.\"	$OpenBSD: DH_set_method.3,v 1.9 2023/11/19 10:34:26 tb Exp $
.\"	OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100
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.\" This file was written by Ulf Moeller <ulf@openssl.org>.
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.Dd $Mdocdate: November 19 2023 $
.Dt DH_SET_METHOD 3
.Os
.Sh NAME
.Nm DH_set_default_method ,
.Nm DH_get_default_method ,
.Nm DH_set_method ,
.Nm DH_new_method ,
.Nm DH_OpenSSL
.Nd select DH method
.Sh SYNOPSIS
.In openssl/dh.h
.Ft void
.Fo DH_set_default_method
.Fa "const DH_METHOD *meth"
.Fc
.Ft const DH_METHOD *
.Fo DH_get_default_method
.Fa void
.Fc
.Ft int
.Fo DH_set_method
.Fa "DH *dh"
.Fa "const DH_METHOD *meth"
.Fc
.Ft DH *
.Fo DH_new_method
.Fa "ENGINE *engine"
.Fc
.Ft const DH_METHOD *
.Fo DH_OpenSSL
.Fa void
.Fc
.Sh DESCRIPTION
A
.Vt DH_METHOD
object contains pointers to the functions
used for Diffie-Hellman operations.
By default, the internal implementation returned by
.Fn DH_OpenSSL
is used.
By selecting another method, alternative implementations
such as hardware accelerators may be used.
.Pp
.Fn DH_set_default_method
selects
.Fa meth
as the default method for all
.Vt DH
structures created later.
.Pp
.Fn DH_get_default_method
returns a pointer to the current default method.
.Pp
.Fn DH_set_method
selects
.Fa meth
to perform all operations using the key
.Fa dh .
This replaces the
.Vt DH_METHOD
used by the
.Fa dh
key.
It is possible to have
.Vt DH
keys that only work with certain
.Vt DH_METHOD
implementations,
and in such cases attempting to change the
.Vt DH_METHOD
for the key can have unexpected results.
.Pp
.Fn DH_new_method
allocates and initializes a
.Vt DH
structure.
The
.Fa engine
argument is ignored and
the default method controlled by
.Fn DH_set_default_method
is used.
.Pp
The
.Vt DH_METHOD
structure is defined as follows:
.Bd -literal
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;
.Ed
.Sh RETURN VALUES
.Fn DH_OpenSSL
and
.Fn DH_get_default_method
return pointers to the respective
.Vt DH_METHOD .
.Pp
.Fn DH_set_method
returns 1 on success or 0 on failure.
Currently, it cannot fail.
.Pp
.Fn DH_new_method
returns
.Dv NULL
and sets an error code that can be obtained by
.Xr ERR_get_error 3
if the allocation fails.
Otherwise it returns a pointer to the newly allocated structure.
.Sh SEE ALSO
.Xr DH_new 3
.Sh HISTORY
.Fn DH_set_default_method ,
.Fn DH_get_default_method ,
.Fn DH_set_method ,
.Fn DH_new_method
and
.Fn DH_OpenSSL
first appeared in OpenSSL 0.9.5 and have been available since
.Ox 2.7 .