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=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
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. 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. Files dh1024.pem
and dh512.pem contain 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
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();
...
/* Set up ephemeral DH parameters. */
DH *dh_2048 = NULL;
FILE *paramfile;
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. */
}
SSL_CTX_set_options(ctx, SSL_OP_SINGLE_DH_USE);
...
=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
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