.\"	$OpenBSD: DH_set_method.3,v 1.3 2016/11/12 19:23:16 jmc Exp $
.\"
.Dd $Mdocdate: November 12 2016 $
.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 ,
.Nm DH_set_default_openssl_method ,
.Nm DH_get_default_openssl_method
.Nd select DH method
.Sh SYNOPSIS
.In openssl/dh.h
.In openssl/engine.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
specifies the functions that OpenSSL uses for Diffie-Hellman operations.
By modifying the method, alternative implementations such as hardware
accelerators may be used.
See the
.Sx CAVEATS
section for how these DH API functions are affected by the use of
.Xr engine 3
API calls.
.Pp
Initially, the default
.Vt DH_METHOD
is the OpenSSL internal implementation as returned by
.Fn DH_OpenSSL .
.Pp
.Fn DH_set_default_method
makes
.Fa meth
the default method for all
.Vt DH
structures created later.
.Sy NB :
This is true only whilst no
.Vt ENGINE
has been set as a default for DH, so this function is no longer
recommended.
.Pp
.Fn DH_get_default_method
returns a pointer to the current default
.Vt DH_METHOD .
However, the meaningfulness of this result is dependent on whether the
.Xr engine 3
API is being used, so this function is no longer recommended.
.Pp
.Fn DH_set_method
selects
.Fa meth
to perform all operations using the key
.Fa dh .
This will replace the
.Vt DH_METHOD
used by the
.Fa dh
key and if the previous method was supplied by an
.Vt ENGINE ,
the handle to that
.Vt ENGINE
will be released during the change.
It is possible to have
.Vt DH
keys that only work with certain
.Vt DH_METHOD
implementations (e.g. from an
.Vt ENGINE
module that supports embedded hardware-protected keys),
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 so that
.Fa engine
will be used for the DH operations.
If
.Fa engine
is
.Dv NULL ,
the default
.Vt ENGINE
for DH operations is used and, if no default
.Vt ENGINE
is set, the
.Vt DH_METHOD
controlled by
.Fn DH_set_default_method
is used.
.Sh THE DH_METHOD STRUCTURE
.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
.Sy DH_METHOD Ns s.
.Pp
.Fn DH_set_method
returns non-zero if the provided
.Fa meth
was successfully set as the method for
.Fa dh
(including unloading the
.Vt ENGINE
handle if the previous method was supplied by an
.Vt ENGINE ) .
.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 3 ,
.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
were added in OpenSSL 0.9.4.
.Pp
.Fn DH_set_default_openssl_method
and
.Fn DH_get_default_openssl_method
replaced
.Fn DH_set_default_method
and
.Fn DH_get_default_method
respectively, and
.Fn DH_set_method
and
.Fn DH_new_method
were altered to use
.Vt ENGINE Ns s
rather than
.Vt DH_METHOD Ns s
during development of the engine version of OpenSSL 0.9.6.
For 0.9.7, the handling of defaults in the
.Xr engine 3
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
.Xr engine 3
API now transparently overrides the behaviour of defaults in the
DH API without requiring changing these function prototypes.
.Sh CAVEATS
As of version 0.9.7,
.Vt DH_METHOD
implementations are grouped together with other algorithmic APIs
(e.g. RSA_METHOD, EVP_CIPHER) in
.Vt ENGINE
modules.
If a default
.Vt ENGINE
is specified for DH functionality using an
.Xr engine 3
API function, that will override any DH defaults set using the DH API
.Pq i.e. Fn DH_set_default_method .
For this reason, the
.Xr engine 3
API is the recommended way to control default implementations
for use in DH and other cryptographic algorithms.