File Coverage

src/rsa/rsa_oaep_unpad.c

Criterion	Covered	Total	%
statement	34	35	97.1
branch	6	8	75.0
condition			n/a
subroutine			n/a
pod			n/a
total	40	43	93.0

/*
 * Copyright (c) 2018 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"

/*
 * 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;
}