SSL_CTX_set_tmp_dh_callback manpage

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SSL_CTX_set_tmp_dh_callback(3)	    OpenSSL	SSL_CTX_set_tmp_dh_callback(3)

       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,
		   DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
	long SSL_set_tmp_dh(SSL *ssl, DH *dh)

       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.

       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 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
       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.

       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);
	} else {
	  /* Error. */
	if (dh_2048 == NULL) {
	 /* Error. */
	if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1) {
	  /* Error. */

       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.

       ssl(3), SSL_CTX_set_cipher_list(3), SSL_CTX_set_tmp_rsa_callback(3),
       SSL_CTX_set_options(3), ciphers(1), dhparam(1)

1.0.2k				  2017-01-26	SSL_CTX_set_tmp_dh_callback(3)