Commit fd179ede authored by Niels Möller's avatar Niels Möller
Browse files

New function curve25519_eh_to_x.

parent 2e24e947
2014-09-02 Niels Möller <>
* curve25519-eh-to-x.c (curve25519_eh_to_x): New file, new
function. The curve25519 transform currently done by ecc_eh_to_a,
but which should eventually be eliminted from that function.
* (hogweed_SOURCES): Added curve25519-eh-to-x.c.
* ecc-internal.h (curve25519_eh_to_x): Declare it.
* curve25519-mul.c (curve25519_mul): Use it.
* curve25519-mul-g.c (curve25519_mul_g): Likewise.
2014-08-29 Niels Möller <>
* testsuite/testutils.c (test_ecc_mul_j): Renamed, to ...
......@@ -174,7 +174,7 @@ hogweed_SOURCES = sexp.c sexp-format.c \
ecc-point.c ecc-scalar.c ecc-point-mul.c ecc-point-mul-g.c \
ecc-ecdsa-sign.c ecdsa-sign.c \
ecc-ecdsa-verify.c ecdsa-verify.c ecdsa-keygen.c \
curve25519-mul-g.c curve25519-mul.c \
curve25519-mul-g.c curve25519-mul.c curve25519-eh-to-x.c \
HEADERS = aes.h arcfour.h arctwo.h asn1.h blowfish.h \
/* curve25519-x.c
Copyright (C) 2014 Niels Möller
This file is part of GNU Nettle.
GNU Nettle is free software: you can redistribute it and/or
modify it under the terms of either:
* the GNU Lesser General Public License as published by the Free
Software Foundation; either version 3 of the License, or (at your
option) any later version.
* the GNU General Public License as published by the Free
Software Foundation; either version 2 of the License, or (at your
option) any later version.
or both in parallel, as here.
GNU Nettle is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
General Public License for more details.
You should have received copies of the GNU General Public License and
the GNU Lesser General Public License along with this program. If
not, see
# include "config.h"
#include <string.h>
#include "curve25519.h"
#include "ecc.h"
#include "ecc-internal.h"
/* Transform a point on the twisted Edwards curve to the curve25519
Montgomery curve, and return the x coordinate. */
curve25519_eh_to_x (mp_limb_t *xp, const mp_limb_t *p,
mp_limb_t *scratch)
#define vp (p + ecc->size)
#define wp (p + 2*ecc->size)
#define t0 scratch
#define t1 (scratch + ecc->size)
#define t2 (scratch + 2*ecc->size)
const struct ecc_curve *ecc = &nettle_curve25519;
mp_limb_t cy;
/* If u = U/W and v = V/W are the coordiantes of the point on the
Edwards curve we get the curve25519 x coordinate as
x = (1+v) / (1-v) = (W + V) / (W - V)
/* NOTE: For the infinity point, this subtraction gives zero (mod
p), which isn't invertible. For curve25519, the desired output is
x = 0, and we should be fine, since ecc_modp_inv returns 0
in this case. */
ecc_modp_sub (ecc, t0, wp, vp);
/* Needs 3*size scratch, for a total of 5*size */
ecc_modp_inv (ecc, t1, t0, t2);
ecc_modp_add (ecc, t0, wp, vp);
ecc_modp_mul (ecc, t2, t0, t1);
cy = mpn_sub_n (xp, t2, ecc->p, ecc->size);
cnd_copy (cy, xp, t2, ecc->size);
#undef vp
#undef wp
#undef t0
#undef t1
#undef t2
......@@ -64,7 +64,7 @@ curve25519_mul_g (uint8_t *r, const uint8_t *n)
mpn_set_base256_le (x, ecc_size, t, CURVE25519_SIZE);
ecc_mul_g_eh (&nettle_curve25519, p, x, scratch_out);
ecc_eh_to_a (&nettle_curve25519, 1, x, p, scratch_out);
curve25519_eh_to_x (x, p, scratch_out);
mpn_get_base256_le (r, CURVE25519_SIZE, x, ecc_size);
gmp_free_limbs (scratch, itch);
......@@ -82,7 +82,7 @@ curve25519_mul (uint8_t *q, const uint8_t *n, const uint8_t *p)
mpn_set_base256_le (s, ecc->size, t, CURVE25519_SIZE);
ecc_mul_a_eh (ecc, x, s, x, scratch_out);
ecc_eh_to_a (ecc, 1, s, x, scratch_out);
curve25519_eh_to_x (s, x, scratch_out);
mpn_get_base256_le (q, CURVE25519_SIZE, s, ecc->size);
gmp_free_limbs (scratch, itch);
......@@ -64,6 +64,7 @@
#define sec_tabselect _nettle_sec_tabselect
#define sec_modinv _nettle_sec_modinv
#define ecc_25519_sqrt _nettle_ecc_25519_sqrt
#define curve25519_eh_to_x _nettle_curve25519_eh_to_x
#define ECC_MAX_SIZE ((521 + GMP_NUMB_BITS - 1) / GMP_NUMB_BITS)
......@@ -266,6 +267,10 @@ sec_modinv (mp_limb_t *vp, mp_limb_t *ap, mp_size_t n,
ecc_25519_sqrt(mp_limb_t *rp, const mp_limb_t *ap);
curve25519_eh_to_x (mp_limb_t *xp, const mp_limb_t *p,
mp_limb_t *scratch);
/* Current scratch needs: */
#define ECC_MODINV_ITCH(size) (3*(size))
#define ECC_J_TO_A_ITCH(size) (5*(size))
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