Eliminate conditional final subtraction in Montgomery multiplication.

diff --git a/crypto/bn/bn_mont.c b/crypto/bn/bn_mont.c
index a4612d6639e1a1d2e51a3f368548c2e2370b4aec..594d95940fdf86d99eaae4c7d4bc51712694e842 100644 (file)
--- a/crypto/bn/bn_mont.c
+++ b/crypto/bn/bn_mont.c
@@ -240,20 +240,67 @@ static int BN_from_montgomery_word(BIGNUM *ret, BIGNUM *r, 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;
                return(1);
                }
        al=r->top-ri;
-       if (bn_wexpand(ret,al) == NULL) return(0);
+
+#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) */
        ret->neg=r->neg;
+
+       rp=ret->d;
+       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);
+               /* if (al==ri && !v) || al>ri)  nrp=rp; */
+               /* in other words if subtraction result is real, then
+                * trick unconditional memcpy below to make "refresh"
+                * instead of real 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=~(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));
+               }
+
+       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) return(0);
        ret->top=al;
+       ret->neg=r->neg;
 
        rp=ret->d;
        ap=&(r->d[ri]);
@@ -274,12 +321,12 @@ static int BN_from_montgomery_word(BIGNUM *ret, BIGNUM *r, BN_MONT_CTX *mont)
        al+=4;
        for (; i<al; i++)
                rp[i]=ap[i];
-#endif
 
        if (BN_ucmp(ret, &(mont->N)) >= 0)
                {
                if (!BN_usub(ret,ret,&(mont->N))) return(0);
                }
+#endif
        bn_check_top(ret);
 
        return(1);