.\"	$OpenBSD: EC_GFp_simple_method.3,v 1.12 2023/04/27 08:47:04 tb Exp $
.\"	OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100
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.\" This file was written by Matt Caswell <matt@openssl.org>.
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.Dd $Mdocdate: April 27 2023 $
.Dt EC_GFP_SIMPLE_METHOD 3
.Os
.Sh NAME
.Nm EC_GFp_simple_method ,
.Nm EC_GFp_mont_method ,
.Nm EC_METHOD_get_field_type
.Nd obtain EC_METHOD objects
.Sh SYNOPSIS
.In openssl/ec.h
.Ft const EC_METHOD *
.Fn EC_GFp_simple_method void
.Ft const EC_METHOD *
.Fn EC_GFp_mont_method void
.Ft int
.Fo EC_METHOD_get_field_type
.Fa "const EC_METHOD *meth"
.Fc
.Sh DESCRIPTION
The elliptic curve library provides a number of different
implementations through a single common interface.
Each implementation is optimised for different scenarios.
An implementation is represented by an
.Vt EC_METHOD
structure.
.Pp
When constructing a curve using
.Xr EC_GROUP_new 3 ,
an implementation method must be provided.
The functions described here all return a const pointer to an
.Sy EC_METHOD
structure that can be passed to
.Xr EC_GROUP_new 3 .
It is important that the correct implementation type for the form
of curve selected is used.
.Pp
For Fp curves the lowest common denominator implementation is the
.Fn EC_GFp_simple_method
implementation.
All other implementations are based on this one.
.Fn EC_GFp_mont_method
adds the use of Montgomery multiplication (see
.Xr BN_mod_mul_montgomery 3 ) .
.Pp
.Fn EC_METHOD_get_field_type
identifies what type of field the
.Vt EC_METHOD
structure supports.
If the field type is Fp, then the value
.Dv NID_X9_62_prime_field
is returned.
If the field type is F2^m, then the value
.Dv NID_X9_62_characteristic_two_field
is returned.
These values are defined in the
.In openssl/objects.h
header file.
.Sh RETURN VALUES
All
.Fn EC_GFp*
functions always return a const pointer to an
.Vt EC_METHOD
structure.
.Pp
.Fn EC_METHOD_get_field_type
returns an integer that identifies the type of field the
.Vt EC_METHOD
structure supports.
.Sh SEE ALSO
.Xr BN_mod_mul_montgomery 3 ,
.Xr d2i_ECPKParameters 3 ,
.Xr EC_GROUP_copy 3 ,
.Xr EC_GROUP_new 3 ,
.Xr EC_KEY_new 3 ,
.Xr EC_POINT_add 3 ,
.Xr EC_POINT_new 3
.Sh HISTORY
.Fn EC_GFp_simple_method
and
.Fn EC_GFp_mont_method
first appeared in OpenSSL 0.9.7 and have been available since
.Ox 3.2 .
.Pp
.Fn EC_METHOD_get_field_type
first appeared in OpenSSL 0.9.8 and has been available since
.Ox 4.5 .