_ Git - BearSSL/blob - src/rsa/rsa_i15_pub.c

   1 /*
   2  * Copyright (c) 2017 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_rsa.h */
  28 uint32_t
  29 br_rsa_i15_public(unsigned char *x, size_t xlen,
  30         const br_rsa_public_key *pk)
  31 {
  32         const unsigned char *n;
  33         size_t nlen;
  34         uint16_t m[1 + ((BR_MAX_RSA_SIZE + 14) / 15)];
  35         uint16_t a[1 + ((BR_MAX_RSA_SIZE + 14) / 15)];
  36         uint16_t t1[1 + ((BR_MAX_RSA_SIZE + 14) / 15)];
  37         uint16_t t2[1 + ((BR_MAX_RSA_SIZE + 14) / 15)];
  38         uint16_t m0i;
  39         uint32_t r;
  40
  41         /*
  42          * Get the actual length of the modulus, and see if it fits within
  43          * our stack buffer. We also check that the length of x[] is valid.
  44          */
  45         n = pk->n;
  46         nlen = pk->nlen;
  47         while (nlen > 0 && *n == 0) {
  48                 n ++;
  49                 nlen --;
  50         }
  51         if (nlen == 0 || nlen > (BR_MAX_RSA_SIZE >> 3) || xlen != nlen) {
  52                 return 0;
  53         }
  54         br_i15_decode(m, n, nlen);
  55         m0i = br_i15_ninv15(m[1]);
  56
  57         /*
  58          * Note: if m[] is even, then m0i == 0. Otherwise, m0i must be
  59          * an odd integer.
  60          */
  61         r = m0i & 1;
  62
  63         /*
  64          * Decode x[] into a[]; we also check that its value is proper.
  65          */
  66         r &= br_i15_decode_mod(a, x, xlen, m);
  67
  68         /*
  69          * Compute the modular exponentiation.
  70          */
  71         br_i15_modpow(a, pk->e, pk->elen, m, m0i, t1, t2);
  72
  73         /*
  74          * Encode the result.
  75          */
  76         br_i15_encode(x, xlen, a);
  77         return r;
  78 }