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DESCRIPTION

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.

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. */
}