circomlib/src/babyjub.js

const F1Field = require("ffjavascript").F1Field;
const Scalar = require("ffjavascript").Scalar;
const utils = require("ffjavascript").utils;

exports.addPoint = addPoint;
exports.mulPointEscalar = mulPointEscalar;
exports.inCurve = inCurve;
exports.inSubgroup = inSubgroup;
exports.packPoint = packPoint;
exports.unpackPoint = unpackPoint;


exports.p = Scalar.fromString("21888242871839275222246405745257275088548364400416034343698204186575808495617");
const F = new F1Field(exports.p);
exports.F = F;

exports.Generator = [
    F.e("995203441582195749578291179787384436505546430278305826713579947235728471134"),
    F.e("5472060717959818805561601436314318772137091100104008585924551046643952123905")
];
exports.Base8 = [
    F.e("5299619240641551281634865583518297030282874472190772894086521144482721001553"),
    F.e("16950150798460657717958625567821834550301663161624707787222815936182638968203")
];
exports.order = Scalar.fromString("21888242871839275222246405745257275088614511777268538073601725287587578984328");
exports.subOrder = Scalar.shiftRight(exports.order, 3);
exports.A = F.e("168700");
exports.D = F.e("168696");


function addPoint(a,b) {

    const res = [];

    /* does the equivalent of:
     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);
    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);
    */

    const beta = F.mul(a[0],b[1]);
    const gamma = F.mul(a[1],b[0]);
    const delta = F.mul(
        F.sub(a[1], F.mul(exports.A, a[0])),
        F.add(b[0], b[1])
    );
    const tau = F.mul(beta, gamma);
    const dtau = F.mul(exports.D, tau);

    res[0] = F.div(
        F.add(beta, gamma),
        F.add(F.one, dtau)
    );

    res[1] = F.div(
        F.add(delta, F.sub(F.mul(exports.A,beta), gamma)),
        F.sub(F.one, dtau)
    );

    return res;
}

function mulPointEscalar(base, e) {
    let res = [F.e("0"),F.e("1")];
    let rem = e;
    let exp = base;

    while (! Scalar.isZero(rem)) {
        if (Scalar.isOdd(rem)) {
            res = addPoint(res, exp);
        }
        exp = addPoint(exp, exp);
        rem = Scalar.shiftRight(rem, 1);
    }

    return res;
}

function inSubgroup(P) {
    if (!inCurve(P)) return false;
    const res= mulPointEscalar(P, exports.subOrder);
    return (F.isZero(res[0]) && F.eq(res[1], F.one));
}

function inCurve(P) {

    const x2 = F.square(P[0]);
    const y2 = F.square(P[1]);

    if (!F.eq(
        F.add(F.mul(exports.A, x2), y2),
        F.add(F.one, F.mul(F.mul(x2, y2), exports.D)))) return false;

    return true;
}

function packPoint(P) {
    const buff = utils.leInt2Buff(P[1], 32);
    if (F.lt(P[0], F.zero)) {
        buff[31] = buff[31] | 0x80;
    }
    return buff;
}

function unpackPoint(_buff) {
    const buff = Buffer.from(_buff);
    let sign = false;
    const P = new Array(2);
    if (buff[31] & 0x80) {
        sign = true;
        buff[31] = buff[31] & 0x7F;
    }
    P[1] = utils.leBuff2int(buff);
    if (Scalar.gt(P[1], exports.p)) return null;

    const y2 = F.square(P[1]);

    let x = F.sqrt(F.div(
        F.sub(F.one, y2),
        F.sub(exports.A, F.mul(exports.D, y2))));

    if (x == null) return null;

    if (sign) x = F.neg(x);

    P[0] = x;

    return P;
}