_ Git - BearSSL/blob - src/rsa/rsa_oaep_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 /*
  28  * Hash some data. This is put as a separate function so that stack
  29  * allocation of the hash function context is done only for the duration
  30  * of the hash.
  31  */
  32 static void
  33 hash_data(const br_hash_class *dig, void *dst, const void *src, size_t len)
  34 {
  35         br_hash_compat_context hc;
  36
  37         hc.vtable = dig;
  38         dig->init(&hc.vtable);
  39         dig->update(&hc.vtable, src, len);
  40         dig->out(&hc.vtable, dst);
  41 }
  42
  43 /* see inner.h */
  44 size_t
  45 br_rsa_oaep_pad(const br_prng_class **rnd, const br_hash_class *dig,
  46         const void *label, size_t label_len,
  47         const br_rsa_public_key *pk,
  48         void *dst, size_t dst_max_len,
  49         const void *src, size_t src_len)
  50 {
  51         size_t k, hlen;
  52         unsigned char *buf;
  53
  54         hlen = br_digest_size(dig);
  55
  56         /*
  57          * Compute actual modulus length (in bytes).
  58          */
  59         k = pk->nlen;
  60         while (k > 0 && pk->n[k - 1] == 0) {
  61                 k --;
  62         }
  63
  64         /*
  65          * An error is reported if:
  66          *  - the modulus is too short;
  67          *  - the source message length is too long;
  68          *  - the destination buffer is too short.
  69          */
  70         if (k < ((hlen << 1) + 2)
  71                 || src_len > (k - (hlen << 1) - 2)
  72                 || dst_max_len < k)
  73         {
  74                 return 0;
  75         }
  76
  77         /*
  78          * Apply padding. At this point, things cannot fail.
  79          */
  80         buf = dst;
  81
  82         /*
  83          * Assemble: DB = lHash || PS || 0x01 || M
  84          * We first place the source message M with memmove(), so that
  85          * overlaps between source and destination buffers are supported.
  86          */
  87         memmove(buf + k - src_len, src, src_len);
  88         hash_data(dig, buf + 1 + hlen, label, label_len);
  89         memset(buf + 1 + (hlen << 1), 0, k - src_len - (hlen << 1) - 2);
  90         buf[k - src_len - 1] = 0x01;
  91
  92         /*
  93          * Make the random seed.
  94          */
  95         (*rnd)->generate(rnd, buf + 1, hlen);
  96
  97         /*
  98          * Mask DB with the mask generated from the seed.
  99          */
 100         br_mgf1_xor(buf + 1 + hlen, k - hlen - 1, dig, buf + 1, hlen);
 101
 102         /*
 103          * Mask the seed with the mask generated from the masked DB.
 104          */
 105         br_mgf1_xor(buf + 1, hlen, dig, buf + 1 + hlen, k - hlen - 1);
 106
 107         /*
 108          * Padding result: EM = 0x00 || maskedSeed || maskedDB.
 109          */
 110         buf[0] = 0x00;
 111         return k;
 112 }