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/* ecc-ecdsa-sign.c */
/* nettle, low-level cryptographics library
*
* Copyright (C) 2013 Niels Möller
*
* The nettle library is free software; you can redistribute it and/or modify
* it under the terms of the GNU Lesser General Public License as published by
* the Free Software Foundation; either version 2.1 of the License, or (at your
* option) any later version.
*
* The nettle library is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
* or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
* License for more details.
*
* You should have received a copy of the GNU Lesser General Public License
* along with the nettle library; see the file COPYING.LIB. If not, write to
* the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
* MA 02111-1301, USA.
*/
/* Development of Nettle's ECC support was funded by the .SE Internet Fund. */
#if HAVE_CONFIG_H
# include "config.h"
#endif
#include <assert.h>
#include <stdlib.h>
#include "ecdsa.h"
#include "ecc-internal.h"
/* Low-level ECDSA signing */
mp_size_t
ecc_ecdsa_sign_itch (const struct ecc_curve *ecc)
{
/* Needs 3*ecc->size + scratch for ecc_mul_g. */
return ECC_ECDSA_SIGN_ITCH (ecc->size);
}
/* NOTE: Caller should check if r or s is zero. */
void
ecc_ecdsa_sign (const struct ecc_curve *ecc,
const mp_limb_t *zp,
/* Random nonce, must be invertible mod ecc group
order. */
const mp_limb_t *kp,
unsigned length, const uint8_t *digest,
mp_limb_t *rp, mp_limb_t *sp,
mp_limb_t *scratch)
{
mp_limb_t cy;
#define P scratch
#define kinv scratch /* Needs 5*ecc->size for computation */
#define hp (scratch + ecc->size) /* NOTE: ecc->size + 1 limbs! */
#define tp (scratch + 2*ecc->size)
/* Procedure, according to RFC 6090, "KT-I". q denotes the group
order.
1. k <-- uniformly random, 0 < k < q
2. R <-- (r_x, r_y) = k g
3. s1 <-- r_x mod q
4. s2 <-- (h + z*s1)/k mod q.
*/
ecc_mul_g (ecc, P, kp, P + 3*ecc->size);
/* x coordinate only */
ecc_j_to_a (ecc, 3, rp, P, P + 3*ecc->size);
/* We need to reduce x coordinate mod ecc->q. It should already
be < 2*ecc->q, so one subtraction should suffice. */
cy = mpn_sub_n (scratch, rp, ecc->q, ecc->size);
cnd_copy (cy == 0, rp, scratch, ecc->size);
/* Invert k, uses 5 * ecc->size including scratch */
mpn_copyi (hp, kp, ecc->size);
ecc_modq_inv (ecc, kinv, hp, tp);
/* Process hash digest */
ecc_hash (ecc, hp, length, digest);
ecc_modq_mul (ecc, tp, zp, rp);
ecc_modq_add (ecc, hp, hp, tp);
ecc_modq_mul (ecc, tp, hp, kinv);
mpn_copyi (sp, tp, ecc->size);
#undef P
#undef hp
#undef kinv
#undef tp
}
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