SSL_CTX_SET_TMP_DH_CA... BSD Library Functions Manual SSL_CTX_SET_TMP_DH_CA...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 *ssl,
DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength);
long
SSL_set_tmp_dh(SSL *ssl, DH *dh);
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 cer‐
tificate 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 tempo‐
rary DH key inside the server application that is lost when the applica‐
tion is left, it becomes impossible for an attacker to decrypt past ses‐
sions, 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 always enabling 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 there‐
fore generate their own DH parameters during the installation process
using the openssl openssl(1) application. In order to reduce the com‐
puter time needed for this generation, it is possible to use DSA parame‐
ters instead (see openssl(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.pem 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 openssl(1) application. Authors may also generate their own set of
parameters using openssl(1), but a user may not be sure how the parame‐
ters were generated. The generation of DH parameters during installation
is therefore recommended.
An application may either directly specify the DH parameters or can sup‐
ply 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.
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.
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 = 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);
}
SEE ALSOopenssl(1), ssl(3), SSL_CTX_set_cipher_list(3), SSL_CTX_set_options(3),
SSL_CTX_set_tmp_rsa_callback(3)BSD April 26, 2024 BSD
