_ Git - BearSSL/blob - rsa_pss_sig_unpad.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 inner.h */
  28 uint32_t
  29 br_rsa_pss_sig_unpad(const br_hash_class *hf_data,
  30         const br_hash_class *hf_mgf1,
  31         const unsigned char *hash, size_t salt_len,
  32         const br_rsa_public_key *pk, unsigned char *x)
  33 {
  34         size_t u, xlen, hash_len;
  35         br_hash_compat_context hc;
  36         unsigned char *seed, *salt;
  37         unsigned char tmp[64];
  38         uint32_t r, n_bitlen;
  39
  40         hash_len = br_digest_size(hf_data);
  41
  42         /*
  43          * Value r will be set to a non-zero value is any test fails.
  44          */
  45         r = 0;
  46
  47         /*
  48          * The value bit length (as an integer) must be strictly less than
  49          * that of the modulus.
  50          */
  51         for (u = 0; u < pk->nlen; u ++) {
  52                 if (pk->n[u] != 0) {
  53                         break;
  54                 }
  55         }
  56         if (u == pk->nlen) {
  57                 return 0;
  58         }
  59         n_bitlen = BIT_LENGTH(pk->n[u]) + ((uint32_t)(pk->nlen - u - 1) << 3);
  60         n_bitlen --;
  61         if ((n_bitlen & 7) == 0) {
  62                 r |= *x ++;
  63         } else {
  64                 r |= x[0] & (0xFF << (n_bitlen & 7));
  65         }
  66         xlen = (n_bitlen + 7) >> 3;
  67
  68         /*
  69          * Check that the modulus is large enough for the hash value
  70          * length combined with the intended salt length.
  71          */
  72         if (hash_len > xlen || salt_len > xlen
  73                 || (hash_len + salt_len + 2) > xlen)
  74         {
  75                 return 0;
  76         }
  77
  78         /*
  79          * Check value of rightmost byte.
  80          */
  81         r |= x[xlen - 1] ^ 0xBC;
  82
  83         /*
  84          * Generate the mask and XOR it into the first bytes to reveal PS;
  85          * we must also mask out the leading bits.
  86          */
  87         seed = x + xlen - hash_len - 1;
  88         br_mgf1_xor(x, xlen - hash_len - 1, hf_mgf1, seed, hash_len);
  89         if ((n_bitlen & 7) != 0) {
  90                 x[0] &= 0xFF >> (8 - (n_bitlen & 7));
  91         }
  92
  93         /*
  94          * Check that all padding bytes have the expected value.
  95          */
  96         for (u = 0; u < (xlen - hash_len - salt_len - 2); u ++) {
  97                 r |= x[u];
  98         }
  99         r |= x[xlen - hash_len - salt_len - 2] ^ 0x01;
 100
 101         /*
 102          * Recompute H.
 103          */
 104         salt = x + xlen - hash_len - salt_len - 1;
 105         hf_data->init(&hc.vtable);
 106         memset(tmp, 0, 8);
 107         hf_data->update(&hc.vtable, tmp, 8);
 108         hf_data->update(&hc.vtable, hash, hash_len);
 109         hf_data->update(&hc.vtable, salt, salt_len);
 110         hf_data->out(&hc.vtable, tmp);
 111
 112         /*
 113          * Check that the recomputed H value matches the one appearing
 114          * in the string.
 115          */
 116         for (u = 0; u < hash_len; u ++) {
 117                 r |= tmp[u] ^ x[(xlen - salt_len - 1) + u];
 118         }
 119
 120         return EQ0(r);
 121 }