ec-field-test.c: Make the field-element type use internal format.

[secnet] / x25519.c

/* -*-c-*-
 *
 * The X25519 key-agreement algorithm
 *
 * (c) 2017 Straylight/Edgeware
 */

/*----- Licensing notice --------------------------------------------------*
 *
 * This file is part of secnet.
 * See README for full list of copyright holders.
 *
 * secnet is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version d of the License, or
 * (at your option) any later version.
 *
 * secnet 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 a copy of the GNU General Public License
 * version 3 along with secnet; if not, see
 * https://www.gnu.org/licenses/gpl.html.
 *
 * This file was originally part of Catacomb, but has been automatically
 * modified for incorporation into secnet: see `import-catacomb-crypto'
 * for details.
 *
 * Catacomb is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Library General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or (at your option) any later version.
 *
 * Catacomb 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 Library General Public License for more details.
 *
 * You should have received a copy of the GNU Library General Public
 * License along with Catacomb; if not, write to the Free
 * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
 * MA 02111-1307, USA.
 */

/*----- Header files ------------------------------------------------------*/

#include "fake-mLib-bits.h"

#include "montladder.h"
#include "f25519.h"
#include "x25519.h"

/*----- Important constants -----------------------------------------------*/

const octet x25519_base[32] = { 9, 0, /* ... */ };

#define A0 121665

/*----- Main code ---------------------------------------------------------*/

/* --- @x25519@ --- *
 *
 * Arguments:	@octet zz[X25519_OUTSZ]@ = where to put the result
 *		@const octet k[X25519_KEYSZ]@ = pointer to private key
 *		@const octet qx[X25519_PUBSZ]@ = pointer to public value
 *
 * Returns:	---
 *
 * Use:		Calculates X25519 of @k@ and @qx@.
 *
 *		Note that there is disagreement over whether the most
 *		significant bit of @qx@ (i.e., the value @qx[31]&0x80@)
 *		should be ignored or counted towards the represented value.
 *		Historically implementations respected the bit; later
 *		convention seems to be to ignore it.  This implementation
 *		honours the bit: a caller who wants to ignore the bit can
 *		easily clear it, while caller who wants to respect it has a
 *		difficult job if this function ignores it.
 */

void x25519(octet zz[X25519_OUTSZ],
	    const octet k[X25519_KEYSZ],
	    const octet qx[X25519_PUBSZ])
{
  uint32 kw[8];
  f25519 x1;

  /* Load and clamp the key.  The low bits are cleared to kill the small
   * subgroups on the curve and its twist, and a high bit is set to guard
   * against careless implementations, though this isn't one of those.
   */
  kw[0] = LOAD32_L(k +  0); kw[1] = LOAD32_L(k +  4);
  kw[2] = LOAD32_L(k +  8); kw[3] = LOAD32_L(k + 12);
  kw[4] = LOAD32_L(k + 16); kw[5] = LOAD32_L(k + 20);
  kw[6] = LOAD32_L(k + 24); kw[7] = LOAD32_L(k + 28);
  kw[0] &= 0xfffffff8; kw[7] = (kw[7]&0x3fffffff) | 0x40000000;

  /* And run the ladder. */
  f25519_load(&x1, qx);
#define MULA0(z, x) do { f25519_mulconst((z), (x), A0); } while (0)
  MONT_LADDER(f25519, MULA0, kw, 8, 32, &x1, &x1);
#undef MULA0
  f25519_store(zz, &x1);
}

/*----- That's all, folks -------------------------------------------------*/