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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 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 dependent 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()
       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.

1.0.2h                            2016-05-03                  DH_set_method(3)

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