lilcrypto/ct.c

/*
 * Copyright (c) 2024 Lucas Gabriel Vuotto <lucas@lgv5.net>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

#include "lilcrypto.h"


uint32_t
lc_ct_cmp(const uint8_t *x, const uint8_t *y, size_t l)
{
	uint32_t	r = 0;

	for (; l > 0; l--)
		r |= *x++ ^ *y++;

	return lc_ct_mask32(r);
}

uint32_t
lc_ct_mask32(uint32_t x)
{
	/* Ensures that if any bit is set, then LSB is set. */
	x |= x >> 16;
	x |= x >> 8;
	x |= x >> 4;
	x |= x >> 2;
	x |= x >> 1;

	return UINT32_MAX + (x & 1);
}