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       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. 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	cipher suites
       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_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.

       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();

	DH *dh_2048 = NULL;
	FILE *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(7), SSL_CTX_set_cipher_list(3), SSL_CTX_set_options(3), ciphers(1),

       Copyright 2001-2019 The OpenSSL Project Authors.	All Rights Reserved.

       Licensed	under the OpenSSL license (the "License").  You	may not	use
       this file except	in compliance with the License.	 You can obtain	a copy
       in the file LICENSE in the source distribution or at

1.1.1k				  2021-03-25	SSL_CTX_SET_TMP_DH_CALLBACK(3)


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