=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,

                              DH *(*tmp_dh_callback)(SSL *ssl, int is_export,

                                                     int keylength));

 long SSL_set_tmp_dh(SSL *ssl, DH *dh)

=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.

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)> 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)> 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 cipher suites

are advised to either use SSL_CTX_set_tmp_dh() 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 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 EXAMPLES

Setup DH parameters with a key length of 2048 bits. (Error handling

partly left out.)

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();

 DH *dh_2048 = NULL;

 FILE *paramfile = fopen("dh_param_2048.pem", "r");

 if (paramfile) {

     dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);

     fclose(paramfile);

 } else {

     /* Error. */

 }

 if (dh_2048 == NULL)

     /* Error. */

 if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1)

     /* Error. */

 ...

=head1 SEE ALSO

L<ssl(7)>, L<SSL_CTX_set_cipher_list(3)>,

L<SSL_CTX_set_options(3)>,

L<ciphers(1)>, L<dhparam(1)>

=head1 COPYRIGHT

Copyright 2001-2019 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (the "License").  You may not use

this file except in compliance with the License.  You can obtain a copy

in the file LICENSE in the source distribution or at

L<https://www.openssl.org/source/license.html>.

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