status-im
/
secp256k1
mirror of https://github.com/status-im/secp256k1.git
2
0
Fork 0
Code Issues Projects Releases Wiki Activity

Doubling formula using fe_half

This commit is contained in:
Pieter Wuille 2021-12-22 19:39:22 -05:00 committed by Peter Dettman
parent 2cbb4b1a42
commit 557b31fac3
1 changed files with 27 additions and 30 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

57
src/group_impl.h
View File

@ -271,37 +271,36 @@ static int secp256k1_ge_is_valid_var(const secp256k1_ge *a) {
}
static SECP256K1_INLINE void secp256k1_gej_double(secp256k1_gej *r, const secp256k1_gej *a) {
/* Operations: 3 mul, 4 sqr, 0 normalize, 12 mul_int/add/negate.
*
* Note that there is an implementation described at
* https://hyperelliptic.org/EFD/g1p/auto-shortw-jacobian-0.html#doubling-dbl-2009-l
* which trades a multiply for a square, but in practice this is actually slower,
* mainly because it requires more normalizations.
*/
secp256k1_fe t1,t2,t3,t4;
secp256k1_fe l, s, t, q;
r->infinity = a->infinity;
secp256k1_fe_mul(&r->z, &a->z, &a->y);
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, &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) */
secp256k1_fe_mul(&t3, &t3, &a->x); /* T3 = 2*X*Y^2 (1) */
r->x = t3;
secp256k1_fe_mul_int(&r->x, 4); /* X' = 8*X*Y^2 (4) */
secp256k1_fe_negate(&r->x, &r->x, 4); /* X' = -8*X*Y^2 (5) */
secp256k1_fe_add(&r->x, &t2); /* X' = 9*X^4 - 8*X*Y^2 (6) */
secp256k1_fe_negate(&t2, &t2, 1); /* T2 = -9*X^4 (2) */
secp256k1_fe_mul_int(&t3, 6); /* T3 = 12*X*Y^2 (6) */
secp256k1_fe_add(&t3, &t2); /* T3 = 12*X*Y^2 - 9*X^4 (8) */
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) */
/* Formula used:
* L = (3/2) * X1^2
* S = Y1^2
* T = X1*S
* X3 = L^2 - 2*T
* Y3 = L*(T - X3) - S^2
* Z3 = Y1*Z1
*/
secp256k1_fe_mul(&r->z, &a->z, &a->y); /* Z3 = Y1*Z1 (1) */
secp256k1_fe_sqr(&l, &a->x); /* L = X1^2 (1) */
secp256k1_fe_mul_int(&l, 3); /* L = 3*X1^2 (3) */
secp256k1_fe_half(&l); /* L = 3/2*X1^2 (2) */
secp256k1_fe_sqr(&s, &a->y); /* S = Y1^2 (1) */
secp256k1_fe_mul(&t, &a->x, &s); /* T = X1*S (1) */
q = t;
secp256k1_fe_add(&q, &t); /* Q = 2*T (2) */
secp256k1_fe_negate(&r->x, &q, 2); /* X3 = -2*T (3) */
secp256k1_fe_sqr(&q, &l); /* Q = L^2 (1) */
secp256k1_fe_add(&r->x, &q); /* X3 = L^2 - 2*T (4) */
secp256k1_fe_negate(&q, &r->x, 4); /* Q = -X3 (5) */
secp256k1_fe_add(&q, &t); /* Q = T-X3 (6) */
secp256k1_fe_mul(&q, &q, &l); /* Q = L*(T-X3) (1) */
secp256k1_fe_sqr(&s, &s);
secp256k1_fe_negate(&r->y, &s, 1); /* Y3 = -S^2 (2) */
secp256k1_fe_add(&r->y, &q); /* Y3 = L*(T-X3) - S^2 (3) */
}
static void secp256k1_gej_double_var(secp256k1_gej *r, const secp256k1_gej *a, secp256k1_fe *rzr) {
@ -325,8 +324,6 @@ static void secp256k1_gej_double_var(secp256k1_gej *r, const secp256k1_gej *a, s
if (rzr != NULL) {
*rzr = a->y;
secp256k1_fe_normalize_weak(rzr);
secp256k1_fe_mul_int(rzr, 2);
}
secp256k1_gej_double(r, a);