Eliminate conditional final subtraction in Montgomery multiplication

[from HEAD].

crypto/bn/bn_mont.c

@@ -175,7 +175,6 @@ 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;
@@ -227,19 +226,70 @@ int BN_from_montgomery(BIGNUM *ret, const BIGNUM *a, BN_MONT_CTX *mont,
 		}
 	bn_fix_top(r);
 	
-	/* mont->ri will be a multiple of the word size */
-#if 0
-	BN_rshift(ret,r,mont->ri);
-#else
-	ret->neg = r->neg;
-	x=ri;
+	/* 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;
+
 	rp=ret->d;
-	ap= &(r->d[x]);
-	if (r->top < x)
-		al=0;
-	else
-		al=r->top-x;
+	ap=&(r->d[ri]);
+
+	{
+	size_t m1,m2;
+
+	v=bn_sub_words(rp,ap,np,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)
 		{
@@ -257,7 +307,7 @@ int BN_from_montgomery(BIGNUM *ret, const BIGNUM *a, BN_MONT_CTX *mont,
 	al+=4;
 	for (; i<al; i++)
 		rp[i]=ap[i];
-#endif
+# endif
 #else /* !MONT_WORD */ 
 	BIGNUM *t1,*t2;
 
@@ -277,11 +327,14 @@ 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:
 	BN_CTX_end(ctx);
 	return(retn);