.\"	$OpenBSD: EC_GFp_simple_method.3,v 1.3 2016/11/15 09:25:45 jmc Exp $
.\"
.Dd $Mdocdate: November 15 2016 $
.Dt EC_GFP_SIMPLE_METHOD 3
.Os
.Sh NAME
.Nm EC_GFp_simple_method ,
.Nm EC_GFp_mont_method ,
.Nm EC_GFp_nist_method ,
.Nm EC_GFp_nistp224_method ,
.Nm EC_GFp_nistp256_method ,
.Nm EC_GFp_nistp521_method ,
.Nm EC_GF2m_simple_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 const EC_METHOD *
.Fn EC_GFp_nist_method void
.Ft const EC_METHOD *
.Fn EC_GFp_nistp224_method void
.Ft const EC_METHOD *
.Fn EC_GFp_nistp256_method void
.Ft const EC_METHOD *
.Fn EC_GFp_nistp521_method void
.Ft const EC_METHOD *
.Fn EC_GF2m_simple_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.
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 F2^m curves there is only one implementation choice,
.Fn EC_GF2_simple_method .
.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 ) .
.Fn EC_GFp_nist_method
offers an implementation optimised for use with NIST recommended
curves.
NIST curves are available through
.Xr EC_GROUP_new_by_curve_name 3 .
.Pp
The functions
.Fn EC_GFp_nistp224_method ,
.Fn EC_GFp_nistp256_method ,
and
.Fn EC_GFp_nistp521_method
offer 64-bit optimised implementations for the NIST P224, P256 and
P521 curves respectively.
Note, however, that these implementations are not available on all
platforms.
.Pp
.Fn EC_METHOD_get_field_type
identifies what type of field the
.Vt EC_METHOD
structure supports, which will be either F2^m or Fp.
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/obj_mac.h
header file.
.Sh RETURN VALUES
All
.Fn EC_GFp*
functions and
.Fn EC_GF2m_simple_method
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 crypto 3 ,
.Xr d2i_ECPKParameters 3 ,
.Xr ec 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