Shade/packages/shade-recovery/tests/shamir.test.ts

import { describe, test, expect } from 'bun:test';
import fc from 'fast-check';
import {
  combineShares,
  decodeShare,
  encodeShare,
  splitSecret,
  type ShamirShare,
} from '../src/shamir.js';

const cryptoRandom = (n: number): Uint8Array => {
  const out = new Uint8Array(n);
  globalThis.crypto.getRandomValues(out);
  return out;
};

describe('Shamir Secret Sharing', () => {
  test('split + combine roundtrip restores the secret (k=3, n=5)', () => {
    const secret = new Uint8Array([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]);
    const shares = splitSecret(secret, 3, 5, cryptoRandom);
    expect(shares.length).toBe(5);
    // Pick first 3 shares — any 3 should work.
    const subset = shares.slice(0, 3);
    const combined = combineShares(subset);
    expect(Array.from(combined)).toEqual(Array.from(secret));
  });

  test('any threshold-sized subset reconstructs', () => {
    const secret = cryptoRandom(32);
    const shares = splitSecret(secret, 3, 5, cryptoRandom);
    // All 10 possible 3-subsets must reconstruct.
    for (let i = 0; i < 5; i++) {
      for (let j = i + 1; j < 5; j++) {
        for (let k = j + 1; k < 5; k++) {
          const combined = combineShares([shares[i]!, shares[j]!, shares[k]!]);
          expect(Array.from(combined)).toEqual(Array.from(secret));
        }
      }
    }
  });

  test('exactly threshold shares reconstruct (k=2, n=2)', () => {
    const secret = new Uint8Array([0xde, 0xad, 0xbe, 0xef]);
    const shares = splitSecret(secret, 2, 2, cryptoRandom);
    const combined = combineShares(shares);
    expect(Array.from(combined)).toEqual(Array.from(secret));
  });

  test('k-1 shares yield a wrong (random-looking) result', () => {
    const secret = cryptoRandom(32);
    const shares = splitSecret(secret, 3, 5, cryptoRandom);
    const truncated = shares.slice(0, 2); // 2 < k=3
    const combined = combineShares(truncated);
    // We can't reliably assert "looks random", but we can assert
    // it's not equal to the secret (passing 2 shares to a polynomial
    // of degree 2 yields a different polynomial with prob ≈ 1).
    expect(Array.from(combined)).not.toEqual(Array.from(secret));
  });

  test('split rejects k < 1, n < k, n > 255', () => {
    expect(() => splitSecret(new Uint8Array([1]), 0, 5, cryptoRandom)).toThrow();
    expect(() => splitSecret(new Uint8Array([1]), 6, 5, cryptoRandom)).toThrow();
    expect(() => splitSecret(new Uint8Array([1]), 1, 256, cryptoRandom)).toThrow();
  });

  test('split rejects empty secret', () => {
    expect(() => splitSecret(new Uint8Array(0), 1, 1, cryptoRandom)).toThrow();
  });

  test('combine rejects empty share set', () => {
    expect(() => combineShares([])).toThrow();
  });

  test('combine rejects duplicate x-coordinates', () => {
    const secret = new Uint8Array([1, 2, 3]);
    const shares = splitSecret(secret, 2, 3, cryptoRandom);
    const dup: ShamirShare[] = [shares[0]!, { x: shares[0]!.x, y: shares[1]!.y }];
    expect(() => combineShares(dup)).toThrow(/duplicate x-coordinate/);
  });

  test('combine rejects mismatched share lengths', () => {
    const a: ShamirShare = { x: 1, y: new Uint8Array([1, 2, 3]) };
    const b: ShamirShare = { x: 2, y: new Uint8Array([1, 2]) };
    expect(() => combineShares([a, b])).toThrow(/length mismatch/);
  });

  test('encode + decode share roundtrip', () => {
    const share: ShamirShare = { x: 7, y: new Uint8Array([1, 2, 3, 4, 5]) };
    const bytes = encodeShare(share);
    expect(bytes.length).toBe(6);
    expect(bytes[0]).toBe(7);
    const decoded = decodeShare(bytes);
    expect(decoded.x).toBe(7);
    expect(Array.from(decoded.y)).toEqual([1, 2, 3, 4, 5]);
  });

  test('encodeShare rejects x out of range', () => {
    expect(() => encodeShare({ x: 0, y: new Uint8Array([1]) })).toThrow();
    expect(() => encodeShare({ x: 256, y: new Uint8Array([1]) })).toThrow();
  });

  test('decodeShare rejects x=0', () => {
    const bad = new Uint8Array([0, 1, 2, 3]);
    expect(() => decodeShare(bad)).toThrow();
  });

  test('property: random-secret roundtrip preserves bytes for arbitrary k/n', () => {
    fc.assert(
      fc.property(
        fc.uint8Array({ minLength: 1, maxLength: 64 }),
        fc.integer({ min: 1, max: 8 }),
        fc.integer({ min: 0, max: 8 }),
        (secret, k, extra) => {
          const n = k + extra;
          if (n > 16) return;
          const shares = splitSecret(secret, k, n, cryptoRandom);
          // Pick the first k shares — any k will do.
          const reconstructed = combineShares(shares.slice(0, k));
          expect(Array.from(reconstructed)).toEqual(Array.from(secret));
        },
      ),
      { numRuns: 50 },
    );
  });

  test('property: any k-1 share subset yields a different output than the secret', () => {
    // This is a probabilistic statement: with random secrets and
    // random polynomials, P(reconstruction collides with the secret
    // by accident) is ≈ 1/256^len, vanishingly small for 32-byte
    // secrets.
    fc.assert(
      fc.property(
        fc.uint8Array({ minLength: 16, maxLength: 32 }),
        fc.integer({ min: 2, max: 6 }),
        (secret, k) => {
          const n = k + 2;
          if (n > 16) return;
          const shares = splitSecret(secret, k, n, cryptoRandom);
          const subset = shares.slice(0, k - 1); // k-1 < threshold
          const combined = combineShares(subset);
          expect(Array.from(combined)).not.toEqual(Array.from(secret));
        },
      ),
      { numRuns: 30 },
    );
  });
});