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Use smaller value in felem_diff

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This commit is contained in:
Olivier Chéron 2020-01-14 21:11:51 +01:00
parent 44a1651d26
commit 2579d1e7aa
1 changed files with 10 additions and 4 deletions
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14
cbits/include64/p256/p256_gf.h
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@ -229,8 +229,14 @@ static void felem_sum(felem out, const felem in, const felem in2) {
felem_reduce_carry(out, carry);
}
/* zero31 is 0 mod p. */
static const felem zero31 = { 0xffffffffffffc0, 0x1f7ffffffffffe0, 0xf7ffffffffffe1, 0x1f00fffffffffe1, 0xfffffffeffffe1 };
#define two53m3 (((limb)1) << 53) - (((limb)1) << 3)
#define two54m52p48m2 (((limb)1) << 54) - (((limb)1) << 52) + (((limb)1) << 48) - (((limb)1) << 2)
#define two53m2p0 (((limb)1) << 53) - (((limb)1) << 2) + (((limb)1) << 0)
#define two54m52p41m2 (((limb)1) << 54) - (((limb)1) << 52) + (((limb)1) << 41) - (((limb)1) << 2)
#define two53m21m2p0 (((limb)1) << 53) - (((limb)1) << 21) - (((limb)1) << 2) + (((limb)1) << 0)
/* zero53 is 0 mod p. */
static const felem zero53 = { two53m3, two54m52p48m2, two53m2p0, two54m52p41m2, two53m21m2p0 };
/* felem_diff sets out = in-in2.
*
@ -243,7 +249,7 @@ static void felem_diff(felem out, const felem in, const felem in2) {
for (i = 0;; i++) {
out[i] = in[i] - in2[i];
out[i] += zero31[i];
out[i] += zero53[i];
out[i] += carry;
carry = out[i] >> 51;
out[i] &= kBottom51Bits;
@ -253,7 +259,7 @@ static void felem_diff(felem out, const felem in, const felem in2) {
break;
out[i] = in[i] - in2[i];
out[i] += zero31[i];
out[i] += zero53[i];
out[i] += carry;
carry = out[i] >> 52;
out[i] &= kBottom52Bits;