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-Hellman operations. By modifying the method,
       alternative implementations such as hardware accelera-
       tors 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
       implementation, 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 recommended.

       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 allo-
       cated 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 implemen-
       tations for use in DH and other cryptographic algo-
       rithms.

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()
       respectively, and DH_set_method() and DH_new_method()
       were altered to use ENGINEs rather than DH_METHODs dur-
       ing development of the engine version 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 transparently overrides the behaviour of
       defaults in the DH API without requiring changing these
       function prototypes.



