Commit 1a56614a authored by Andy Polyakov's avatar Andy Polyakov
Browse files

Eliminate conditional final subtraction in Montgomery multiplication 

[from HEAD].
parent 693c33e4

crypto/bn/bn_mont.c

+71 −13
Original line number Diff line number Diff line
@@ -176,7 +176,6 @@ int BN_from_montgomery(BIGNUM *ret, const BIGNUM *a, BN_MONT_CTX *mont, 


	max=(nl+al+1); /* allow for overflow (no?) XXX */

	if (bn_wexpand(r,max) == NULL) goto err;

	if (bn_wexpand(ret,max) == NULL) goto err;



	r->neg=a->neg^n->neg;

	np=n->d;
@@ -228,19 +227,76 @@ int BN_from_montgomery(BIGNUM *ret, const BIGNUM *a, BN_MONT_CTX *mont, 
		}

	bn_correct_top(r);

	

	/* mont->ri will be a multiple of the word size */

#if 0

	BN_rshift(ret,r,mont->ri);

#else

	/* mont->ri will be a multiple of the word size and below code

	 * is kind of BN_rshift(ret,r,mont->ri) equivalent */

	if (r->top <= ri)

		{

		ret->top=0;

		retn=1;

		goto err;

		}

	al=r->top-ri;



# define BRANCH_FREE 1

# if BRANCH_FREE

	if (bn_wexpand(ret,ri) == NULL) goto err;

	x=0-(((al-ri)>>(sizeof(al)*8-1))&1);

	ret->top=x=(ri&~x)|(al&x);	/* min(ri,al) */

	ret->neg=r->neg;

	x=ri;



	rp=ret->d;

	ap= &(r->d[x]);

	if (r->top < x)

		al=0;

	else

		al=r->top-x;

	ap=&(r->d[ri]);

	nrp=ap;



	/* This 'if' denotes violation of 2*M<r^(n-1) boundary condition

	 * formulated by C.D.Walter in "Montgomery exponentiation needs

	 * no final subtractions." Incurred branch can disclose only

	 * information about modulus length, which is not really secret. */

	if ((mont->N.d[ri-1]>>(BN_BITS2-2))!=0)

		{

		size_t m1,m2;



		v=bn_sub_words(rp,ap,mont->N.d,ri);

		/* this -----------------------^^ works even in al<ri case

		 * thanks to zealous zeroing of top of the vector in the

		 * beginning. */



		/* if (al==ri && !v) || al>ri) nrp=rp; else nrp=ap; */

		/* in other words if subtraction result is real, then

		 * trick unconditional memcpy below to perform in-place

		 * "refresh" instead of actual copy. */

		m1=0-(size_t)(((al-ri)>>(sizeof(al)*8-1))&1);	/* al<ri */

		m2=0-(size_t)(((ri-al)>>(sizeof(al)*8-1))&1);	/* al>ri */

		m1|=m2;			/* (al!=ri) */

		m1|=(0-(size_t)v);	/* (al!=ri || v) */

		m1&=~m2;		/* (al!=ri || v) && !al>ri */

		nrp=(BN_ULONG *)(((size_t)rp&~m1)|((size_t)ap&m1));

		}



	/* 'i<ri' is chosen to eliminate dependency on input data, even

	 * though it results in redundant copy in al<ri case. */

	for (i=0,ri-=4; i<ri; i+=4)

		{

		BN_ULONG t1,t2,t3,t4;

		

		t1=nrp[i+0];

		t2=nrp[i+1];

		t3=nrp[i+2];	ap[i+0]=0;

		t4=nrp[i+3];	ap[i+1]=0;

		rp[i+0]=t1;	ap[i+2]=0;

		rp[i+1]=t2;	ap[i+3]=0;

		rp[i+2]=t3;

		rp[i+3]=t4;

		}

	for (ri+=4; i<ri; i++)

		rp[i]=nrp[i], ap[i]=0;

# else

	if (bn_wexpand(ret,al) == NULL) goto err;

	ret->top=al;

	ret->neg=r->neg;



	rp=ret->d;

	ap=&(r->d[ri]);

	al-=4;

	for (i=0; i<al; i+=4)

		{
@@ -278,10 +334,12 @@ int BN_from_montgomery(BIGNUM *ret, const BIGNUM *a, BN_MONT_CTX *mont, 
	if (!BN_rshift(ret,t2,mont->ri)) goto err;

#endif /* MONT_WORD */



#if !defined(BRANCH_FREE) || BRANCH_FREE==0

	if (BN_ucmp(ret, &(mont->N)) >= 0)

		{

		if (!BN_usub(ret,ret,&(mont->N))) goto err;

		}

#endif

	retn=1;

	bn_check_top(ret);

 err: