bn_mont.c: simplify BN_from_montgomery_word.
authorAndy Polyakov <appro@openssl.org>
Mon, 17 Oct 2011 17:24:28 +0000 (17:24 +0000)
committerAndy Polyakov <appro@openssl.org>
Mon, 17 Oct 2011 17:24:28 +0000 (17:24 +0000)
crypto/bn/bn_mont.c

index 36ac6d8..5bf1f02 100644 (file)
@@ -179,24 +179,19 @@ err:
 static int BN_from_montgomery_word(BIGNUM *ret, BIGNUM *r, BN_MONT_CTX *mont)
        {
        BIGNUM *n;
-       BN_ULONG *ap,*np,*rp,n0,v,*nrp;
-       int al,nl,max,i,x,ri;
+       BN_ULONG *ap,*np,*rp,n0,v,carry;
+       int nl,max,i;
 
        n= &(mont->N);
-       /* mont->ri is the size of mont->N in bits (rounded up
-          to the word size) */
-       al=ri=mont->ri/BN_BITS2;
-
        nl=n->top;
-       if ((al == 0) || (nl == 0)) { ret->top=0; return(1); }
+       if (nl == 0) { ret->top=0; return(1); }
 
-       max=(nl+al+1); /* allow for overflow (no?) XXX */
+       max=(2*nl); /* carry is stored separately */
        if (bn_wexpand(r,max) == NULL) return(0);
 
        r->neg^=n->neg;
        np=n->d;
        rp=r->d;
-       nrp= &(r->d[nl]);
 
        /* clear the top words of T */
 #if 1
@@ -212,7 +207,7 @@ static int BN_from_montgomery_word(BIGNUM *ret, BIGNUM *r, BN_MONT_CTX *mont)
 #ifdef BN_COUNT
        fprintf(stderr,"word BN_from_montgomery_word %d * %d\n",nl,nl);
 #endif
-       for (i=0; i<nl; i++)
+       for (carry=0, i=0; i<nl; i++, rp++)
                {
 #ifdef __TANDEM
                 {
@@ -230,61 +225,33 @@ static int BN_from_montgomery_word(BIGNUM *ret, BIGNUM *r, BN_MONT_CTX *mont)
 #else
                v=bn_mul_add_words(rp,np,nl,(rp[0]*n0)&BN_MASK2);
 #endif
-               nrp++;
-               rp++;
-               if (((nrp[-1]+=v)&BN_MASK2) >= v)
-                       continue;
+               if ((rp[nl] = (rp[nl]+v+carry)&BN_MASK2) < v)
+                       carry = 1;
                else
-                       {
-                       if (((++nrp[0])&BN_MASK2) != 0) continue;
-                       if (((++nrp[1])&BN_MASK2) != 0) continue;
-                       for (x=2; (((++nrp[x])&BN_MASK2) == 0); x++) ;
-                       }
-               }
-       bn_correct_top(r);
-
-       /* 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;
-               return(1);
+                       carry = 0;
                }
-       al=r->top-ri;
 
-#define BRANCH_FREE 1
-#if BRANCH_FREE
-       if (bn_wexpand(ret,ri) == NULL) return(0);
-       x=0-(((al-ri)>>(sizeof(al)*8-1))&1);
-       ret->top=x=(ri&~x)|(al&x);      /* min(ri,al) */
+       if (bn_wexpand(ret,nl) == NULL) return(0);
+       ret->top=nl;
        ret->neg=r->neg;
 
        rp=ret->d;
-       ap=&(r->d[ri]);
+       ap=&(r->d[nl]);
 
+#define BRANCH_FREE 1
+#if BRANCH_FREE
        {
-       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. */
+       BN_ULONG *nrp;
+       size_t m;
 
-       /* if (al==ri && !v) || al>ri) nrp=rp; else nrp=ap; */
-       /* in other words if subtraction result is real, then
+       v=bn_sub_words(rp,ap,np,nl)-carry;
+       /* 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 *)(((PTR_SIZE_INT)rp&~m1)|((PTR_SIZE_INT)ap&m1));
-       }
+       m=(0-(size_t)v);
+       nrp=(BN_ULONG *)(((PTR_SIZE_INT)rp&~m)|((PTR_SIZE_INT)ap&m));
 
-       /* '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)
+       for (i=0,nl-=4; i<nl; i+=4)
                {
                BN_ULONG t1,t2,t3,t4;
                
@@ -297,40 +264,15 @@ static int BN_from_montgomery_word(BIGNUM *ret, BIGNUM *r, BN_MONT_CTX *mont)
                rp[i+2]=t3;
                rp[i+3]=t4;
                }
-       for (ri+=4; i<ri; i++)
+       for (nl+=4; i<nl; i++)
                rp[i]=nrp[i], ap[i]=0;
-       bn_correct_top(r);
-       bn_correct_top(ret);
+       }
 #else
-       if (bn_wexpand(ret,al) == NULL) return(0);
-       ret->top=al;
-       ret->neg=r->neg;
-
-       rp=ret->d;
-       ap=&(r->d[ri]);
-       al-=4;
-       for (i=0; i<al; i+=4)
-               {
-               BN_ULONG t1,t2,t3,t4;
-               
-               t1=ap[i+0];
-               t2=ap[i+1];
-               t3=ap[i+2];
-               t4=ap[i+3];
-               rp[i+0]=t1;
-               rp[i+1]=t2;
-               rp[i+2]=t3;
-               rp[i+3]=t4;
-               }
-       al+=4;
-       for (; i<al; i++)
-               rp[i]=ap[i];
-
-       if (BN_ucmp(ret, &(mont->N)) >= 0)
-               {
-               if (!BN_usub(ret,ret,&(mont->N))) return(0);
-               }
+       if (bn_sub_words (rp,ap,np,nl)-carry)
+               memcpy(rp,ap,nl*sizeof(BN_ULONG));
 #endif
+       bn_correct_top(r);
+       bn_correct_top(ret);
        bn_check_top(ret);
 
        return(1);