DateKeys-App/src/lib/dkc/bls12381.contrast.test.ts

// Contrast test of bls12381.ts against an audited implementation.
//
// checkCompressedPoint is our own code. It is kept because it matches the Go
// reference on every edge case, is 1.3 KB gzip and adds no runtime
// dependency. Its assurance comes from this file: on every run it is compared
// with the Go reference on 157 frozen edge cases and with @noble/curves 2.4.0
// (Cure53 2024; Trail of Bits 2026 review, whose BLS findings were fixed in
// 2.3.0) on a deterministic corpus. noble is a runtime dependency since phase
// 2, for the IBE core and the release verification only: the guards of
// src/lib/dependencies.test.ts fail if any other file of the library or the
// page imports it.

import { bls12_381 } from '@noble/curves/bls12-381.js';
import { readFileSync } from 'node:fs';
import { describe, expect, it } from 'vitest';
import { checkCompressedPoint, type Group } from './bls12381.ts';

type Verdict = 'point' | 'identity' | 'invalid';

const SIZE: Record<Group, number> = { G1: 48, G2: 96 };
const P = bls12_381.fields.Fp.ORDER;
const R = bls12_381.fields.Fr.ORDER;

// The audited oracle. noble also decodes uncompressed encodings, which the
// protocol never uses for a public key, so the length is checked first, as
// the Go reference does; with that check noble >= 2.3.0 matches Go on every
// frozen edge case.
function noble(group: Group, bytes: Uint8Array): Verdict {
  if (bytes.length !== SIZE[group]) return 'invalid';
  try {
    const point = bls12_381[group].Point.fromBytes(bytes);
    point.assertValidity();
    return point.is0() ? 'identity' : 'point';
  } catch {
    return 'invalid';
  }
}

const hex = (s: string): Uint8Array => Uint8Array.from(s.match(/../g) ?? [], (b) => parseInt(b, 16));
const toHex = (b: Uint8Array): string => Array.from(b, (v) => v.toString(16).padStart(2, '0')).join('');

// splitmix64: a fixed-seed generator, so every run checks the same corpus.
function rng(seed: bigint): (n: number) => Uint8Array {
  let state = seed;
  const M = (1n << 64n) - 1n;
  const next = (): bigint => {
    state = (state + 0x9e3779b97f4a7c15n) & M;
    let z = state;
    z = ((z ^ (z >> 30n)) * 0xbf58476d1ce4e5b9n) & M;
    z = ((z ^ (z >> 27n)) * 0x94d049bb133111ebn) & M;
    return z ^ (z >> 31n);
  };
  return (n) => {
    const out = new Uint8Array(n);
    for (let i = 0; i < n; i += 8) {
      let w = next();
      for (let j = 0; j < 8 && i + j < n; j++, w >>= 8n) out[i + j] = Number(w & 0xffn);
    }
    return out;
  };
}

const big = (b: Uint8Array): bigint => b.reduce((acc, v) => (acc << 8n) | BigInt(v), 0n);

function be48(x: bigint): Uint8Array {
  const out = new Uint8Array(48);
  for (let i = 47; i >= 0; i--, x >>= 8n) out[i] = Number(x & 0xffn);
  return out;
}

function modPow(base: bigint, exp: bigint): bigint {
  let result = 1n;
  let b = base % P;
  for (let e = exp; e > 0n; e >>= 1n) {
    if (e & 1n) result = (result * b) % P;
    b = (b * b) % P;
  }
  return result;
}

// A G1 encoding of a point on y^2 = x^3 + 4 chosen from a random x: the
// cofactor is about 2^126, so it is outside the prime-order subgroup.
function onCurveOutsideSubgroupG1(random: (n: number) => Uint8Array): Uint8Array {
  for (;;) {
    const x = big(random(48)) % P;
    const rhs = (x * x * x + 4n) % P;
    if (modPow(rhs, (P - 1n) / 2n) !== 1n) continue;
    const y = modPow(rhs, (P + 1n) / 4n);
    const out = be48(x);
    out[0]! |= 0x80 | (y > (P - 1n) / 2n ? 0x20 : 0);
    return out;
  }
}

interface Case {
  label: string;
  group: Group;
  bytes: Uint8Array;
}

function corpus(): Case[] {
  const random = rng(20260926n);
  const cases: Case[] = [];
  const scalar = (): bigint => (big(random(40)) % (R - 1n)) + 1n;
  for (const [group, valid, other] of [
    ['G1', 60, 120],
    ['G2', 20, 40],
  ] as const) {
    const G = bls12_381[group].Point;
    for (let i = 0; i < valid; i++) {
      const bytes = G.BASE.multiply(scalar()).toBytes(true);
      cases.push({ label: `${group} valid ${i}`, group, bytes });
      const negated = new Uint8Array(bytes);
      negated[0]! ^= 0x20;
      cases.push({ label: `${group} valid ${i} negated`, group, bytes: negated });
      const flipped = new Uint8Array(bytes);
      const bit = Number(big(random(2)) % BigInt(SIZE[group] * 8 - 3)) + 3;
      flipped[bit >> 3]! ^= 0x80 >> (bit & 7);
      cases.push({ label: `${group} valid ${i} bit ${bit} flipped`, group, bytes: flipped });
    }
    for (let i = 0; i < other; i++) {
      const bytes = random(SIZE[group]);
      bytes[0] = 0x80 | (bytes[0]! & 0x3f);
      cases.push({ label: `${group} random x ${i}`, group, bytes });
    }
  }
  for (let i = 0; i < 60; i++) {
    cases.push({ label: `G1 on the curve, outside the subgroup ${i}`, group: 'G1', bytes: onCurveOutsideSubgroupG1(random) });
  }
  return cases;
}

describe('bls12381.ts against the Go reference and @noble/curves 2.4.0', () => {
  const file = JSON.parse(readFileSync(new URL('./testing/bls12381-vectors.json', import.meta.url), 'utf8')) as {
    vectors: { label: string; group: Group; hex: string; go: Verdict }[];
  };

  it('matches the Go reference on every frozen edge case', () => {
    expect(file.vectors.length).toBe(157);
    for (const v of file.vectors) expect(checkCompressedPoint(v.group, hex(v.hex)), v.label).toBe(v.go);
  });

  it('noble agrees with the Go reference on every frozen edge case', () => {
    for (const v of file.vectors) expect(noble(v.group, hex(v.hex)), v.label).toBe(v.go);
  });

  it('matches noble on a deterministic corpus', { timeout: 120_000 }, () => {
    const cases = corpus();
    const seen = { point: 0, identity: 0, invalid: 0 };
    for (const c of cases) {
      const want = noble(c.group, c.bytes);
      expect(checkCompressedPoint(c.group, c.bytes), `${c.label}: ${toHex(c.bytes)}`).toBe(want);
      seen[want]++;
    }
    // Both classes that matter are exercised: valid points (and their
    // negations) and encodings that decode to no subgroup point.
    expect(seen.point).toBeGreaterThanOrEqual(160);
    expect(seen.invalid).toBeGreaterThanOrEqual(200);
  });
});