/* eddsa-compress.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.

   or

     * 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

   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

   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 http://www.gnu.org/licenses/.

*/

#if HAVE_CONFIG_H

# include "config.h"

#endif

#include "eddsa.h"

#include "eddsa-internal.h"

#include "ecc-internal.h"

#include "gmp-glue.h"

mp_size_t

_eddsa_compress_itch (const struct ecc_curve *ecc)

{

  return 2*ecc->p.size + ecc->h_to_a_itch;

}

void

_eddsa_compress (const struct ecc_curve *ecc, uint8_t *r, mp_limb_t *p,

     mp_limb_t *scratch)

{

#define xp scratch

#define yp (scratch + ecc->p.size)

#define scratch_out (scratch + 2*ecc->p.size)

  size_t nbytes = 1 + ecc->p.bit_size / 8;

  ecc->h_to_a (ecc, 0, xp, p, scratch_out);

  /* Encoding is the y coordinate and an appended "sign" bit, which is

     the low bit of x. The sign bit is stored as the most significant

     bit of the last byte. */

  mpn_get_base256_le (r, nbytes, yp, ecc->p.size);

  r[nbytes - 1] += (xp[0] & 1) << 7;

}