DHsetmethod(3) OpenSSL DHsetmethod(3)

NAME
       DHsetdefaultmethod, DHgetdefaultmethod, DHsetmethod,
       DHnewmethod, DHOpenSSL - select DH method

SYNOPSIS
        #include 
        #include 

        void DHsetdefaultmethod(const DHMETHOD *meth);

        const DHMETHOD *DHgetdefaultmethod(void);

        int DHsetmethod(DH *dh, const DHMETHOD *meth);

        DH *DHnewmethod(ENGINE *engine);

        const DHMETHOD *DHOpenSSL(void);

DESCRIPTION
       A DDHHMMEETTHHOODD specifies the functions that OpenSSL uses for Diffie-
       Hellman 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 EENNGGIINNEE API calls.

       Initially, the default DHMETHOD is the OpenSSL internal
       implementation, as returned by DHOpenSSL().

       DHsetdefaultmethod() makes mmeetthh the default method for all DH
       structures created later. NNBB: This is true only whilst no ENGINE has
       been set as a default for DH, so this function is no longer
       recommended.

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

       DHsetmethod() selects mmeetthh to perform all operations using the key
       ddhh.  This will replace the DHMETHOD 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 DHMETHOD implementations (eg. from an ENGINE
       module that supports embedded hardware-protected keys), and in such
       cases attempting to change the DHMETHOD for the key can have
       unexpected results.

       DHnewmethod() allocates and initializes a DH structure so that eennggiinnee
       will be used for the DH operations. If eennggiinnee is NULL, the default
       ENGINE for DH operations is used, and if no default ENGINE is set, the
       DHMETHOD controlled by DHsetdefaultmethod() is used.

TTHHEE DDHHMMEETTHHOODD SSTTRRUUCCTTUURREE
        typedef struct dhmethst
        {
            /* name of the implementation */
               const char *name;

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

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

            /* compute r = a ^ p mod m (May be NULL for some implementations) */
               int (*bnmodexp)(DH *dh, BIGNUM *r, BIGNUM *a, const BIGNUM *p,
                                       const BIGNUM *m, BNCTX *ctx,
                                       BNMONTCTX *mctx);

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

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

               int flags;

               char *appdata; /* ?? */

        } DHMETHOD;

RETURN VALUES
       DHOpenSSL() and DHgetdefaultmethod() return pointers to the
       respective DDHHMMEETTHHOODDs.

       DHsetdefaultmethod() returns no value.

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

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

NNOOTTEESS
       As of version 0.9.7, DHMETHOD implementations are grouped together
       with other algorithmic APIs (eg. RSAMETHOD, EVPCIPHER, etc) in EENNGGIINNEE
       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.  DHsetdefaultmethod()). 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), DHnew(3)

HISTORY       DHsetdefaultmethod(), DHgetdefaultmethod(), DHsetmethod(),
       DHnewmethod() and DHOpenSSL() were added in OpenSSL 0.9.4.

       DHsetdefaultopensslmethod() and DHgetdefaultopensslmethod()
       replaced DHsetdefaultmethod() and DHgetdefaultmethod()
       respectively, and DHsetmethod() and DHnewmethod() were altered to
       use EENNGGIINNEEs rather than DDHHMMEETTHHOODDs during 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.



0.9.7l                            2002-10-28                  DHsetmethod(3)