From: Andy Polyakov <appro@openssl.org>
Date: Sun, 14 Aug 2011 11:31:35 +0000 (+0000)
Subject: bn_div.c: remove duplicate code by merging BN_div and BN_div_no_branch.
X-Git-Tag: OpenSSL-fips-2_0-rc1~214
X-Git-Url: https://git.openssl.org/gitweb/?p=openssl.git;a=commitdiff_plain;h=bf3dfe7feee5f62ba1e4367720819255db039f29

bn_div.c: remove duplicate code by merging BN_div and BN_div_no_branch.
---

diff --git a/crypto/bn/bn_div.c b/crypto/bn/bn_div.c
index 92ef8cb918..034a80a4a6 100644
--- a/crypto/bn/bn_div.c
+++ b/crypto/bn/bn_div.c
@@ -173,15 +173,13 @@ int BN_div(BIGNUM *dv, BIGNUM *rem, const BIGNUM *m, const BIGNUM *d,
 #endif /* OPENSSL_NO_ASM */
 
 
-/* BN_div[_no_branch] computes  dv := num / divisor,  rounding towards
+/* BN_div computes  dv := num / divisor,  rounding towards
  * zero, and sets up rm  such that  dv*divisor + rm = num  holds.
  * Thus:
  *     dv->neg == num->neg ^ divisor->neg  (unless the result is zero)
  *     rm->neg == num->neg                 (unless the remainder is zero)
  * If 'dv' or 'rm' is NULL, the respective value is not returned.
  */
-static int BN_div_no_branch(BIGNUM *dv, BIGNUM *rm, const BIGNUM *num,
-        const BIGNUM *divisor, BN_CTX *ctx);
 int BN_div(BIGNUM *dv, BIGNUM *rm, const BIGNUM *num, const BIGNUM *divisor,
 	   BN_CTX *ctx)
 	{
@@ -190,6 +188,7 @@ int BN_div(BIGNUM *dv, BIGNUM *rm, const BIGNUM *num, const BIGNUM *divisor,
 	BN_ULONG *resp,*wnump;
 	BN_ULONG d0,d1;
 	int num_n,div_n;
+	int no_branch=0;
 
 	/* Invalid zero-padding would have particularly bad consequences
 	 * in the case of 'num', so don't just rely on bn_check_top() for this one
@@ -204,7 +203,7 @@ int BN_div(BIGNUM *dv, BIGNUM *rm, const BIGNUM *num, const BIGNUM *divisor,
 
 	if ((BN_get_flags(num, BN_FLG_CONSTTIME) != 0) || (BN_get_flags(divisor, BN_FLG_CONSTTIME) != 0))
 		{
-		return BN_div_no_branch(dv, rm, num, divisor, ctx);
+		no_branch=1;
 		}
 
 	bn_check_top(dv);
@@ -218,7 +217,7 @@ int BN_div(BIGNUM *dv, BIGNUM *rm, const BIGNUM *num, const BIGNUM *divisor,
 		return(0);
 		}
 
-	if (BN_ucmp(num,divisor) < 0)
+	if (!no_branch && BN_ucmp(num,divisor) < 0)
 		{
 		if (rm != NULL)
 			{ if (BN_copy(rm,num) == NULL) return(0); }
@@ -243,242 +242,25 @@ int BN_div(BIGNUM *dv, BIGNUM *rm, const BIGNUM *num, const BIGNUM *divisor,
 	norm_shift+=BN_BITS2;
 	if (!(BN_lshift(snum,num,norm_shift))) goto err;
 	snum->neg=0;
-	div_n=sdiv->top;
-	num_n=snum->top;
-	loop=num_n-div_n;
-	/* Lets setup a 'window' into snum
-	 * This is the part that corresponds to the current
-	 * 'area' being divided */
-	wnum.neg   = 0;
-	wnum.d     = &(snum->d[loop]);
-	wnum.top   = div_n;
-	/* only needed when BN_ucmp messes up the values between top and max */
-	wnum.dmax  = snum->dmax - loop; /* so we don't step out of bounds */
-
-	/* Get the top 2 words of sdiv */
-	/* div_n=sdiv->top; */
-	d0=sdiv->d[div_n-1];
-	d1=(div_n == 1)?0:sdiv->d[div_n-2];
-
-	/* pointer to the 'top' of snum */
-	wnump= &(snum->d[num_n-1]);
-
-	/* Setup to 'res' */
-	res->neg= (num->neg^divisor->neg);
-	if (!bn_wexpand(res,(loop+1))) goto err;
-	res->top=loop;
-	resp= &(res->d[loop-1]);
-
-	/* space for temp */
-	if (!bn_wexpand(tmp,(div_n+1))) goto err;
 
-	if (BN_ucmp(&wnum,sdiv) >= 0)
+	if (no_branch)
 		{
-		/* If BN_DEBUG_RAND is defined BN_ucmp changes (via
-		 * bn_pollute) the const bignum arguments =>
-		 * clean the values between top and max again */
-		bn_clear_top2max(&wnum);
-		bn_sub_words(wnum.d, wnum.d, sdiv->d, div_n);
-		*resp=1;
-		}
-	else
-		res->top--;
-	/* if res->top == 0 then clear the neg value otherwise decrease
-	 * the resp pointer */
-	if (res->top == 0)
-		res->neg = 0;
-	else
-		resp--;
-
-	for (i=0; i<loop-1; i++, wnump--, resp--)
-		{
-		BN_ULONG q,l0;
-		/* the first part of the loop uses the top two words of
-		 * snum and sdiv to calculate a BN_ULONG q such that
-		 * | wnum - sdiv * q | < sdiv */
-#if defined(BN_DIV3W) && !defined(OPENSSL_NO_ASM)
-		BN_ULONG bn_div_3_words(BN_ULONG*,BN_ULONG,BN_ULONG);
-		q=bn_div_3_words(wnump,d1,d0);
-#else
-		BN_ULONG n0,n1,rem=0;
-
-		n0=wnump[0];
-		n1=wnump[-1];
-		if (n0 == d0)
-			q=BN_MASK2;
-		else 			/* n0 < d0 */
-			{
-#ifdef BN_LLONG
-			BN_ULLONG t2;
-
-#if defined(BN_LLONG) && defined(BN_DIV2W) && !defined(bn_div_words)
-			q=(BN_ULONG)(((((BN_ULLONG)n0)<<BN_BITS2)|n1)/d0);
-#else
-			q=bn_div_words(n0,n1,d0);
-#ifdef BN_DEBUG_LEVITTE
-			fprintf(stderr,"DEBUG: bn_div_words(0x%08X,0x%08X,0x%08\
-X) -> 0x%08X\n",
-				n0, n1, d0, q);
-#endif
-#endif
-
-#ifndef REMAINDER_IS_ALREADY_CALCULATED
-			/*
-			 * rem doesn't have to be BN_ULLONG. The least we
-			 * know it's less that d0, isn't it?
-			 */
-			rem=(n1-q*d0)&BN_MASK2;
-#endif
-			t2=(BN_ULLONG)d1*q;
-
-			for (;;)
-				{
-				if (t2 <= ((((BN_ULLONG)rem)<<BN_BITS2)|wnump[-2]))
-					break;
-				q--;
-				rem += d0;
-				if (rem < d0) break; /* don't let rem overflow */
-				t2 -= d1;
-				}
-#else /* !BN_LLONG */
-			BN_ULONG t2l,t2h;
-
-			q=bn_div_words(n0,n1,d0);
-#ifdef BN_DEBUG_LEVITTE
-			fprintf(stderr,"DEBUG: bn_div_words(0x%08X,0x%08X,0x%08\
-X) -> 0x%08X\n",
-				n0, n1, d0, q);
-#endif
-#ifndef REMAINDER_IS_ALREADY_CALCULATED
-			rem=(n1-q*d0)&BN_MASK2;
-#endif
-
-#if defined(BN_UMULT_LOHI)
-			BN_UMULT_LOHI(t2l,t2h,d1,q);
-#elif defined(BN_UMULT_HIGH)
-			t2l = d1 * q;
-			t2h = BN_UMULT_HIGH(d1,q);
-#else
+		/* Since we don't know whether snum is larger than sdiv,
+		 * we pad snum with enough zeroes without changing its
+		 * value. 
+		 */
+		if (snum->top <= sdiv->top+1) 
 			{
-			BN_ULONG ql, qh;
-			t2l=LBITS(d1); t2h=HBITS(d1);
-			ql =LBITS(q);  qh =HBITS(q);
-			mul64(t2l,t2h,ql,qh); /* t2=(BN_ULLONG)d1*q; */
+			if (bn_wexpand(snum, sdiv->top + 2) == NULL) goto err;
+			for (i = snum->top; i < sdiv->top + 2; i++) snum->d[i] = 0;
+			snum->top = sdiv->top + 2;
 			}
-#endif
-
-			for (;;)
-				{
-				if ((t2h < rem) ||
-					((t2h == rem) && (t2l <= wnump[-2])))
-					break;
-				q--;
-				rem += d0;
-				if (rem < d0) break; /* don't let rem overflow */
-				if (t2l < d1) t2h--; t2l -= d1;
-				}
-#endif /* !BN_LLONG */
-			}
-#endif /* !BN_DIV3W */
-
-		l0=bn_mul_words(tmp->d,sdiv->d,div_n,q);
-		tmp->d[div_n]=l0;
-		wnum.d--;
-		/* ingore top values of the bignums just sub the two 
-		 * BN_ULONG arrays with bn_sub_words */
-		if (bn_sub_words(wnum.d, wnum.d, tmp->d, div_n+1))
+		else
 			{
-			/* Note: As we have considered only the leading
-			 * two BN_ULONGs in the calculation of q, sdiv * q
-			 * might be greater than wnum (but then (q-1) * sdiv
-			 * is less or equal than wnum)
-			 */
-			q--;
-			if (bn_add_words(wnum.d, wnum.d, sdiv->d, div_n))
-				/* we can't have an overflow here (assuming
-				 * that q != 0, but if q == 0 then tmp is
-				 * zero anyway) */
-				(*wnump)++;
+			if (bn_wexpand(snum, snum->top + 1) == NULL) goto err;
+			snum->d[snum->top] = 0;
+			snum->top ++;
 			}
-		/* store part of the result */
-		*resp = q;
-		}
-	bn_correct_top(snum);
-	if (rm != NULL)
-		{
-		/* Keep a copy of the neg flag in num because if rm==num
-		 * BN_rshift() will overwrite it.
-		 */
-		int neg = num->neg;
-		BN_rshift(rm,snum,norm_shift);
-		if (!BN_is_zero(rm))
-			rm->neg = neg;
-		bn_check_top(rm);
-		}
-	BN_CTX_end(ctx);
-	return(1);
-err:
-	bn_check_top(rm);
-	BN_CTX_end(ctx);
-	return(0);
-	}
-
-
-/* BN_div_no_branch is a special version of BN_div. It does not contain
- * branches that may leak sensitive information.
- */
-static int BN_div_no_branch(BIGNUM *dv, BIGNUM *rm, const BIGNUM *num, 
-	const BIGNUM *divisor, BN_CTX *ctx)
-	{
-	int norm_shift,i,loop;
-	BIGNUM *tmp,wnum,*snum,*sdiv,*res;
-	BN_ULONG *resp,*wnump;
-	BN_ULONG d0,d1;
-	int num_n,div_n;
-
-	bn_check_top(dv);
-	bn_check_top(rm);
-	/* bn_check_top(num); */ /* 'num' has been checked in BN_div() */
-	bn_check_top(divisor);
-
-	if (BN_is_zero(divisor))
-		{
-		BNerr(BN_F_BN_DIV_NO_BRANCH,BN_R_DIV_BY_ZERO);
-		return(0);
-		}
-
-	BN_CTX_start(ctx);
-	tmp=BN_CTX_get(ctx);
-	snum=BN_CTX_get(ctx);
-	sdiv=BN_CTX_get(ctx);
-	if (dv == NULL)
-		res=BN_CTX_get(ctx);
-	else	res=dv;
-	if (sdiv == NULL || res == NULL) goto err;
-
-	/* First we normalise the numbers */
-	norm_shift=BN_BITS2-((BN_num_bits(divisor))%BN_BITS2);
-	if (!(BN_lshift(sdiv,divisor,norm_shift))) goto err;
-	sdiv->neg=0;
-	norm_shift+=BN_BITS2;
-	if (!(BN_lshift(snum,num,norm_shift))) goto err;
-	snum->neg=0;
-
-	/* Since we don't know whether snum is larger than sdiv,
-	 * we pad snum with enough zeroes without changing its
-	 * value. 
-	 */
-	if (snum->top <= sdiv->top+1) 
-		{
-		if (bn_wexpand(snum, sdiv->top + 2) == NULL) goto err;
-		for (i = snum->top; i < sdiv->top + 2; i++) snum->d[i] = 0;
-		snum->top = sdiv->top + 2;
-		}
-	else
-		{
-		if (bn_wexpand(snum, snum->top + 1) == NULL) goto err;
-		snum->d[snum->top] = 0;
-		snum->top ++;
 		}
 
 	div_n=sdiv->top;
@@ -504,12 +286,27 @@ static int BN_div_no_branch(BIGNUM *dv, BIGNUM *rm, const BIGNUM *num,
 	/* Setup to 'res' */
 	res->neg= (num->neg^divisor->neg);
 	if (!bn_wexpand(res,(loop+1))) goto err;
-	res->top=loop-1;
+	res->top=loop-no_branch;
 	resp= &(res->d[loop-1]);
 
 	/* space for temp */
 	if (!bn_wexpand(tmp,(div_n+1))) goto err;
 
+	if (!no_branch)
+		{
+		if (BN_ucmp(&wnum,sdiv) >= 0)
+			{
+			/* If BN_DEBUG_RAND is defined BN_ucmp changes (via
+			 * bn_pollute) the const bignum arguments =>
+			 * clean the values between top and max again */
+			bn_clear_top2max(&wnum);
+			bn_sub_words(wnum.d, wnum.d, sdiv->d, div_n);
+			*resp=1;
+			}
+		else
+			res->top--;
+		}
+
 	/* if res->top == 0 then clear the neg value otherwise decrease
 	 * the resp pointer */
 	if (res->top == 0)
@@ -642,7 +439,7 @@ X) -> 0x%08X\n",
 			rm->neg = neg;
 		bn_check_top(rm);
 		}
-	bn_correct_top(res);
+	if (no_branch)	bn_correct_top(res);
 	BN_CTX_end(ctx);
 	return(1);
 err:
@@ -650,5 +447,4 @@ err:
 	BN_CTX_end(ctx);
 	return(0);
 	}
-
 #endif