SSL_CTX_set_tmp_dh_callback(3OpenSSSSL_CTX_set_tmp_dh_callback(3)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));
DESCRIPTIONSSL_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 VALUESSSL_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 ALSOssl(3), SSL_CTX_set_cipher_list(3),
SSL_CTX_set_tmp_rsa_callback(3), SSL_CTX_set_options(3),
ciphers(1), dhparam(1)13/Sep/2001 0.9.6SSL_CTX_set_tmp_dh_callback(3)
