eddsa-expand.c

   1 /* eddsa-expand.c
   2
   3    Copyright (C) 2014 Niels Möller
   4
   5    This file is part of GNU Nettle.
   6
   7    GNU Nettle is free software: you can redistribute it and/or
   8    modify it under the terms of either:
   9
  10      * the GNU Lesser General Public License as published by the Free
  11        Software Foundation; either version 3 of the License, or (at your
  12        option) any later version.
  13
  14    or
  15
  16      * the GNU General Public License as published by the Free
  17        Software Foundation; either version 2 of the License, or (at your
  18        option) any later version.
  19
  20    or both in parallel, as here.
  21
  22    GNU Nettle is distributed in the hope that it will be useful,
  23    but WITHOUT ANY WARRANTY; without even the implied warranty of
  24    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  25    General Public License for more details.
  26
  27    You should have received copies of the GNU General Public License and
  28    the GNU Lesser General Public License along with this program.  If
  29    not, see http://www.gnu.org/licenses/.
  30 */
  31
  32 #if HAVE_CONFIG_H
  33 # include "config.h"
  34 #endif
  35
  36 #include <assert.h>
  37 #include <string.h>
  38
  39 #include "eddsa.h"
  40 #include "eddsa-internal.h"
  41
  42 #include "ecc.h"
  43 #include "ecc-internal.h"
  44
  45 /* Expands a private key, generating the secret scalar K2 and leaving
  46    the key K1 for nonce generation, at the end of the digest. */
  47 void
  48 _eddsa_expand_key (const struct ecc_curve *ecc,
  49                    const struct ecc_eddsa *eddsa,
  50                    void *ctx,
  51                    const uint8_t *key,
  52                    uint8_t *digest,
  53                    mp_limb_t *k2)
  54 {
  55   size_t nbytes = 1 + ecc->p.bit_size / 8;
  56
  57   eddsa->update (ctx, nbytes, key);
  58   eddsa->digest (ctx, 2*nbytes, digest);
  59
  60   /* For ed448, ignores the most significant byte. */
  61   mpn_set_base256_le (k2, ecc->p.size, digest, (ecc->p.bit_size + 7) / 8);
  62
  63   /* Clear low c bits */
  64   k2[0] &= eddsa->low_mask;
  65
  66   /* Clear higher bits. */
  67   k2[ecc->p.size - 1] &= eddsa->high_bit - 1;
  68
  69   /* Set bit number bit_size - 1 (bit 254 for curve25519, bit 447 for
  70      curve448) */
  71   k2[ecc->p.size - 1] |= eddsa->high_bit;
  72 }