_ 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 /*
  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 + 14) / 15)))
  32
  33 /* see bearssl_rsa.h */
  34 uint32_t
  35 br_rsa_i15_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         uint16_t tmp[1 + TLEN];
  41         uint16_t *m, *a, *t;
  42         size_t fwlen;
  43         long z;
  44         uint16_t m0i;
  45         uint32_t r;
  46
  47         /*
  48          * Get the actual length of the modulus, and see if it fits within
  49          * our stack buffer. We also check that the length of x[] is valid.
  50          */
  51         n = pk->n;
  52         nlen = pk->nlen;
  53         while (nlen > 0 && *n == 0) {
  54                 n ++;
  55                 nlen --;
  56         }
  57         if (nlen == 0 || nlen > (BR_MAX_RSA_SIZE >> 3) || xlen != nlen) {
  58                 return 0;
  59         }
  60         z = (long)nlen << 3;
  61         fwlen = 1;
  62         while (z > 0) {
  63                 z -= 15;
  64                 fwlen ++;
  65         }
  66         /*
  67          * Round up length to an even number.
  68          */
  69         fwlen += (fwlen & 1);
  70
  71         /*
  72          * The modulus gets decoded into m[].
  73          * The value to exponentiate goes into a[].
  74          * The temporaries for modular exponentiations are in t[].
  75          *
  76          * We want the first value word of each integer to be aligned
  77          * on a 32-bit boundary.
  78          */
  79         m = tmp;
  80         if (((uintptr_t)m & 2) == 0) {
  81                 m ++;
  82         }
  83         a = m + fwlen;
  84         t = m + 2 * fwlen;
  85
  86         /*
  87          * Decode the modulus.
  88          */
  89         br_i15_decode(m, n, nlen);
  90         m0i = br_i15_ninv15(m[1]);
  91
  92         /*
  93          * Note: if m[] is even, then m0i == 0. Otherwise, m0i must be
  94          * an odd integer.
  95          */
  96         r = m0i & 1;
  97
  98         /*
  99          * Decode x[] into a[]; we also check that its value is proper.
 100          */
 101         r &= br_i15_decode_mod(a, x, xlen, m);
 102
 103         /*
 104          * Compute the modular exponentiation.
 105          */
 106         br_i15_modpow_opt(a, pk->e, pk->elen, m, m0i, t, TLEN - 2 * fwlen);
 107
 108         /*
 109          * Encode the result.
 110          */
 111         br_i15_encode(x, xlen, a);
 112         return r;
 113 }