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SSL_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) );
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 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() 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
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. 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
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 can compile in DH parameters. Files dh512.pem,
dh1024.pem, dh2048.pem, and dh4096 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 -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 different 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() functions
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 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 =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_callback(3) SSL_CTX_set_options(3)
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