/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#include "ecp.h"
#include "mpi.h"
#include "mplogic.h"
#include "mpi-priv.h"

#define ECP521_DIGITS ECL_CURVE_DIGITS(521)

/* Fast modular reduction for p521 = 2^521 - 1.  a can be r. Uses
 * algorithm 2.31 from Hankerson, Menezes, Vanstone. Guide to 
 * Elliptic Curve Cryptography. */
static mp_err
ec_GFp_nistp521_mod(const mp_int *a, mp_int *r, const GFMethod *meth)
{
	mp_err res = MP_OKAY;
	int a_bits = mpl_significant_bits(a);
	int i;

	/* m1, m2 are statically-allocated mp_int of exactly the size we need */
	mp_int m1;

	mp_digit s1[ECP521_DIGITS] = { 0 };

	MP_SIGN(&m1) = MP_ZPOS;
	MP_ALLOC(&m1) = ECP521_DIGITS;
	MP_USED(&m1) = ECP521_DIGITS;
	MP_DIGITS(&m1) = s1;

	if (a_bits < 521) {
		if (a==r) return MP_OKAY;
		return mp_copy(a, r);
	}
	/* for polynomials larger than twice the field size or polynomials 
	 * not using all words, use regular reduction */
	if (a_bits > (521*2)) {
		MP_CHECKOK(mp_mod(a, &meth->irr, r));
	} else {
#define FIRST_DIGIT (ECP521_DIGITS-1)
		for (i = FIRST_DIGIT; i < MP_USED(a)-1; i++) {
			s1[i-FIRST_DIGIT] = (MP_DIGIT(a, i) >> 9) 
				| (MP_DIGIT(a, 1+i) << (MP_DIGIT_BIT-9));
		}
		s1[i-FIRST_DIGIT] = MP_DIGIT(a, i) >> 9;

		if ( a != r ) {
			MP_CHECKOK(s_mp_pad(r,ECP521_DIGITS));
			for (i = 0; i < ECP521_DIGITS; i++) {
				MP_DIGIT(r,i) = MP_DIGIT(a, i);
			}
		}
		MP_USED(r) = ECP521_DIGITS;
		MP_DIGIT(r,FIRST_DIGIT) &=  0x1FF;

		MP_CHECKOK(s_mp_add(r, &m1));
		if (MP_DIGIT(r, FIRST_DIGIT) & 0x200) {
			MP_CHECKOK(s_mp_add_d(r,1));
			MP_DIGIT(r,FIRST_DIGIT) &=  0x1FF;
		} else if (s_mp_cmp(r, &meth->irr) == 0) {
			mp_zero(r);
		}
		s_mp_clamp(r);
	}

  CLEANUP:
	return res;
}

/* Compute the square of polynomial a, reduce modulo p521. Store the
 * result in r.  r could be a.  Uses optimized modular reduction for p521. 
 */
static mp_err
ec_GFp_nistp521_sqr(const mp_int *a, mp_int *r, const GFMethod *meth)
{
	mp_err res = MP_OKAY;

	MP_CHECKOK(mp_sqr(a, r));
	MP_CHECKOK(ec_GFp_nistp521_mod(r, r, meth));
  CLEANUP:
	return res;
}

/* Compute the product of two polynomials a and b, reduce modulo p521.
 * Store the result in r.  r could be a or b; a could be b.  Uses
 * optimized modular reduction for p521. */
static mp_err
ec_GFp_nistp521_mul(const mp_int *a, const mp_int *b, mp_int *r,
					const GFMethod *meth)
{
	mp_err res = MP_OKAY;

	MP_CHECKOK(mp_mul(a, b, r));
	MP_CHECKOK(ec_GFp_nistp521_mod(r, r, meth));
  CLEANUP:
	return res;
}

/* Divides two field elements. If a is NULL, then returns the inverse of
 * b. */
static mp_err
ec_GFp_nistp521_div(const mp_int *a, const mp_int *b, mp_int *r,
		   const GFMethod *meth)
{
	mp_err res = MP_OKAY;
	mp_int t;

	/* If a is NULL, then return the inverse of b, otherwise return a/b. */
	if (a == NULL) {
		return mp_invmod(b, &meth->irr, r);
	} else {
		/* MPI doesn't support divmod, so we implement it using invmod and 
		 * mulmod. */
		MP_CHECKOK(mp_init(&t));
		MP_CHECKOK(mp_invmod(b, &meth->irr, &t));
		MP_CHECKOK(mp_mul(a, &t, r));
		MP_CHECKOK(ec_GFp_nistp521_mod(r, r, meth));
	  CLEANUP:
		mp_clear(&t);
		return res;
	}
}

/* Wire in fast field arithmetic and precomputation of base point for
 * named curves. */
mp_err
ec_group_set_gfp521(ECGroup *group, ECCurveName name)
{
	if (name == ECCurve_NIST_P521) {
		group->meth->field_mod = &ec_GFp_nistp521_mod;
		group->meth->field_mul = &ec_GFp_nistp521_mul;
		group->meth->field_sqr = &ec_GFp_nistp521_sqr;
		group->meth->field_div = &ec_GFp_nistp521_div;
	}
	return MP_OKAY;
}
