NAME    [Toc]    [Back]

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

SYNOPSIS    [Toc]    [Back]

        #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    [Toc]    [Back]

       A DH_METHOD 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 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    [Toc]    [Back]

        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    [Toc]    [Back]

       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    [Toc]    [Back]

       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    [Toc]    [Back]

       dh(3), DH_new(3)

HISTORY    [Toc]    [Back]

       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 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.


OpenBSD 3.6                 2002-09-14                          3