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
LIBRARY
libcrypto, -lcrypto
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,
DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
long SSL_set_tmp_dh(SSL *ssl, DH *dh)
DESCRIPTION
SSL_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.
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
openssl_dhparam(1) application. This application guarantees
that "strong" primes are used.
Files dh2048.pem, and dh4096.pem 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
openssl_dhparam(1) application. Generation of custom DH parameters
during installation should still be preferred to stop an attacker from
specializing on a commonly used group. Files dh1024.pem and dh512.pem contain
old parameters that must not be used by applications.
An application may either directly specify the DH parameters or can supply the
DH parameters via a callback function.
Previous versions of the callback used
is_export and
keylength
parameters to control parameter generation for export and non-export cipher
suites. Modern servers that do not support export ciphersuites are advised to
either use
SSL_CTX_set_tmp_dh() or alternatively, use the callback but
ignore
keylength and
is_export and simply supply at least
2048-bit parameters in the callback.
EXAMPLES
Setup DH parameters with a key length of 2048 bits. (Error handling partly left
out.)
Command-line parameter generation:
$ openssl dhparam -out dh_param_2048.pem 2048
Code for setting up parameters during server initialization:
...
SSL_CTX ctx = SSL_CTX_new();
...
/* Set up ephemeral DH parameters. */
DH *dh_2048 = NULL;
FILE *paramfile;
paramfile = fopen("dh_param_2048.pem", "r");
if (paramfile) {
dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
fclose(paramfile);
} else {
/* Error. */
}
if (dh_2048 == NULL) {
/* Error. */
}
if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1) {
/* Error. */
}
...
RETURN VALUES
SSL_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 ALSO
ssl(3),
SSL_CTX_set_cipher_list(3),
SSL_CTX_set_tmp_rsa_callback(3),
SSL_CTX_set_options(3),
openssl_ciphers(1),
openssl_dhparam(1)