SSL_set_tmp_dh
SSL_CTX_set_tmp_dh_callback(3) OpenSSL SSL_CTX_set_tmp_dh_callback(3)
NAME
SSL_CTX_set_tmp_dh_callback, SSL_CTX_set_tmp_dh, SSL_set_tmp_dh_call-
back, SSL_set_tmp_dh - handle DH keys for ephemeral key exchange
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));
DESCRIPTION
SSL_CTX_set_tmp_dh_callback() sets the callback function for ctx to be
used when a DH parameters are required to tmp_dh_callback. The call-
back is inherited by all ssl objects created from ctx.
SSL_CTX_set_tmp_dh() sets DH parameters to be used to be dh. The key
is inherited by all ssl objects created from ctx.
SSL_set_tmp_dh_callback() sets the callback only for ssl.
SSL_set_tmp_dh() sets the parameters only for ssl.
These functions apply to SSL/TLS servers only.
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 connec-
tion 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 sup-
plied via callback and/or when the SSL_OP_SINGLE_DH_USE option of
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 parame-
ters. DH parameters can be reused, as the actual key is newly gener-
ated 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 installa-
tion process using the openssl dhparam(1) application. In order to
reduce the computer time needed for this generation, it is possible to
use DSA parameters instead (see 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 -C option of the
dhparam(1) application. Authors may also generate their own set of
parameters using dhparam(1), but a user may not be sure how the parame-
ters were generated. The generation of DH parameters during installa-
tion 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 dif-
ferent key lengths.
The tmp_dh_callback is called with the keylength needed and the
is_export information. The is_export flag is set, when the ephemeral DH
key exchange is performed with an export cipher.
EXAMPLES
Handle DH parameters for key lengths of 512 and 1024 bits. (Error han-
dling 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);
}
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.
SEE ALSO
ssl(3), SSL_CTX_set_cipher_list(3), SSL_CTX_set_tmp_rsa_callback(3),
SSL_CTX_set_options(3), ciphers(1), dhparam(1)
0.9.7a 2001-09-07 SSL_CTX_set_tmp_dh_callback(3)
