text: /* 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
  }
  