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SSL_CTX_set_tmp_dh_callback(3)
Contents
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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
#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) );
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.
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.
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.
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);
} }
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)
SSL_CTX_set_tmp_dh_callback(3)
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