_ Git - BearSSL/blob - src/rsa/rsa_oaep_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 /*
  28  * Hash some data and XOR the result into the provided buffer. This is put
  29  * as a separate function so that stack allocation of the hash function
  30  * context is done only for the duration of the hash.
  31  */
  32 static void
  33 xor_hash_data(const br_hash_class *dig, void *dst, const void *src, size_t len)
  34 {
  35         br_hash_compat_context hc;
  36         unsigned char tmp[64];
  37         unsigned char *buf;
  38         size_t u, hlen;
  39
  40         hc.vtable = dig;
  41         dig->init(&hc.vtable);
  42         dig->update(&hc.vtable, src, len);
  43         dig->out(&hc.vtable, tmp);
  44         buf = dst;
  45         hlen = br_digest_size(dig);
  46         for (u = 0; u < hlen; u ++) {
  47                 buf[u] ^= tmp[u];
  48         }
  49 }
  50
  51 /* see inner.h */
  52 uint32_t
  53 br_rsa_oaep_unpad(const br_hash_class *dig,
  54         const void *label, size_t label_len,
  55         void *data, size_t *len)
  56 {
  57         size_t u, k, hlen;
  58         unsigned char *buf;
  59         uint32_t r, s, zlen;
  60
  61         hlen = br_digest_size(dig);
  62         k = *len;
  63         buf = data;
  64
  65         /*
  66          * There must be room for the padding.
  67          */
  68         if (k < ((hlen << 1) + 2)) {
  69                 return 0;
  70         }
  71
  72         /*
  73          * Unmask the seed, then the DB value.
  74          */
  75         br_mgf1_xor(buf + 1, hlen, dig, buf + 1 + hlen, k - hlen - 1);
  76         br_mgf1_xor(buf + 1 + hlen, k - hlen - 1, dig, buf + 1, hlen);
  77
  78         /*
  79          * Hash the label and XOR it with the value in the array; if
  80          * they are equal then these should yield only zeros.
  81          */
  82         xor_hash_data(dig, buf + 1 + hlen, label, label_len);
  83
  84         /*
  85          * At that point, if the padding was correct, when we should
  86          * have: 0x00 || seed || 0x00 ... 0x00 0x01 || M
  87          * Padding is valid as long as:
  88          *  - There is at least hlen+1 leading bytes of value 0x00.
  89          *  - There is at least one non-zero byte.
  90          *  - The first (leftmost) non-zero byte has value 0x01.
  91          *
  92          * Ultimately, we may leak the resulting message length, i.e.
  93          * the position of the byte of value 0x01, but we must take care
  94          * to do so only if the number of zero bytes has been verified
  95          * to be at least hlen+1.
  96          *
  97          * The loop below counts the number of bytes of value 0x00, and
  98          * checks that the next byte has value 0x01, in constant-time.
  99          *
 100          *  - If the initial byte (before the seed) is not 0x00, then
 101          *    r and s are set to 0, and stay there.
 102          *  - Value r is 1 until the first non-zero byte is reached
 103          *    (after the seed); it switches to 0 at that point.
 104          *  - Value s is set to 1 if and only if the data encountered
 105          *    at the time of the transition of r from 1 to 0 has value
 106          *    exactly 0x01.
 107          *  - Value zlen counts the number of leading bytes of value zero
 108          *    (after the seed).
 109          */
 110         r = 1 - ((buf[0] + 0xFF) >> 8);
 111         s = 0;
 112         zlen = 0;
 113         for (u = hlen + 1; u < k; u ++) {
 114                 uint32_t w, nz;
 115
 116                 w = buf[u];
 117
 118                 /*
 119                  * nz == 1 only for the first non-zero byte.
 120                  */
 121                 nz = r & ((w + 0xFF) >> 8);
 122                 s |= nz & EQ(w, 0x01);
 123                 r &= NOT(nz);
 124                 zlen += r;
 125         }
 126
 127         /*
 128          * Padding is correct only if s == 1, _and_ zlen >= hlen.
 129          */
 130         s &= GE(zlen, (uint32_t)hlen);
 131
 132         /*
 133          * At that point, padding was verified, and we are now allowed
 134          * to make conditional jumps.
 135          */
 136         if (s) {
 137                 size_t plen;
 138
 139                 plen = 2 + hlen + zlen;
 140                 k -= plen;
 141                 memmove(data, data + plen, k);
 142                 *len = k;
 143         }
 144         return s;
 145 }