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more minor improvement around 2^256-38.

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NIIBE Yutaka
2014-04-17 12:21:00 +09:00
parent 4f1343e154
commit 62a59b56fe
4 changed files with 37 additions and 18 deletions
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8
ChangeLog
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@@ -1,3 +1,11 @@
2014-04-17 Niibe Yutaka <gniibe@fsij.org>
* src/mod25638.c (mod25638_add, mod25638_sub): Simplify.
(n25638): Remove.
(mod25638_neg): New.
* src/ecc-edwards.c (point_double): Use mod25638_neg.
2014-04-16 Niibe Yutaka <gniibe@fsij.org>
* VERSION: 1.1.3.

8
src/ecc-edwards.c
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@@ -127,7 +127,6 @@ mod25519_is_neg (const bn256 *a)
static void
point_double (ptc *X, const ptc *A)
{
uint32_t borrow;
bn256 b[1], d[1], e[1];
/* Compute: B = (X1 + Y1)^2 */
@@ -143,12 +142,7 @@ point_double (ptc *X, const ptc *A)
/* E = aC; where a = -1 */
/* Compute: E - D = -(C+D) : Y3_tmp */
mod25638_add (X->y, e, d);
/* Negation: it can result borrow, as it is in redundant representation. */
borrow = bn256_sub (X->y, n25638, X->y);
if (borrow)
bn256_add (X->y, X->y, n25638); /* carry ignored */
else
bn256_add (X->x, X->y, n25638); /* dummy calculation */
mod25638_neg (X->y, X->y);
/* Compute: F = E + D = D - C; where a = -1 : E */
mod25638_sub (e, d, e);

37
src/mod25638.c
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@@ -68,17 +68,13 @@
256 224 192 160 128 96 64 32 0
2^256
1 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
2^256 - 32
0 ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffe0
2^256 - 32 - 4
0 ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffdc
2^256 - 32 - 4 - 2
0 ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffda
2^256 - 16
0 ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff fffffff0
2^256 - 16 - 2
0 ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffee
2^256 - 16 - 2 - 1
0 ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffed
*/
const bn256 n25638[1] = {
{{0xffffffda, 0xffffffff, 0xffffffff, 0xffffffff,
0xffffffff, 0xffffffff, 0xffffffff, 0xffffffff }} };
const bn256 p25519[1] = {
{{ 0xffffffed, 0xffffffff, 0xffffffff, 0xffffffff,
0xffffffff, 0xffffffff, 0xffffffff, 0x7fffffff }} };
@@ -93,6 +89,27 @@ const bn256 p25519[1] = {
* 256-bit.
*/
/**
* @brief X = -A mod 2^256-38
*/
void
mod25638_neg (bn256 *X, const bn256 *A)
{
int i;
uint32_t borrow;
uint32_t *px;
const uint32_t *pa;
px = X->word;
pa = A->word;
for (i = 0; i < BN256_WORDS; i++)
*px++ = ~*pa++;
borrow = bn256_sub_uint (X, X, 37);
X->word[0] -= borrow * 38;
}
/**
* @brief X = (A + B) mod 2^256-38
*/

2
src/mod25638.h
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@@ -1,6 +1,6 @@
extern const bn256 n25638[1];
extern const bn256 p25519[1];
void mod25638_neg (bn256 *X, const bn256 *A);
void mod25638_add (bn256 *X, const bn256 *A, const bn256 *B);
void mod25638_sub (bn256 *X, const bn256 *A, const bn256 *B);
void mod25638_mul (bn256 *X, const bn256 *A, const bn256 *B);