rsa_i31_pub.c

/*
 * Copyright (c) 2016 Thomas Pornin <[email protected]>
 *
 * 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"

/*
 * As a strict minimum, we need four buffers that can hold a
 * modular integer.
 */
#define TLEN   (4 * (2 + ((BR_MAX_RSA_SIZE + 30) / 31)))

/* see bearssl_rsa.h */
uint32_t
br_rsa_i31_public(unsigned char *x, size_t xlen,
	const br_rsa_public_key *pk)
{
	const unsigned char *n;
	size_t nlen;
	uint32_t tmp[1 + TLEN];
	uint32_t *m, *a, *t;
	size_t fwlen;
	long z;
	uint32_t m0i, r;

	/*
	 * Get the actual length of the modulus, and see if it fits within
	 * our stack buffer. We also check that the length of x[] is valid.
	 */
	n = pk->n;
	nlen = pk->nlen;
	while (nlen > 0 && *n == 0) {
		n ++;
		nlen --;
	}
	if (nlen == 0 || nlen > (BR_MAX_RSA_SIZE >> 3) || xlen != nlen) {
		return 0;
	}
	z = (long)nlen << 3;
	fwlen = 1;
	while (z > 0) {
		z -= 31;
		fwlen ++;
	}
	/*
	 * Round up length to an even number.
	 */
	fwlen += (fwlen & 1);

	/*
	 * The modulus gets decoded into m[].
	 * The value to exponentiate goes into a[].
	 * The temporaries for modular exponentiation are in t[].
	 */
	m = tmp;
	a = m + fwlen;
	t = m + 2 * fwlen;

	/*
	 * Decode the modulus.
	 */
	br_i31_decode(m, n, nlen);
	m0i = br_i31_ninv31(m[1]);

	/*
	 * Note: if m[] is even, then m0i == 0. Otherwise, m0i must be
	 * an odd integer.
	 */
	r = m0i & 1;

	/*
	 * Decode x[] into a[]; we also check that its value is proper.
	 */
	r &= br_i31_decode_mod(a, x, xlen, m);

	/*
	 * Compute the modular exponentiation.
	 */
	br_i31_modpow_opt(a, pk->e, pk->elen, m, m0i, t, TLEN - 2 * fwlen);

	/*
	 * Encode the result.
	 */
	br_i31_encode(x, xlen, a);
	return r;
}