_ Git - BearSSL/blob - src/rsa/rsa_i31_pub.c

   1 /*
   2  * Copyright (c) 2016 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 /*
  28  * As a strict minimum, we need four buffers that can hold a
  29  * modular integer.
  30  */
  31 #define TLEN   (4 * (2 + ((BR_MAX_RSA_SIZE + 30) / 31)))
  32
  33 /* see bearssl_rsa.h */
  34 uint32_t
  35 br_rsa_i31_public(unsigned char *x, size_t xlen,
  36         const br_rsa_public_key *pk)
  37 {
  38         const unsigned char *n;
  39         size_t nlen;
  40         uint32_t tmp[1 + TLEN];
  41         uint32_t *m, *a, *t;
  42         size_t fwlen;
  43         long z;
  44         uint32_t m0i, r;
  45
  46         /*
  47          * Get the actual length of the modulus, and see if it fits within
  48          * our stack buffer. We also check that the length of x[] is valid.
  49          */
  50         n = pk->n;
  51         nlen = pk->nlen;
  52         while (nlen > 0 && *n == 0) {
  53                 n ++;
  54                 nlen --;
  55         }
  56         if (nlen == 0 || nlen > (BR_MAX_RSA_SIZE >> 3) || xlen != nlen) {
  57                 return 0;
  58         }
  59         z = (long)nlen << 3;
  60         fwlen = 1;
  61         while (z > 0) {
  62                 z -= 31;
  63                 fwlen ++;
  64         }
  65         /*
  66          * Round up length to an even number.
  67          */
  68         fwlen += (fwlen & 1);
  69
  70         /*
  71          * The modulus gets decoded into m[].
  72          * The value to exponentiate goes into a[].
  73          * The temporaries for modular exponentiation are in t[].
  74          */
  75         m = tmp;
  76         a = m + fwlen;
  77         t = m + 2 * fwlen;
  78
  79         /*
  80          * Decode the modulus.
  81          */
  82         br_i31_decode(m, n, nlen);
  83         m0i = br_i31_ninv31(m[1]);
  84
  85         /*
  86          * Note: if m[] is even, then m0i == 0. Otherwise, m0i must be
  87          * an odd integer.
  88          */
  89         r = m0i & 1;
  90
  91         /*
  92          * Decode x[] into a[]; we also check that its value is proper.
  93          */
  94         r &= br_i31_decode_mod(a, x, xlen, m);
  95
  96         /*
  97          * Compute the modular exponentiation.
  98          */
  99         br_i31_modpow_opt(a, pk->e, pk->elen, m, m0i, t, TLEN - 2 * fwlen);
 100
 101         /*
 102          * Encode the result.
 103          */
 104         br_i31_encode(x, xlen, a);
 105         return r;
 106 }