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Linux 2.6 - man page for ssl_ctx_set_tmp_dh_callback (linux section 3ssl)

SSL_CTX_set_tmp_dh_callback(3SSL)	     OpenSSL		SSL_CTX_set_tmp_dh_callback(3SSL)

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

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

       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,
       when the DH parameters are supplied via callback and/or when the SSL_OP_SINGLE_DH_USE
       option of SSL_CTX_set_options(3) is set. It will immediately create a DH key, when DH
       parameters are supplied via SSL_CTX_set_tmp_dh() and SSL_OP_SINGLE_DH_USE is not set. In
       this case, it may happen that a key is generated on initialization without later being
       needed, while on the other hand the computer time during the negotiation is being saved.

       If "strong" primes were used to generate the DH parameters, it is not strictly necessary
       to generate a new key for each handshake but it does improve forward secrecy. If it is not
       assured, that "strong" primes were used (see especially the section about DSA parameters
       below), SSL_OP_SINGLE_DH_USE must be used in order to prevent small subgroup attacks.
       Always using SSL_OP_SINGLE_DH_USE has an impact on the computer time needed during
       negotiation, but it is not very large, so application authors/users should consider to
       always enable this option.

       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. In order to reduce the computer time
       needed for this generation, it is possible to use DSA parameters instead (see dhparam(1)),
       but in this case SSL_OP_SINGLE_DH_USE is mandatory.

       Application authors may compile in DH parameters. Files dh512.pem, dh1024.pem, dh2048.pem,
       and dh4096 in the 'apps' directory of 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.  Authors may also generate their own set of parameters using dhparam(1), but
       a user may not be sure how the parameters were generated. The generation of DH parameters
       during installation is therefore recommended.

       An application may either directly specify the DH parameters or can supply the DH
       parameters via a callback function. The callback approach has the advantage, that the
       callback may supply DH parameters for different key lengths.

       The tmp_dh_callback is called with the keylength needed and the is_export information. The
       is_export flag is set, when the ephemeral DH key exchange is performed with an export

       Handle DH parameters for key lengths of 512 and 1024 bits. (Error handling partly left

	/* Set up ephemeral DH stuff */
	DH *dh_512 = NULL;
	DH *dh_1024 = NULL;
	FILE *paramfile;

	/* "openssl dhparam -out dh_param_512.pem -2 512" */
	paramfile = fopen("dh_param_512.pem", "r");
	if (paramfile) {
	  dh_512 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
	/* "openssl dhparam -out dh_param_1024.pem -2 1024" */
	paramfile = fopen("dh_param_1024.pem", "r");
	if (paramfile) {
	  dh_1024 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);

	/* "openssl dhparam -C -2 512" etc... */
	DH *get_dh512() { ... }
	DH *get_dh1024() { ... }

	DH *tmp_dh_callback(SSL *s, int is_export, int keylength)
	   DH *dh_tmp=NULL;

	   switch (keylength) {
	   case 512:
	     if (!dh_512)
	       dh_512 = get_dh512();
	     dh_tmp = dh_512;
	   case 1024:
	     if (!dh_1024)
	       dh_1024 = get_dh1024();
	     dh_tmp = dh_1024;
	     /* Generating a key on the fly is very costly, so use what is there */

       SSL_CTX_set_tmp_dh_callback() and SSL_set_tmp_dh_callback() do not return diagnostic

       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)

1.0.0e					    2001-09-07		SSL_CTX_set_tmp_dh_callback(3SSL)

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