_ Git - BearSSL/blob - src/ec/ec_keygen.c

   1 /*
   2  * Copyright (c) 2018 Thomas Pornin <pornin@bolet.org>
   3  *
   4  * Permission is hereby granted, free of charge, to any person obtaining
   5  * a copy of this software and associated documentation files (the
   6  * "Software"), to deal in the Software without restriction, including
   7  * without limitation the rights to use, copy, modify, merge, publish,
   8  * distribute, sublicense, and/or sell copies of the Software, and to
   9  * permit persons to whom the Software is furnished to do so, subject to
  10  * the following conditions:
  11  *
  12  * The above copyright notice and this permission notice shall be
  13  * included in all copies or substantial portions of the Software.
  14  *
  15  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
  16  * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
  17  * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
  18  * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
  19  * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
  20  * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
  21  * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  22  * SOFTWARE.
  23  */
  24
  25 #include "inner.h"
  26
  27 /* see bearssl_ec.h */
  28 size_t
  29 br_ec_keygen(const br_prng_class **rng_ctx,
  30         const br_ec_impl *impl, br_ec_private_key *sk,
  31         void *kbuf, int curve)
  32 {
  33         const unsigned char *order;
  34         unsigned char *buf;
  35         size_t len;
  36         unsigned mask;
  37
  38         if (curve < 0 || curve >= 32
  39                 || ((impl->supported_curves >> curve) & 1) == 0)
  40         {
  41                 return 0;
  42         }
  43         order = impl->order(curve, &len);
  44         while (len > 0 && *order == 0) {
  45                 order ++;
  46                 len --;
  47         }
  48         if (kbuf == NULL || len == 0) {
  49                 return len;
  50         }
  51         mask = order[0];
  52         mask |= (mask >> 1);
  53         mask |= (mask >> 2);
  54         mask |= (mask >> 4);
  55
  56         /*
  57          * We generate sequences of random bits of the right size, until
  58          * the value is strictly lower than the curve order (we also
  59          * check for all-zero values, which are invalid).
  60          */
  61         buf = kbuf;
  62         for (;;) {
  63                 size_t u;
  64                 unsigned cc, zz;
  65
  66                 (*rng_ctx)->generate(rng_ctx, buf, len);
  67                 buf[0] &= mask;
  68                 cc = 0;
  69                 u = len;
  70                 zz = 0;
  71                 while (u -- > 0) {
  72                         cc = ((unsigned)(buf[u] - order[u] - cc) >> 8) & 1;
  73                         zz |= buf[u];
  74                 }
  75                 if (cc != 0 && zz != 0) {
  76                         break;
  77                 }
  78         }
  79
  80         if (sk != NULL) {
  81                 sk->curve = curve;
  82                 sk->x = buf;
  83                 sk->xlen = len;
  84         }
  85         return len;
  86 }