Commit d4122504 authored by Andy Polyakov's avatar Andy Polyakov
Browse files

Clarifying comment.

parent 2c4226c4

crypto/bn/bn_nist.c

+9 −1
Original line number Diff line number Diff line
@@ -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));