xref: /dragonfly/crypto/libressl/crypto/bn/bn_sqrt.c (revision de0e0e4d)

/* $OpenBSD: bn_sqrt.c,v 1.11 2022/06/20 15:02:21 tb Exp $ */
/* Written by Lenka Fibikova <fibikova@exp-math.uni-essen.de>
 * and Bodo Moeller for the OpenSSL project. */
/* ====================================================================
 * Copyright (c) 1998-2000 The OpenSSL Project.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * 3. All advertising materials mentioning features or use of this
 *    software must display the following acknowledgment:
 *    "This product includes software developed by the OpenSSL Project
 *    for use in the OpenSSL Toolkit. (http://www.openssl.org/)"
 *
 * 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to
 *    endorse or promote products derived from this software without
 *    prior written permission. For written permission, please contact
 *    openssl-core@openssl.org.
 *
 * 5. Products derived from this software may not be called "OpenSSL"
 *    nor may "OpenSSL" appear in their names without prior written
 *    permission of the OpenSSL Project.
 *
 * 6. Redistributions of any form whatsoever must retain the following
 *    acknowledgment:
 *    "This product includes software developed by the OpenSSL Project
 *    for use in the OpenSSL Toolkit (http://www.openssl.org/)"
 *
 * THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY
 * EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE OpenSSL PROJECT OR
 * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
 * OF THE POSSIBILITY OF SUCH DAMAGE.
 * ====================================================================
 *
 * This product includes cryptographic software written by Eric Young
 * (eay@cryptsoft.com).  This product includes software written by Tim
 * Hudson (tjh@cryptsoft.com).
 *
 */

#include <openssl/err.h>

#include "bn_lcl.h"

BIGNUM *
BN_mod_sqrt(BIGNUM *in, const BIGNUM *a, const BIGNUM *p, BN_CTX *ctx)
/* Returns 'ret' such that
 *      ret^2 == a (mod p),
 * using the Tonelli/Shanks algorithm (cf. Henri Cohen, "A Course
 * in Algebraic Computational Number Theory", algorithm 1.5.1).
 * 'p' must be prime!
 */
{
	BIGNUM *ret = in;
	int err = 1;
	int r;
	BIGNUM *A, *b, *q, *t, *x, *y;
	int e, i, j;

	if (!BN_is_odd(p) || BN_abs_is_word(p, 1)) {
		if (BN_abs_is_word(p, 2)) {
			if (ret == NULL)
				ret = BN_new();
			if (ret == NULL)
				goto end;
			if (!BN_set_word(ret, BN_is_bit_set(a, 0))) {
				if (ret != in)
					BN_free(ret);
				return NULL;
			}
			bn_check_top(ret);
			return ret;
		}

		BNerror(BN_R_P_IS_NOT_PRIME);
		return (NULL);
	}

	if (BN_is_zero(a) || BN_is_one(a)) {
		if (ret == NULL)
			ret = BN_new();
		if (ret == NULL)
			goto end;
		if (!BN_set_word(ret, BN_is_one(a))) {
			if (ret != in)
				BN_free(ret);
			return NULL;
		}
		bn_check_top(ret);
		return ret;
	}

	BN_CTX_start(ctx);
	if ((A = BN_CTX_get(ctx)) == NULL)
		goto end;
	if ((b = BN_CTX_get(ctx)) == NULL)
		goto end;
	if ((q = BN_CTX_get(ctx)) == NULL)
		goto end;
	if ((t = BN_CTX_get(ctx)) == NULL)
		goto end;
	if ((x = BN_CTX_get(ctx)) == NULL)
		goto end;
	if ((y = BN_CTX_get(ctx)) == NULL)
		goto end;

	if (ret == NULL)
		ret = BN_new();
	if (ret == NULL)
		goto end;

	/* A = a mod p */
	if (!BN_nnmod(A, a, p, ctx))
		goto end;

	/* now write  |p| - 1  as  2^e*q  where  q  is odd */
	e = 1;
	while (!BN_is_bit_set(p, e))
		e++;
	/* we'll set  q  later (if needed) */

	if (e == 1) {
		/* The easy case:  (|p|-1)/2  is odd, so 2 has an inverse
		 * modulo  (|p|-1)/2,  and square roots can be computed
		 * directly by modular exponentiation.
		 * We have
		 *     2 * (|p|+1)/4 == 1   (mod (|p|-1)/2),
		 * so we can use exponent  (|p|+1)/4,  i.e.  (|p|-3)/4 + 1.
		 */
		if (!BN_rshift(q, p, 2))
			goto end;
		q->neg = 0;
		if (!BN_add_word(q, 1))
			goto end;
		if (!BN_mod_exp_ct(ret, A, q, p, ctx))
			goto end;
		err = 0;
		goto vrfy;
	}

	if (e == 2) {
		/* |p| == 5  (mod 8)
		 *
		 * In this case  2  is always a non-square since
		 * Legendre(2,p) = (-1)^((p^2-1)/8)  for any odd prime.
		 * So if  a  really is a square, then  2*a  is a non-square.
		 * Thus for
		 *      b := (2*a)^((|p|-5)/8),
		 *      i := (2*a)*b^2
		 * we have
		 *     i^2 = (2*a)^((1 + (|p|-5)/4)*2)
		 *         = (2*a)^((p-1)/2)
		 *         = -1;
		 * so if we set
		 *      x := a*b*(i-1),
		 * then
		 *     x^2 = a^2 * b^2 * (i^2 - 2*i + 1)
		 *         = a^2 * b^2 * (-2*i)
		 *         = a*(-i)*(2*a*b^2)
		 *         = a*(-i)*i
		 *         = a.
		 *
		 * (This is due to A.O.L. Atkin,
		 * <URL: http://listserv.nodak.edu/scripts/wa.exe?A2=ind9211&L=nmbrthry&O=T&P=562>,
		 * November 1992.)
		 */

		/* t := 2*a */
		if (!BN_mod_lshift1_quick(t, A, p))
			goto end;

		/* b := (2*a)^((|p|-5)/8) */
		if (!BN_rshift(q, p, 3))
			goto end;
		q->neg = 0;
		if (!BN_mod_exp_ct(b, t, q, p, ctx))
			goto end;

		/* y := b^2 */
		if (!BN_mod_sqr(y, b, p, ctx))
			goto end;

		/* t := (2*a)*b^2 - 1*/
		if (!BN_mod_mul(t, t, y, p, ctx))
			goto end;
		if (!BN_sub_word(t, 1))
			goto end;

		/* x = a*b*t */
		if (!BN_mod_mul(x, A, b, p, ctx))
			goto end;
		if (!BN_mod_mul(x, x, t, p, ctx))
			goto end;

		if (!BN_copy(ret, x))
			goto end;
		err = 0;
		goto vrfy;
	}

	/* e > 2, so we really have to use the Tonelli/Shanks algorithm.
	 * First, find some  y  that is not a square. */
	if (!BN_copy(q, p)) /* use 'q' as temp */
		goto end;
	q->neg = 0;
	i = 2;
	do {
		/* For efficiency, try small numbers first;
		 * if this fails, try random numbers.
		 */
		if (i < 22) {
			if (!BN_set_word(y, i))
				goto end;
		} else {
			if (!BN_pseudo_rand(y, BN_num_bits(p), 0, 0))
				goto end;
			if (BN_ucmp(y, p) >= 0) {
				if (p->neg) {
					if (!BN_add(y, y, p))
						goto end;
				} else {
					if (!BN_sub(y, y, p))
						goto end;
				}
			}
			/* now 0 <= y < |p| */
			if (BN_is_zero(y))
				if (!BN_set_word(y, i))
					goto end;
		}

		r = BN_kronecker(y, q, ctx); /* here 'q' is |p| */
		if (r < -1)
			goto end;
		if (r == 0) {
			/* m divides p */
			BNerror(BN_R_P_IS_NOT_PRIME);
			goto end;
		}
	} while (r == 1 && ++i < 82);

	if (r != -1) {
		/* Many rounds and still no non-square -- this is more likely
		 * a bug than just bad luck.
		 * Even if  p  is not prime, we should have found some  y
		 * such that r == -1.
		 */
		BNerror(BN_R_TOO_MANY_ITERATIONS);
		goto end;
	}

	/* Here's our actual 'q': */
	if (!BN_rshift(q, q, e))
		goto end;

	/* Now that we have some non-square, we can find an element
	 * of order  2^e  by computing its q'th power. */
	if (!BN_mod_exp_ct(y, y, q, p, ctx))
		goto end;
	if (BN_is_one(y)) {
		BNerror(BN_R_P_IS_NOT_PRIME);
		goto end;
	}

	/* Now we know that (if  p  is indeed prime) there is an integer
	 * k,  0 <= k < 2^e,  such that
	 *
	 *      a^q * y^k == 1   (mod p).
	 *
	 * As  a^q  is a square and  y  is not,  k  must be even.
	 * q+1  is even, too, so there is an element
	 *
	 *     X := a^((q+1)/2) * y^(k/2),
	 *
	 * and it satisfies
	 *
	 *     X^2 = a^q * a     * y^k
	 *         = a,
	 *
	 * so it is the square root that we are looking for.
	 */

	/* t := (q-1)/2  (note that  q  is odd) */
	if (!BN_rshift1(t, q))
		goto end;

	/* x := a^((q-1)/2) */
	if (BN_is_zero(t)) { /* special case: p = 2^e + 1 */
		if (!BN_nnmod(t, A, p, ctx))
			goto end;
		if (BN_is_zero(t)) {
			/* special case: a == 0  (mod p) */
			BN_zero(ret);
			err = 0;
			goto end;
		} else if (!BN_one(x))
			goto end;
	} else {
		if (!BN_mod_exp_ct(x, A, t, p, ctx))
			goto end;
		if (BN_is_zero(x)) {
			/* special case: a == 0  (mod p) */
			BN_zero(ret);
			err = 0;
			goto end;
		}
	}

	/* b := a*x^2  (= a^q) */
	if (!BN_mod_sqr(b, x, p, ctx))
		goto end;
	if (!BN_mod_mul(b, b, A, p, ctx))
		goto end;

	/* x := a*x    (= a^((q+1)/2)) */
	if (!BN_mod_mul(x, x, A, p, ctx))
		goto end;

	while (1) {
		/* Now  b  is  a^q * y^k  for some even  k  (0 <= k < 2^E
		 * where  E  refers to the original value of  e,  which we
		 * don't keep in a variable),  and  x  is  a^((q+1)/2) * y^(k/2).
		 *
		 * We have  a*b = x^2,
		 *    y^2^(e-1) = -1,
		 *    b^2^(e-1) = 1.
		 */

		if (BN_is_one(b)) {
			if (!BN_copy(ret, x))
				goto end;
			err = 0;
			goto vrfy;
		}

		/* Find the smallest i with 0 < i < e such that b^(2^i) = 1. */
		for (i = 1; i < e; i++) {
			if (i == 1) {
				if (!BN_mod_sqr(t, b, p, ctx))
					goto end;
			} else {
				if (!BN_mod_sqr(t, t, p, ctx))
					goto end;
			}
			if (BN_is_one(t))
				break;
		}
		if (i >= e) {
			BNerror(BN_R_NOT_A_SQUARE);
			goto end;
		}

		/* t := y^2^(e - i - 1) */
		if (!BN_copy(t, y))
			goto end;
		for (j = e - i - 1; j > 0; j--) {
			if (!BN_mod_sqr(t, t, p, ctx))
				goto end;
		}
		if (!BN_mod_mul(y, t, t, p, ctx))
			goto end;
		if (!BN_mod_mul(x, x, t, p, ctx))
			goto end;
		if (!BN_mod_mul(b, b, y, p, ctx))
			goto end;
		e = i;
	}

vrfy:
	if (!err) {
		/* verify the result -- the input might have been not a square
		 * (test added in 0.9.8) */

		if (!BN_mod_sqr(x, ret, p, ctx))
			err = 1;

		if (!err && 0 != BN_cmp(x, A)) {
			BNerror(BN_R_NOT_A_SQUARE);
			err = 1;
		}
	}

end:
	if (err) {
		if (ret != NULL && ret != in) {
			BN_clear_free(ret);
		}
		ret = NULL;
	}
	BN_CTX_end(ctx);
	bn_check_top(ret);
	return ret;
}