_ Git - BearSSL/blob - src/rsa/rsa_pkcs1_sig_unpad.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 /* see bearssl_rsa.h */
  28 uint32_t
  29 br_rsa_pkcs1_sig_unpad(const unsigned char *sig, size_t sig_len,
  30         const unsigned char *hash_oid, size_t hash_len,
  31         unsigned char *hash_out)
  32 {
  33         static const unsigned char pad1[] = {
  34                 0x00, 0x01, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF
  35         };
  36
  37         unsigned char pad2[43];
  38         size_t u, x2, x3, pad_len, zlen;
  39
  40         if (sig_len < 11) {
  41                 return 0;
  42         }
  43
  44         /*
  45          * Expected format:
  46          *  00 01 FF ... FF 00 30 x1 30 x2 06 x3 OID [ 05 00 ] 04 x4 HASH
  47          *
  48          * with the following rules:
  49          *
  50          *  -- Total length is that of the modulus and the signature
  51          *     (this was already verified by br_rsa_i31_public()).
  52          *
  53          *  -- There are at least eight bytes of value 0xFF.
  54          *
  55          *  -- x4 is equal to the hash length (hash_len).
  56          *
  57          *  -- x3 is equal to the encoded OID value length (so x3 is the
  58          *     first byte of hash_oid[]).
  59          *
  60          *  -- If the "05 00" is present, then x2 == x3 + 4; otherwise,
  61          *     x2 == x3 + 2.
  62          *
  63          *  -- x1 == x2 + x4 + 4.
  64          *
  65          * So the total length after the last "FF" is either x3 + x4 + 11
  66          * (with the "05 00") or x3 + x4 + 9 (without the "05 00").
  67          */
  68
  69         /*
  70          * Check the "00 01 FF .. FF 00" with at least eight 0xFF bytes.
  71          * The comparison is valid because we made sure that the signature
  72          * is at least 11 bytes long.
  73          */
  74         if (memcmp(sig, pad1, sizeof pad1) != 0) {
  75                 return 0;
  76         }
  77         for (u = sizeof pad1; u < sig_len; u ++) {
  78                 if (sig[u] != 0xFF) {
  79                         break;
  80                 }
  81         }
  82
  83         /*
  84          * Remaining length is sig_len - u bytes (including the 00 just
  85          * after the last FF). This must be equal to one of the two
  86          * possible values (depending on whether the "05 00" sequence is
  87          * present or not).
  88          */
  89         if (hash_oid == NULL) {
  90                 if (sig_len - u != hash_len + 1 || sig[u] != 0x00) {
  91                         return 0;
  92                 }
  93         } else {
  94                 x3 = hash_oid[0];
  95                 pad_len = x3 + 9;
  96                 memset(pad2, 0, pad_len);
  97                 zlen = sig_len - u - hash_len;
  98                 if (zlen == pad_len) {
  99                         x2 = x3 + 2;
 100                 } else if (zlen == pad_len + 2) {
 101                         x2 = x3 + 4;
 102                         pad_len = zlen;
 103                         pad2[pad_len - 4] = 0x05;
 104                 } else {
 105                         return 0;
 106                 }
 107                 pad2[1] = 0x30;
 108                 pad2[2] = x2 + hash_len + 4;
 109                 pad2[3] = 0x30;
 110                 pad2[4] = x2;
 111                 pad2[5] = 0x06;
 112                 memcpy(pad2 + 6, hash_oid, x3 + 1);
 113                 pad2[pad_len - 2] = 0x04;
 114                 pad2[pad_len - 1] = hash_len;
 115                 if (memcmp(pad2, sig + u, pad_len) != 0) {
 116                         return 0;
 117                 }
 118         }
 119         memcpy(hash_out, sig + sig_len - hash_len, hash_len);
 120         return 1;
 121 }