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/*
* Copyright (c) 2018 Thomas Pornin <pornin@bolet.org>
*
* 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"
/*
* Hash some data and XOR the result into the provided buffer. This is put
* as a separate function so that stack allocation of the hash function
* context is done only for the duration of the hash.
*/
static void
xor_hash_data(const br_hash_class *dig, void *dst, const void *src, size_t len)
{
br_hash_compat_context hc;
unsigned char tmp[64];
unsigned char *buf;
size_t u, hlen;
hc.vtable = dig;
dig->init(&hc.vtable);
dig->update(&hc.vtable, src, len);
dig->out(&hc.vtable, tmp);
buf = dst;
hlen = br_digest_size(dig);
for (u = 0; u < hlen; u ++) {
buf[u] ^= tmp[u];
}
}
/* see inner.h */
uint32_t
br_rsa_oaep_unpad(const br_hash_class *dig,
const void *label, size_t label_len,
void *data, size_t *len)
{
size_t u, k, hlen;
unsigned char *buf;
uint32_t r, s, zlen;
hlen = br_digest_size(dig);
k = *len;
buf = data;
/*
* There must be room for the padding.
*/
if (k < ((hlen << 1) + 2)) {
return 0;
}
/*
* Unmask the seed, then the DB value.
*/
br_mgf1_xor(buf + 1, hlen, dig, buf + 1 + hlen, k - hlen - 1);
br_mgf1_xor(buf + 1 + hlen, k - hlen - 1, dig, buf + 1, hlen);
/*
* Hash the label and XOR it with the value in the array; if
* they are equal then these should yield only zeros.
*/
xor_hash_data(dig, buf + 1 + hlen, label, label_len);
/*
* At that point, if the padding was correct, when we should
* have: 0x00 || seed || 0x00 ... 0x00 0x01 || M
* Padding is valid as long as:
* - There is at least hlen+1 leading bytes of value 0x00.
* - There is at least one non-zero byte.
* - The first (leftmost) non-zero byte has value 0x01.
*
* Ultimately, we may leak the resulting message length, i.e.
* the position of the byte of value 0x01, but we must take care
* to do so only if the number of zero bytes has been verified
* to be at least hlen+1.
*
* The loop below counts the number of bytes of value 0x00, and
* checks that the next byte has value 0x01, in constant-time.
*
* - If the initial byte (before the seed) is not 0x00, then
* r and s are set to 0, and stay there.
* - Value r is 1 until the first non-zero byte is reached
* (after the seed); it switches to 0 at that point.
* - Value s is set to 1 if and only if the data encountered
* at the time of the transition of r from 1 to 0 has value
* exactly 0x01.
* - Value zlen counts the number of leading bytes of value zero
* (after the seed).
*/
r = 1 - ((buf[0] + 0xFF) >> 8);
s = 0;
zlen = 0;
for (u = hlen + 1; u < k; u ++) {
uint32_t w, nz;
w = buf[u];
/*
* nz == 1 only for the first non-zero byte.
*/
nz = r & ((w + 0xFF) >> 8);
s |= nz & EQ(w, 0x01);
r &= NOT(nz);
zlen += r;
}
/*
* Padding is correct only if s == 1, _and_ zlen >= hlen.
*/
s &= GE(zlen, (uint32_t)hlen);
/*
* At that point, padding was verified, and we are now allowed
* to make conditional jumps.
*/
if (s) {
size_t plen;
plen = 2 + hlen + zlen;
k -= plen;
memmove(buf, buf + plen, k);
*len = k;
}
return s;
}
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