SSL_CTX_set_tmp_dh_callback(3)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
The SSL_CTX_set_tmp_dh_callback() function sets the callback function
for ctx to be used when DH parameters are required to tmp_dh_callback.
The callback is inherited by all ssl objects created from ctx.
The SSL_CTX_set_tmp_dh() function sets DH parameters to be used to be
dh. The key is inherited by all ssl objects created from ctx. The
SSL_set_tmp_dh_callback() function sets the callback only for ssl.
The SSL_set_tmp_dh() function 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() is set. It will immediately create a DH key, when
DH parameters are supplied via SSL_CTX_set_tmp_dh() and SSL_OP_SIN‐
GLE_DH_USE is not set. In this case, it may happen that a key is gener‐
ated 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
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 com‐
puter time needed during negotiation. Because it is not very large,
application authors and users should consider always enabling this
option.
Because generating DH parameters is extremely time consuming, an appli‐
cation 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. Applica‐
tions 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 can compile in DH parameters. Files dh512.pem,
dh1024.pem, dh2048.pem, and dh4096 in the 'apps' directory of the cur‐
rent version of the OpenSSL distribution contain the 'SKIP' DH parame‐
ters, which use safe primes and were generated verifiably pseudo-ran‐
domly. These files can be converted into C code using the -C option of
the dhparam application. Authors may also generate their own set of
parameters using dhparam, but a user may not be sure how the parameters
were generated. We recommend the generation of DH parameters during
installation.
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 option is set when the ephemeral
DH key exchange is performed with an export cipher.
RETURN VALUES
The SSL_CTX_set_tmp_dh_callback() and SSL_set_tmp_dh_callback() func‐
tions do not return diagnostic output.
The SSL_CTX_set_tmp_dh() and SSL_set_tmp_dh() functions return 1 on
success and 0 on failure. Check the error queue to find out the reason
of failure.
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 =3D fopen("dh_param_512.pem", "r");
if (paramfile) {
dh_512 =3D PEM_read_DHparams(paramfile, NULL, NULL, NULL);
fclose(paramfile);
}
/* "openssl dhparam -out dh_param_1024.pem -2 1024" */
paramfile =3D fopen("dh_param_1024.pem", "r");
if (paramfile) {
dh_1024 =3D 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=3DNULL;
switch (keylength) {
case 512:
if (!dh_512)
dh_512 =3D get_dh512();
dh_tmp =3D dh_512;
break;
case 1024:
if (!dh_1024)
dh_1024 =3D get_dh1024();
dh_tmp =3D 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);
} }
SEE ALSO
Files: ciphers(1)dhparam(1)
Functions: ssl(3)SSL_CTX_set_cipher_list(3) SSL_CTX_set_tmp_rsa_call‐
back(3)SSL_CTX_set_options(3)SSL_CTX_set_tmp_dh_callback(3)
