140 lines
3.8 KiB
JavaScript
140 lines
3.8 KiB
JavaScript
import { getCurveFromName, Scalar } from "ffjavascript";
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export default async function buildBabyJub() {
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const bn128 = await getCurveFromName("bn128", true);
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return new BabyJub(bn128.Fr);
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}
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class BabyJub {
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constructor(F) {
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this.F = F;
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this.p = Scalar.fromString("21888242871839275222246405745257275088548364400416034343698204186575808495617");
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this.pm1d2 = Scalar.div(Scalar.sub(this.p, Scalar.e(1)), Scalar.e(2));
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this.Generator = [
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F.e("995203441582195749578291179787384436505546430278305826713579947235728471134"),
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F.e("5472060717959818805561601436314318772137091100104008585924551046643952123905")
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];
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this.Base8 = [
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F.e("5299619240641551281634865583518297030282874472190772894086521144482721001553"),
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F.e("16950150798460657717958625567821834550301663161624707787222815936182638968203")
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];
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this.order = Scalar.fromString("21888242871839275222246405745257275088614511777268538073601725287587578984328");
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this.subOrder = Scalar.shiftRight(this.order, 3);
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this.A = F.e("168700");
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this.D = F.e("168696");
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}
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addPoint(a,b) {
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const F = this.F;
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const res = [];
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/* does the equivalent of:
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res[0] = bigInt((a[0]*b[1] + b[0]*a[1]) * bigInt(bigInt("1") + d*a[0]*b[0]*a[1]*b[1]).inverse(q)).affine(q);
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res[1] = bigInt((a[1]*b[1] - cta*a[0]*b[0]) * bigInt(bigInt("1") - d*a[0]*b[0]*a[1]*b[1]).inverse(q)).affine(q);
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*/
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const beta = F.mul(a[0],b[1]);
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const gamma = F.mul(a[1],b[0]);
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const delta = F.mul(
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F.sub(a[1], F.mul(this.A, a[0])),
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F.add(b[0], b[1])
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);
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const tau = F.mul(beta, gamma);
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const dtau = F.mul(this.D, tau);
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res[0] = F.div(
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F.add(beta, gamma),
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F.add(F.one, dtau)
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);
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res[1] = F.div(
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F.add(delta, F.sub(F.mul(this.A,beta), gamma)),
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F.sub(F.one, dtau)
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);
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return res;
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}
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mulPointEscalar(base, e) {
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const F = this.F;
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let res = [F.e("0"),F.e("1")];
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let rem = e;
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let exp = base;
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while (! Scalar.isZero(rem)) {
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if (Scalar.isOdd(rem)) {
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res = this.addPoint(res, exp);
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}
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exp = this.addPoint(exp, exp);
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rem = Scalar.shiftRight(rem, 1);
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}
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return res;
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}
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inSubgroup(P) {
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const F = this.F;
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if (!this.inCurve(P)) return false;
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const res= this.mulPointEscalar(P, this.subOrder);
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return (F.isZero(res[0]) && F.eq(res[1], F.one));
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}
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inCurve(P) {
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const F = this.F;
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const x2 = F.square(P[0]);
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const y2 = F.square(P[1]);
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if (!F.eq(
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F.add(F.mul(this.A, x2), y2),
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F.add(F.one, F.mul(F.mul(x2, y2), this.D)))) return false;
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return true;
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}
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packPoint(P) {
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const F = this.F;
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const buff = new Uint8Array(32);
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F.toRprLE(buff, 0, P[1]);
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const n = F.toObject(P[0]);
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if (Scalar.gt(n, this.pm1d2)) {
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buff[31] = buff[31] | 0x80;
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}
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return buff;
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}
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unpackPoint(buff) {
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const F = this.F;
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let sign = false;
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const P = new Array(2);
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if (buff[31] & 0x80) {
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sign = true;
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buff[31] = buff[31] & 0x7F;
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}
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P[1] = F.fromRprLE(buff, 0);
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if (Scalar.gt(F.toObject(P[1]), this.p)) return null;
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const y2 = F.square(P[1]);
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const x2 = F.div(
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F.sub(F.one, y2),
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F.sub(this.A, F.mul(this.D, y2))
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);
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const x2h = F.exp(x2, F.half);
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if (! F.eq(F.one, x2h)) return null;
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let x = F.sqrt(x2);
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if (x == null) return null;
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if (sign) x = F.neg(x);
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P[0] = x;
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return P;
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}
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}
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