4746f25ac6
[skip ci] Reviewed-by: Matt Caswell <matt@openssl.org> (Merged from https://github.com/openssl/openssl/pull/7829)
135 lines
5 KiB
Text
135 lines
5 KiB
Text
=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 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 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(7)>, L<SSL_CTX_set_cipher_list(3)>,
|
|
L<SSL_CTX_set_options(3)>,
|
|
L<ciphers(1)>, L<dhparam(1)>
|
|
|
|
=head1 COPYRIGHT
|
|
|
|
Copyright 2001-2016 The OpenSSL Project Authors. All Rights Reserved.
|
|
|
|
Licensed under the Apache License 2.0 (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>.
|
|
|
|
=cut
|