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
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
callback 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 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
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
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 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
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 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);
}
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)