=pod

=head1 NAME

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

=head1 SYNOPSIS

 #include <openssl/dh.h>

 void DH_set_default_method(DH_METHOD *meth);

 DH_METHOD *DH_get_default_method(void);

 DH_METHOD *DH_set_method(DH *dh, DH_METHOD *meth);

 DH *DH_new_method(DH_METHOD *meth);

 DH_METHOD *DH_OpenSSL(void);

=head1 DESCRIPTION

A B<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.

Initially, the default is to use the OpenSSL internal implementation.
DH_OpenSSL() returns a pointer to that method.

DH_set_default_method() makes B<meth> the default method for all B<DH>
structures created later.

DH_get_default_method() returns a pointer to the current default
method.

DH_set_method() selects B<meth> for all operations using the structure B<dh>.

DH_get_method() returns a pointer to the method currently selected
for B<dh>.

DH_new_method() allocates and initializes a B<DH> structure so that
B<method> will be used for the DH operations. If B<method> is B<NULL>,
the default method is used.

=head1 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;

=head1 RETURN VALUES

DH_OpenSSL(), DH_get_default_method() and DH_get_method() return
pointers to the respective B<DH_METHOD>s.

DH_set_default_method() returns no value.

DH_set_method() returns a pointer to the B<DH_METHOD> previously
associated with B<dh>.

DH_new_method() returns B<NULL> and sets an error code that can be
obtained by L<ERR_get_error(3)|ERR_get_error(3)> if the allocation fails. Otherwise it
returns a pointer to the newly allocated structure.

=head1 SEE ALSO

L<dh(3)|dh(3)>, L<DH_new(3)|DH_new(3)>

=head1 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.

=cut
