/*
* Copyright (c) 2017 Thomas Pornin
*
* Permission is hereby granted, free of charge, to any person obtaining
* a copy of this software and associated documentation files (the
* "Software"), to deal in the Software without restriction, including
* without limitation the rights to use, copy, modify, merge, publish,
* distribute, sublicense, and/or sell copies of the Software, and to
* permit persons to whom the Software is furnished to do so, subject to
* the following conditions:
*
* The above copyright notice and this permission notice shall be
* included in all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
* NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
* BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
* ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
* CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
* SOFTWARE.
*/
#include "inner.h"
/* see bearssl_rsa.h */
uint32_t
br_rsa_pkcs1_sig_unpad(const unsigned char *sig, size_t sig_len,
const unsigned char *hash_oid, size_t hash_len,
unsigned char *hash_out)
{
static const unsigned char pad1[] = {
0x00, 0x01, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF
};
unsigned char pad2[43];
size_t u, x2, x3, pad_len, zlen;
if (sig_len < 11) {
return 0;
}
/*
* Expected format:
* 00 01 FF ... FF 00 30 x1 30 x2 06 x3 OID [ 05 00 ] 04 x4 HASH
*
* with the following rules:
*
* -- Total length is that of the modulus and the signature
* (this was already verified by br_rsa_i31_public()).
*
* -- There are at least eight bytes of value 0xFF.
*
* -- x4 is equal to the hash length (hash_len).
*
* -- x3 is equal to the encoded OID value length (so x3 is the
* first byte of hash_oid[]).
*
* -- If the "05 00" is present, then x2 == x3 + 4; otherwise,
* x2 == x3 + 2.
*
* -- x1 == x2 + x4 + 4.
*
* So the total length after the last "FF" is either x3 + x4 + 11
* (with the "05 00") or x3 + x4 + 9 (without the "05 00").
*/
/*
* Check the "00 01 FF .. FF 00" with at least eight 0xFF bytes.
* The comparison is valid because we made sure that the signature
* is at least 11 bytes long.
*/
if (memcmp(sig, pad1, sizeof pad1) != 0) {
return 0;
}
for (u = sizeof pad1; u < sig_len; u ++) {
if (sig[u] != 0xFF) {
break;
}
}
/*
* Remaining length is sig_len - u bytes (including the 00 just
* after the last FF). This must be equal to one of the two
* possible values (depending on whether the "05 00" sequence is
* present or not).
*/
if (hash_oid == NULL) {
if (sig_len - u != hash_len + 1 || sig[u] != 0x00) {
return 0;
}
} else {
x3 = hash_oid[0];
pad_len = x3 + 9;
memset(pad2, 0, pad_len);
zlen = sig_len - u - hash_len;
if (zlen == pad_len) {
x2 = x3 + 2;
} else if (zlen == pad_len + 2) {
x2 = x3 + 4;
pad_len = zlen;
pad2[pad_len - 4] = 0x05;
} else {
return 0;
}
pad2[1] = 0x30;
pad2[2] = x2 + hash_len + 4;
pad2[3] = 0x30;
pad2[4] = x2;
pad2[5] = 0x06;
memcpy(pad2 + 6, hash_oid, x3 + 1);
pad2[pad_len - 2] = 0x04;
pad2[pad_len - 1] = hash_len;
if (memcmp(pad2, sig + u, pad_len) != 0) {
return 0;
}
}
memcpy(hash_out, sig + sig_len - hash_len, hash_len);
return 1;
}