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Commentary updates and minor optimization for bn_mont.c.

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This commit is contained in:
Andy Polyakov 2007-06-11 08:53:52 +00:00
parent 6b6443dead
commit c693b5a55c
1 changed files with 13 additions and 7 deletions
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20
crypto/bn/bn_mont.c
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@ -270,18 +270,24 @@ static int BN_from_montgomery_word(BIGNUM *ret, BIGNUM *r, BN_MONT_CTX *mont)
size_t m1,m2;
v=bn_sub_words(rp,ap,mont->N.d,ri);
/* if (al==ri && !v) || al>ri) nrp=rp; */
/* 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 make "refresh"
* instead of real copy. */
* 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=~(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));
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;