DH_set_method

DH_set_method(3)		   OpenSSL		     DH_set_method(3)



NAME
       DH_set_default_method, DH_get_default_method, DH_set_method,
       DH_new_method, DH_OpenSSL - select DH method

SYNOPSIS
	#include <openssl/dh.h>
	#include <openssl/engine.h>

	void DH_set_default_method(const DH_METHOD *meth);

	const DH_METHOD *DH_get_default_method(void);

	int DH_set_method(DH *dh, const DH_METHOD *meth);

	DH *DH_new_method(ENGINE *engine);

	const DH_METHOD *DH_OpenSSL(void);

DESCRIPTION
       A DH_METHOD specifies the functions that OpenSSL uses for Diffie-Hell-
       man operations. By modifying the method, alternative implementations
       such as hardware accelerators may be used. IMPORTANT: See the NOTES
       section for important information about how these DH API functions are
       affected by the use of ENGINE API calls.

       Initially, the default DH_METHOD is the OpenSSL internal implementa-
       tion, as returned by DH_OpenSSL().

       DH_set_default_method() makes meth the default method for all DH
       structures created later. NB: This is true only whilst no ENGINE has
       been set as a default for DH, so this function is no longer recom-
       mended.

       DH_get_default_method() returns a pointer to the current default
       DH_METHOD.  However, the meaningfulness of this result is dependant on
       whether the ENGINE API is being used, so this function is no longer
       recommended.

       DH_set_method() selects meth to perform all operations using the key
       dh.  This will replace the DH_METHOD used by the DH key and if the
       previous method was supplied by an ENGINE, the handle to that ENGINE
       will be released during the change. It is possible to have DH keys
       that only work with certain DH_METHOD implementations (eg. from an
       ENGINE module that supports embedded hardware-protected keys), and in
       such cases attempting to change the DH_METHOD for the key can have
       unexpected results.

       DH_new_method() allocates and initializes a DH structure so that
       engine will be used for the DH operations. If engine is NULL, the
       default ENGINE for DH operations is used, and if no default ENGINE is
       set, the DH_METHOD controlled by DH_set_default_method() is used.

THE DH_METHOD STRUCTURE
	typedef struct dh_meth_st
	{
	    /* name of the implementation */
	       const char *name;

	    /* generate private and public DH values for key agreement */
	       int (*generate_key)(DH *dh);

	    /* compute shared secret */
	       int (*compute_key)(unsigned char *key, BIGNUM *pub_key, DH *dh);

	    /* compute r = a ^ p mod m (May be NULL for some implementations) */
	       int (*bn_mod_exp)(DH *dh, BIGNUM *r, BIGNUM *a, const BIGNUM *p,
				       const BIGNUM *m, BN_CTX *ctx,
				       BN_MONT_CTX *m_ctx);

	    /* called at DH_new */
	       int (*init)(DH *dh);

	    /* called at DH_free */
	       int (*finish)(DH *dh);

	       int flags;

	       char *app_data; /* ?? */

	} DH_METHOD;

RETURN VALUES
       DH_OpenSSL() and DH_get_default_method() return pointers to the
       respective DH_METHODs.

       DH_set_default_method() returns no value.

       DH_set_method() returns non-zero if the provided meth was successfully
       set as the method for dh (including unloading the ENGINE handle if the
       previous method was supplied by an ENGINE).

       DH_new_method() returns NULL and sets an error code that can be
       obtained by ERR_get_error(3) if the allocation fails. Otherwise it
       returns a pointer to the newly allocated structure.

NOTES
       As of version 0.9.7, DH_METHOD implementations are grouped together
       with other algorithmic APIs (eg. RSA_METHOD, EVP_CIPHER, etc) in
       ENGINE modules. If a default ENGINE is specified for DH functionality
       using an ENGINE API function, that will override any DH defaults set
       using the DH API (ie.  DH_set_default_method()). For this reason, the
       ENGINE API is the recommended way to control default implementations
       for use in DH and other cryptographic algorithms.

SEE ALSO
       dh(3), DH_new(3)

HISTORY
       DH_set_default_method(), DH_get_default_method(), DH_set_method(),
       DH_new_method() and DH_OpenSSL() were added in OpenSSL 0.9.4.

       DH_set_default_openssl_method() and DH_get_default_openssl_method()
       replaced DH_set_default_method() and DH_get_default_method() respec-
       tively, and DH_set_method() and DH_new_method() were altered to use
       ENGINEs rather than DH_METHODs during development of the engine ver-
       sion of OpenSSL 0.9.6. For 0.9.7, the handling of defaults in the
       ENGINE API was restructured so that this change was reversed, and
       behaviour of the other functions resembled more closely the previous
       behaviour. The behaviour of defaults in the ENGINE API now transpar-
       ently overrides the behaviour of defaults in the DH API without
       requiring changing these function prototypes.



0.9.7a				  2002-10-28		     DH_set_method(3)