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);
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 by ctx. 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.
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(1) dhparam
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(1) dhparam
application. Generation of
custom DH parameters during installation should still be preferred to stop
an attacker from specializing on a commonly used group. The file
dh1024.pem contains 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.
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.
EXAMPLES
Set up DH parameters with a key length of 2048 bits. Error handling is 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. */ }
SEE ALSO
openssl(1), ssl(3), SSL_CTX_set_cipher_list(3), SSL_CTX_set_options(3), SSL_set_tmp_ecdh(3)
HISTORY
SSL_CTX_set_tmp_dh_callback
() and
SSL_CTX_set_tmp_dh
() appeared before SSLeay 0.8 and
have been available since OpenBSD 2.4.
SSL_set_tmp_dh_callback
() and
SSL_set_tmp_dh
() first appeared in OpenSSL 0.9.2b
and have been available since OpenBSD 2.6.