bn_gcd.c

/* crypto/bn/bn_gcd.c */
/* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
 * All rights reserved.
 *
 * This package is an SSL implementation written
 * by Eric Young (eay@cryptsoft.com).
 * The implementation was written so as to conform with Netscapes SSL.
 * 
 * This library is free for commercial and non-commercial use as long as
 * the following conditions are aheared to.  The following conditions
 * apply to all code found in this distribution, be it the RC4, RSA,
 * lhash, DES, etc., code; not just the SSL code.  The SSL documentation
 * included with this distribution is covered by the same copyright terms
 * except that the holder is Tim Hudson (tjh@cryptsoft.com).
 * 
 * Copyright remains Eric Young's, and as such any Copyright notices in
 * the code are not to be removed.
 * If this package is used in a product, Eric Young should be given attribution
 * as the author of the parts of the library used.
 * This can be in the form of a textual message at program startup or
 * in documentation (online or textual) provided with the package.
 * 
 * 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 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 acknowledgement:
 *    "This product includes cryptographic software written by
 *     Eric Young (eay@cryptsoft.com)"
 *    The word 'cryptographic' can be left out if the rouines from the library
 *    being used are not cryptographic related :-).
 * 4. If you include any Windows specific code (or a derivative thereof) from 
 *    the apps directory (application code) you must include an acknowledgement:
 *    "This product includes software written by Tim Hudson (tjh@cryptsoft.com)"
 * 
 * THIS SOFTWARE IS PROVIDED BY ERIC YOUNG ``AS IS'' AND
 * ANY EXPRESS 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 AUTHOR OR 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.
 * 
 * The licence and distribution terms for any publically available version or
 * derivative of this code cannot be changed.  i.e. this code cannot simply be
 * copied and put under another distribution licence
 * [including the GNU Public Licence.]
 */
/* ====================================================================
 * 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 "cryptlib.h"
#include "bn_lcl.h"

static BIGNUM *euclid(BIGNUM *a, BIGNUM *b);

int BN_gcd(BIGNUM *r, const BIGNUM *in_a, const BIGNUM *in_b, BN_CTX *ctx)
	{
	BIGNUM *a,*b,*t;
	int ret=0;

	bn_check_top(in_a);
	bn_check_top(in_b);

	BN_CTX_start(ctx);
	a = BN_CTX_get(ctx);
	b = BN_CTX_get(ctx);
	if (a == NULL || b == NULL) goto err;

	if (BN_copy(a,in_a) == NULL) goto err;
	if (BN_copy(b,in_b) == NULL) goto err;
	a->neg = 0;
	b->neg = 0;

	if (BN_cmp(a,b) < 0) { t=a; a=b; b=t; }
	t=euclid(a,b);
	if (t == NULL) goto err;

	if (BN_copy(r,t) == NULL) goto err;
	ret=1;
err:
	BN_CTX_end(ctx);
	return(ret);
	}

static BIGNUM *euclid(BIGNUM *a, BIGNUM *b)
	{
	BIGNUM *t;
	int shifts=0;

	bn_check_top(a);
	bn_check_top(b);

	/* 0 <= b <= a */
	while (!BN_is_zero(b))
		{
		/* 0 < b <= a */

		if (BN_is_odd(a))
			{
			if (BN_is_odd(b))
				{
				if (!BN_sub(a,a,b)) goto err;
				if (!BN_rshift1(a,a)) goto err;
				if (BN_cmp(a,b) < 0)
					{ t=a; a=b; b=t; }
				}
			else		/* a odd - b even */
				{
				if (!BN_rshift1(b,b)) goto err;
				if (BN_cmp(a,b) < 0)
					{ t=a; a=b; b=t; }
				}
			}
		else			/* a is even */
			{
			if (BN_is_odd(b))
				{
				if (!BN_rshift1(a,a)) goto err;
				if (BN_cmp(a,b) < 0)
					{ t=a; a=b; b=t; }
				}
			else		/* a even - b even */
				{
				if (!BN_rshift1(a,a)) goto err;
				if (!BN_rshift1(b,b)) goto err;
				shifts++;
				}
			}
		/* 0 <= b <= a */
		}

	if (shifts)
		{
		if (!BN_lshift(a,a,shifts)) goto err;
		}
	return(a);
err:
	return(NULL);
	}


/* solves ax == 1 (mod n) */
BIGNUM *BN_mod_inverse(BIGNUM *in,
	const BIGNUM *a, const BIGNUM *n, BN_CTX *ctx)
	{
	BIGNUM *A,*B,*X,*Y,*M,*D,*T,*R=NULL;
	BIGNUM *ret=NULL;
	int sign;

	bn_check_top(a);
	bn_check_top(n);

	BN_CTX_start(ctx);
	A = BN_CTX_get(ctx);
	B = BN_CTX_get(ctx);
	X = BN_CTX_get(ctx);
	D = BN_CTX_get(ctx);
	M = BN_CTX_get(ctx);
	Y = BN_CTX_get(ctx);
	T = BN_CTX_get(ctx);
	if (T == NULL) goto err;

	if (in == NULL)
		R=BN_new();
	else
		R=in;
	if (R == NULL) goto err;

	BN_one(X);
	BN_zero(Y);
	if (BN_copy(B,a) == NULL) goto err;
	if (BN_copy(A,n) == NULL) goto err;
	A->neg = 0;
	if (B->neg || (BN_ucmp(B, A) >= 0))
		{
		if (!BN_nnmod(B, B, A, ctx)) goto err;
		}
	sign = -1;
	/* From  B = a mod |n|,  A = |n|  it follows that
	 *
	 *      0 <= B < A,
	 *      sign*X*a  ==  B   (mod |n|),
	 *     -sign*Y*a  ==  A   (mod |n|).
	 */

	while (!BN_is_zero(B))
		{
		BIGNUM *tmp;

		/*
		 *      0 < B < A,
		 * (*)  sign*X*a  ==  B   (mod |n|),
		 *     -sign*Y*a  ==  A   (mod |n|)
		 */

		/* (D, M) := (A/B, A%B) ... */
		if (BN_num_bits(A) == BN_num_bits(B))
			{
			if (!BN_one(D)) goto err;
			if (!BN_sub(M,A,B)) goto err;
			}
		else if (BN_num_bits(A) == BN_num_bits(B) + 1)
			{
			/* A/B is 1, 2, or 3 */
			if (!BN_lshift1(T,B)) goto err;
			if (BN_ucmp(A,T) < 0)
				{
				/* A < 2*B, so D=1 */
				if (!BN_one(D)) goto err;
				if (!BN_sub(M,A,B)) goto err;
				}
			else
				{
				/* A >= 2*B, so D=2 or D=3 */
				if (!BN_sub(M,A,T)) goto err;
				if (!BN_add(D,T,B)) goto err; /* use D (:= 3*B) as temp */
				if (BN_ucmp(A,D) < 0)
					{
					/* A < 3*B, so D=2 */
					if (!BN_set_word(D,2)) goto err;
					/* M (= A - 2*B) already has the correct value */
					}
				else
					{
					/* only D=3 remains */
					if (!BN_set_word(D,3)) goto err;
					/* currently  M = A - 2*B,  but we need  M = A - 3*B */
					if (!BN_sub(M,M,B)) goto err;
					}
				}
			}
		else
			{
			if (!BN_div(D,M,A,B,ctx)) goto err;
			}
		
		/* Now
		 *      A = D*B + M;
		 * thus we have
		 * (**) -sign*Y*a  ==  D*B + M   (mod |n|).
		 */
		
		tmp=A; /* keep the BIGNUM object, the value does not matter */

		/* (A, B) := (B, A mod B) ... */
		A=B;
		B=M;
		/* ... so we have  0 <= B < A  again */

		/* Since the former  M  is now  B  and the former  B  is now  A,
		 * (**) translates into
		 *      -sign*Y*a  ==  D*A + B    (mod |n|),
		 * i.e.
		 *      -sign*Y*a - D*A  ==  B    (mod |n|).
		 * Similarly, (*) translates into
		 *       sign*X*a  ==  A          (mod |n|).
		 *
		 * Thus,
		 *  -sign*Y*a - D*sign*X*a  ==  B  (mod |n|),
		 * i.e.
		 *       -sign*(Y + D*X)*a  ==  B  (mod |n|).
		 *
		 * So if we set  (X, Y, sign) := (Y + D*X, X, -sign),  we arrive back at
		 *       sign*X*a  ==  B   (mod |n|),
		 *      -sign*Y*a  ==  A   (mod |n|).
		 * Note that  X  and  Y  stay non-negative all the time.
		 */

		/* most of the time D is very small, so we can optimize tmp := D*X+Y */
		if (BN_is_one(D))
			{
			if (!BN_add(tmp,X,Y)) goto err;
			}
		else
			{
			if (BN_is_word(D,2))
				{
				if (!BN_lshift1(tmp,X)) goto err;
				}
			else if (BN_is_word(D,4))
				{
				if (!BN_lshift(tmp,X,2)) goto err;
				}
			else if (D->top == 1)
				{
				if (!BN_copy(tmp,X)) goto err;
				if (!BN_mul_word(tmp,D->d[0])) goto err;
				}
			else
				{
				if (!BN_mul(tmp,D,X,ctx)) goto err;
				}
			if (!BN_add(tmp,tmp,Y)) goto err;
			}
		
		M=Y; /* keep the BIGNUM object, the value does not matter */
		Y=X;
		X=tmp;
		sign = -sign;
		}

	/*
	 * The while loop (Euclid's algorithm) ends when
	 *      A == gcd(a,n);
	 * we have
	 *      -sign*Y*a  ==  A  (mod |n|),
	 * where  Y  is non-negative.
	 */

	if (sign < 0)
		{
		if (!BN_sub(Y,n,Y)) goto err;
		}
	/* Now  Y*a  ==  A  (mod |n|).  */
	

	if (BN_is_one(A))
		{
		/* Y*a == 1  (mod |n|) */
		if (BN_ucmp(Y,n) < 0)
			{
			if (!BN_copy(R,Y)) goto err;
			}
		else
			{
			if (!BN_nnmod(R,Y,n,ctx)) goto err;
			}
		}
	else
		{
		BNerr(BN_F_BN_MOD_INVERSE,BN_R_NO_INVERSE);
		goto err;
		}
	ret=R;
err:
	if ((ret == NULL) && (in == NULL)) BN_free(R);
	BN_CTX_end(ctx);
	return(ret);
	}