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Clarifying comment.

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This commit is contained in:
Andy Polyakov 2008-04-09 12:06:42 +00:00
parent 2c4226c42b
commit d4122504a2
1 changed files with 9 additions and 1 deletions
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10
crypto/bn/bn_nist.c
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@ -593,7 +593,15 @@ int BN_nist_mod_384(BIGNUM *r, const BIGNUM *a, const BIGNUM *field,
}
carry = bn_add_words(r_d+(128/BN_BITS2), r_d+(128/BN_BITS2),
t_d, BN_NIST_256_TOP);
/* this is equivalent to if (result >= module) */
/*
* we need if (result>=modulus) subtract(result,modulus);
* in n-bit space this can be expressed as
* if (carry || result>=modulus) subtract(result,modulus);
* the catch is that comparison implies subtraction and
* therefore one can write tmp=subtract(result,modulus);
* and then if(carry || !borrow) result=tmp; this's what
* happens below, but without explicit if:-) a.
*/
mask = 0-(size_t)bn_sub_words(c_d,r_d,_nist_p_384,BN_NIST_384_TOP);
mask = ~mask | (0-(size_t)carry);
res = (BN_ULONG *)(((size_t)c_d&mask) | ((size_t)r_d&~mask));