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Merge pull request #120

e3d692f Explain why no y=0 check is necessary for doubling (Pieter Wuille)
f7dc1c6 Optimize doubling: secp256k1 has no y=0 point (Pieter Wuille)
master
Pieter Wuille 8 years ago
parent
commit
0af5b47133
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GPG Key ID: 57896D2FF8F0B657
  1. 19
      src/group_impl.h

19
src/group_impl.h

@ -208,25 +208,21 @@ static int secp256k1_ge_is_valid(const secp256k1_ge_t *a) { @@ -208,25 +208,21 @@ static int secp256k1_ge_is_valid(const secp256k1_ge_t *a) {
}
static void secp256k1_gej_double_var(secp256k1_gej_t *r, const secp256k1_gej_t *a) {
if (a->infinity) {
r->infinity = 1;
return;
}
secp256k1_fe_t t5 = a->y;
secp256k1_fe_normalize(&t5);
if (secp256k1_fe_is_zero(&t5)) {
r->infinity = 1;
// For secp256k1, 2Q is infinity if and only if Q is infinity. This is because if 2Q = infinity,
// Q must equal -Q, or that Q.y == -(Q.y), or Q.y is 0. For a point on y^2 = x^3 + 7 to have
// y=0, x^3 must be -7 mod p. However, -7 has no cube root mod p.
r->infinity = a->infinity;
if (r->infinity) {
return;
}
secp256k1_fe_t t1,t2,t3,t4;
secp256k1_fe_mul(&r->z, &t5, &a->z);
secp256k1_fe_mul(&r->z, &a->y, &a->z);
secp256k1_fe_mul_int(&r->z, 2); /* Z' = 2*Y*Z (2) */
secp256k1_fe_sqr(&t1, &a->x);
secp256k1_fe_mul_int(&t1, 3); /* T1 = 3*X^2 (3) */
secp256k1_fe_sqr(&t2, &t1); /* T2 = 9*X^4 (1) */
secp256k1_fe_sqr(&t3, &t5);
secp256k1_fe_sqr(&t3, &a->y);
secp256k1_fe_mul_int(&t3, 2); /* T3 = 2*Y^2 (2) */
secp256k1_fe_sqr(&t4, &t3);
secp256k1_fe_mul_int(&t4, 2); /* T4 = 8*Y^4 (2) */
@ -241,7 +237,6 @@ static void secp256k1_gej_double_var(secp256k1_gej_t *r, const secp256k1_gej_t * @@ -241,7 +237,6 @@ static void secp256k1_gej_double_var(secp256k1_gej_t *r, const secp256k1_gej_t *
secp256k1_fe_mul(&r->y, &t1, &t3); /* Y' = 36*X^3*Y^2 - 27*X^6 (1) */
secp256k1_fe_negate(&t2, &t4, 2); /* T2 = -8*Y^4 (3) */
secp256k1_fe_add(&r->y, &t2); /* Y' = 36*X^3*Y^2 - 27*X^6 - 8*Y^4 (4) */
r->infinity = 0;
}
static void secp256k1_gej_add_var(secp256k1_gej_t *r, const secp256k1_gej_t *a, const secp256k1_gej_t *b) {

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