
callback and/or when the SSL_OP_SINGLE_DH_USE option of
L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)> is set. It will
immediately create a DH key, when 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 (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.

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. 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.

Application authors may compile in DH parameters. Files dh512.pem,
dh1024.pem, dh2048.pem, and dh4096 in the 'apps' directory of 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.

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.

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_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)>

=cut