From d4122504a2025c90eb9ffc028c4792c4b8e8e748 Mon Sep 17 00:00:00 2001 From: Andy Polyakov Date: Wed, 9 Apr 2008 12:06:42 +0000 Subject: [PATCH] Clarifying comment. --- crypto/bn/bn_nist.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/crypto/bn/bn_nist.c b/crypto/bn/bn_nist.c index 6b88c5fa13..0eabb0332b 100644 --- a/crypto/bn/bn_nist.c +++ b/crypto/bn/bn_nist.c @@ -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)); -- 2.34.1