/usr/man/cat.3/DH_set_method.3

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

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DH_set_method(3)             OpenSSL             DH_set_method(3)

     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

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DH_set_method(3)             OpenSSL             DH_set_method(3)

     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.

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See also DH_OpenSSL(3)
See also DH_get_default_method(3)
See also DH_new_method(3)
See also DH_set_default_method(3)