_ Git - BearSSL/blob - src/rsa/rsa_pss_sig_pad.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_pad(const br_prng_class **rng,
  30         const br_hash_class *hf_data, const br_hash_class *hf_mgf1,
  31         const unsigned char *hash, size_t salt_len,
  32         uint32_t n_bitlen, unsigned char *x)
  33 {
  34         size_t xlen, hash_len;
  35         br_hash_compat_context hc;
  36         unsigned char *salt, *seed;
  37
  38         hash_len = br_digest_size(hf_data);
  39
  40         /*
  41          * The padded string is one bit smaller than the modulus;
  42          * notably, if the modulus length is equal to 1 modulo 8, then
  43          * the padded string will be one _byte_ smaller, and the first
  44          * byte will be set to 0. We apply these transformations here.
  45          */
  46         n_bitlen --;
  47         if ((n_bitlen & 7) == 0) {
  48                 *x ++ = 0;
  49         }
  50         xlen = (n_bitlen + 7) >> 3;
  51
  52         /*
  53          * Check that the modulus is large enough for the hash value
  54          * length combined with the intended salt length.
  55          */
  56         if (hash_len > xlen || salt_len > xlen
  57                 || (hash_len + salt_len + 2) > xlen)
  58         {
  59                 return 0;
  60         }
  61
  62         /*
  63          * Produce a random salt.
  64          */
  65         salt = x + xlen - hash_len - salt_len - 1;
  66         if (salt_len != 0) {
  67                 (*rng)->generate(rng, salt, salt_len);
  68         }
  69
  70         /*
  71          * Compute the seed for MGF1.
  72          */
  73         seed = x + xlen - hash_len - 1;
  74         hf_data->init(&hc.vtable);
  75         memset(seed, 0, 8);
  76         hf_data->update(&hc.vtable, seed, 8);
  77         hf_data->update(&hc.vtable, hash, hash_len);
  78         hf_data->update(&hc.vtable, salt, salt_len);
  79         hf_data->out(&hc.vtable, seed);
  80
  81         /*
  82          * Prepare string PS (padded salt). The salt is already at the
  83          * right place.
  84          */
  85         memset(x, 0, xlen - salt_len - hash_len - 2);
  86         x[xlen - salt_len - hash_len - 2] = 0x01;
  87
  88         /*
  89          * Generate the mask and XOR it into PS.
  90          */
  91         br_mgf1_xor(x, xlen - hash_len - 1, hf_mgf1, seed, hash_len);
  92
  93         /*
  94          * Clear the top bits to ensure the value is lower than the
  95          * modulus.
  96          */
  97         x[0] &= 0xFF >> (((uint32_t)xlen << 3) - n_bitlen);
  98
  99         /*
 100          * The seed (H) is already in the right place. We just set the
 101          * last byte.
 102          */
 103         x[xlen - 1] = 0xBC;
 104
 105         return 1;
 106 }