_ Git - BearSSL/blob - src/rsa/rsa_i32_pkcs1_vrfy.c

   1 /*
   2  * Copyright (c) 2016 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_i32_pkcs1_vrfy(const unsigned char *x, size_t xlen,
  30         const unsigned char *hash_oid, size_t hash_len,
  31         const br_rsa_public_key *pk, 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 sig[BR_MAX_RSA_SIZE >> 3];
  38         unsigned char pad2[43];
  39         size_t u, x2, x3, pad_len, zlen;
  40
  41         if (xlen > (sizeof sig) || xlen < 11) {
  42                 return 0;
  43         }
  44         memcpy(sig, x, xlen);
  45         if (!br_rsa_i32_public(sig, xlen, pk)) {
  46                 return 0;
  47         }
  48
  49         /*
  50          * Expected format:
  51          *  00 01 FF ... FF 00 30 x1 30 x2 06 x3 OID [ 05 00 ] 04 x4 HASH
  52          *
  53          * with the following rules:
  54          *
  55          *  -- Total length is that of the modulus and the signature
  56          *     (this was already verified by br_rsa_i32_public()).
  57          *
  58          *  -- There are at least eight bytes of value 0xFF.
  59          *
  60          *  -- x4 is equal to the hash length (hash_len).
  61          *
  62          *  -- x3 is equal to the encoded OID value length (so x3 is the
  63          *     first byte of hash_oid[]).
  64          *
  65          *  -- If the "05 00" is present, then x2 == x3 + 4; otherwise,
  66          *     x2 == x3 + 2.
  67          *
  68          *  -- x1 == x2 + x4 + 4.
  69          *
  70          * So the total length after the last "FF" is either x3 + x4 + 11
  71          * (with the "05 00") or x3 + x4 + 9 (without the "05 00").
  72          */
  73
  74         /*
  75          * Check the "00 01 FF .. FF 00" with at least eight 0xFF bytes.
  76          * The comparaison is valid because we made sure that the signature
  77          * is at least 11 bytes long.
  78          */
  79         if (memcmp(sig, pad1, sizeof pad1) != 0) {
  80                 return 0;
  81         }
  82         for (u = sizeof pad1; u < xlen; u ++) {
  83                 if (sig[u] != 0xFF) {
  84                         break;
  85                 }
  86         }
  87
  88         /*
  89          * Remaining length is xlen - u bytes (including the 00 just
  90          * after the last FF). This must be equal to one of the two
  91          * possible values (depending on whether the "05 00" sequence is
  92          * present or not).
  93          */
  94         if (hash_oid == NULL) {
  95                 if (xlen - u != hash_len + 1 || sig[u] != 0x00) {
  96                         return 0;
  97                 }
  98         } else {
  99                 x3 = hash_oid[0];
 100                 pad_len = x3 + 9;
 101                 memset(pad2, 0, pad_len);
 102                 zlen = xlen - u - hash_len;
 103                 if (zlen == pad_len) {
 104                         x2 = x3 + 2;
 105                 } else if (zlen == pad_len + 2) {
 106                         x2 = x3 + 4;
 107                         pad_len = zlen;
 108                         pad2[pad_len - 4] = 0x05;
 109                 } else {
 110                         return 0;
 111                 }
 112                 pad2[1] = 0x30;
 113                 pad2[2] = x2 + hash_len + 4;
 114                 pad2[3] = 0x30;
 115                 pad2[4] = x2;
 116                 pad2[5] = 0x06;
 117                 memcpy(pad2 + 6, hash_oid, x3 + 1);
 118                 pad2[pad_len - 2] = 0x04;
 119                 pad2[pad_len - 1] = hash_len;
 120                 if (memcmp(pad2, sig + u, pad_len) != 0) {
 121                         return 0;
 122                 }
 123         }
 124         memcpy(hash_out, sig + xlen - hash_len, hash_len);
 125         return 1;
 126 }