PassKeeper/app/static/js/sharing.js

/**
 * sharing.js — ECDH P-256 cryptography for zero-knowledge item sharing
 *
 * Each user has a P-256 keypair:
 *   - Public key  : stored on server as base64 raw bytes (65-byte uncompressed point)
 *   - Private key : stored on server as JWK, AES-256-GCM encrypted with the user's vault key
 *
 * When Alice shares with Bob:
 *   1. Alice fetches Bob's public key from the server
 *   2. Alice derives an ECDH shared secret: ECDH(Alice_priv, Bob_pub)
 *   3. Alice imports that shared secret as an AES-256-GCM key
 *   4. Alice encrypts the item's plaintext → (enc_data, iv)
 *   5. Alice POST /api/sharing with the ciphertext — server stores the blob
 *
 * When Bob decrypts:
 *   1. Bob fetches Alice's public key from the server (returned in inbox response)
 *   2. Bob derives the same ECDH shared secret: ECDH(Bob_priv, Alice_pub)  ← commutative!
 *   3. Bob decrypts enc_data with the derived key
 *
 * The server only ever sees ciphertext. Zero-knowledge.
 */

const SharingCrypto = (() => {
  const subtle = window.crypto.subtle;

  // ── Helpers (same encoding as crypto.js) ─────────────────────────────────

  function base64ToBytes(b64) {
    const bin = atob(b64);
    const bytes = new Uint8Array(bin.length);
    for (let i = 0; i < bin.length; i++) bytes[i] = bin.charCodeAt(i);
    return bytes;
  }

  function bytesToBase64(bytes) {
    let bin = '';
    bytes.forEach(b => (bin += String.fromCharCode(b)));
    return btoa(bin);
  }

  // ── Key generation ────────────────────────────────────────────────────────

  /**
   * Generate a fresh ECDH P-256 keypair.
   * Both keys are extractable so they can be exported/stored.
   */
  async function generateKeyPair() {
    return subtle.generateKey(
      { name: 'ECDH', namedCurve: 'P-256' },
      true,
      ['deriveBits'],
    );
  }

  /**
   * Export the public key as raw bytes (uncompressed point, 65 bytes) → base64.
   * This is what gets stored on the server and shared with other users.
   */
  async function exportPublicKey(publicKey) {
    const raw = await subtle.exportKey('raw', publicKey);
    return bytesToBase64(new Uint8Array(raw));
  }

  /**
   * Encrypt the private key JWK with the user's vault key (AES-256-GCM).
   * The resulting ciphertext is stored on the server — only the user can decrypt it.
   */
  async function encryptPrivateKey(vaultKey, privateKey) {
    const jwk = await subtle.exportKey('jwk', privateKey);
    const iv = window.crypto.getRandomValues(new Uint8Array(12));
    const plaintext = new TextEncoder().encode(JSON.stringify(jwk));
    const ciphertext = await subtle.encrypt({ name: 'AES-GCM', iv }, vaultKey, plaintext);
    return {
      private_key_enc: bytesToBase64(new Uint8Array(ciphertext)),
      private_key_iv: bytesToBase64(iv),
    };
  }

  /**
   * Decrypt the private key JWK (fetched from server) using the user's vault key.
   * Returns a CryptoKey usable for ECDH deriveBits.
   */
  async function decryptPrivateKey(vaultKey, private_key_enc, private_key_iv) {
    const plaintext = await subtle.decrypt(
      { name: 'AES-GCM', iv: base64ToBytes(private_key_iv) },
      vaultKey,
      base64ToBytes(private_key_enc),
    );
    const jwk = JSON.parse(new TextDecoder().decode(plaintext));
    return subtle.importKey(
      'jwk',
      jwk,
      { name: 'ECDH', namedCurve: 'P-256' },
      false,
      ['deriveBits'],
    );
  }

  /**
   * Import a remote user's public key from its base64 raw representation.
   */
  async function importPublicKey(base64Raw) {
    return subtle.importKey(
      'raw',
      base64ToBytes(base64Raw),
      { name: 'ECDH', namedCurve: 'P-256' },
      false,
      [],  // public keys have no usages in WebCrypto ECDH
    );
  }

  // ── Shared-secret derivation ──────────────────────────────────────────────

  /**
   * Derive an AES-256-GCM CryptoKey from the ECDH shared secret.
   * ECDH is commutative: ECDH(A_priv, B_pub) === ECDH(B_priv, A_pub).
   */
  async function deriveSharedKey(myPrivateKey, theirPublicKey) {
    const bits = await subtle.deriveBits(
      { name: 'ECDH', public: theirPublicKey },
      myPrivateKey,
      256,
    );
    return subtle.importKey(
      'raw',
      bits,
      { name: 'AES-GCM' },
      false,
      ['encrypt', 'decrypt'],
    );
  }

  // ── Encrypt / Decrypt with shared key ────────────────────────────────────

  async function encryptForShare(sharedKey, plaintextObject) {
    const iv = window.crypto.getRandomValues(new Uint8Array(12));
    const plaintext = new TextEncoder().encode(JSON.stringify(plaintextObject));
    const ciphertext = await subtle.encrypt({ name: 'AES-GCM', iv }, sharedKey, plaintext);
    return {
      enc_data: bytesToBase64(new Uint8Array(ciphertext)),
      iv: bytesToBase64(iv),
    };
  }

  async function decryptShare(sharedKey, enc_data, iv) {
    const plaintext = await subtle.decrypt(
      { name: 'AES-GCM', iv: base64ToBytes(iv) },
      sharedKey,
      base64ToBytes(enc_data),
    );
    return JSON.parse(new TextDecoder().decode(plaintext));
  }

  // ── Public API ────────────────────────────────────────────────────────────

  return {
    generateKeyPair,
    exportPublicKey,
    encryptPrivateKey,
    decryptPrivateKey,
    importPublicKey,
    deriveSharedKey,
    encryptForShare,
    decryptShare,
  };
})();

/**
 * SharingSession — holds the decrypted ECDH private key for the tab lifetime.
 * Similar to VaultSession; cleared on sign-out.
 */
const SharingSession = (() => {
  let _privateKey = null;

  function setKey(key) { _privateKey = key; }
  function getKey() { return _privateKey; }
  function clear() { _privateKey = null; }
  function isReady() { return _privateKey !== null; }

  return { setKey, getKey, clear, isReady };
})();