aprhodite/static/crypto.js

/**
 * crypto.js – SexyChat SubtleCrypto wrapper
 *
 * Provides PBKDF2-derived AES-GCM-256 keys for end-to-end encrypted PMs.
 *
 * Key facts:
 *  - Keys are derived deterministically from (password, username) so the
 *    same credentials always produce the same key across sessions.
 *  - Keys are marked `extractable: true` so they can be exported as raw bytes
 *    for the AI transit-encryption flow (sent over HTTPS, never stored).
 *  - The server only ever receives ciphertext + nonce. It never sees a key
 *    for user-to-user conversations.
 */

"use strict";

const SexyChato = (() => {
  const PBKDF2_ITERATIONS = 100_000;
  const KEY_LENGTH_BITS   = 256;
  const SALT_PREFIX       = "sexychat:v1:";

  // ── Base64 helpers ────────────────────────────────────────────────────────

  function bufToBase64(buffer) {
    const bytes = new Uint8Array(buffer);
    let binary  = "";
    for (let i = 0; i < bytes.byteLength; i++) binary += String.fromCharCode(bytes[i]);
    return btoa(binary);
  }

  function base64ToBuf(b64) {
    const binary = atob(b64);
    const buf    = new Uint8Array(binary.length);
    for (let i = 0; i < binary.length; i++) buf[i] = binary.charCodeAt(i);
    return buf.buffer;
  }

  // ── Key derivation ─────────────────────────────────────────────────────────

  /**
   * Derive an AES-GCM key from a password and username.
   * The username acts as a deterministic, per-account salt.
   *
   * @param {string} password
   * @param {string} username
   * @returns {Promise<CryptoKey>}
   */
  async function deriveKey(password, username) {
    const enc  = new TextEncoder();
    const salt = enc.encode(SALT_PREFIX + username.toLowerCase());

    const baseKey = await crypto.subtle.importKey(
      "raw",
      enc.encode(password),
      { name: "PBKDF2" },
      false,
      ["deriveKey"],
    );

    return crypto.subtle.deriveKey(
      {
        name:       "PBKDF2",
        salt,
        iterations: PBKDF2_ITERATIONS,
        hash:       "SHA-256",
      },
      baseKey,
      { name: "AES-GCM", length: KEY_LENGTH_BITS },
      true,           // extractable – needed for transit encryption to AI endpoint
      ["encrypt", "decrypt"],
    );
  }

  // ── Encrypt / decrypt ─────────────────────────────────────────────────────

  /**
   * Encrypt a plaintext string with an AES-GCM key.
   *
   * @param {CryptoKey} key
   * @param {string}    plaintext
   * @returns {Promise<{ ciphertext: string, nonce: string }>}  both base64
   */
  async function encrypt(key, plaintext) {
    const nonce     = crypto.getRandomValues(new Uint8Array(12));
    const encoded   = new TextEncoder().encode(plaintext);
    const cipherBuf = await crypto.subtle.encrypt(
      { name: "AES-GCM", iv: nonce },
      key,
      encoded,
    );
    return {
      ciphertext: bufToBase64(cipherBuf),
      nonce:      bufToBase64(nonce),
    };
  }

  /**
   * Decrypt an AES-GCM ciphertext.
   *
   * @param {CryptoKey} key
   * @param {string}    ciphertext  base64
   * @param {string}    nonce       base64
   * @returns {Promise<string>}  plaintext
   */
  async function decrypt(key, ciphertext, nonce) {
    const cipherBuf = base64ToBuf(ciphertext);
    const nonceBuf  = base64ToBuf(nonce);
    const plainBuf  = await crypto.subtle.decrypt(
      { name: "AES-GCM", iv: nonceBuf },
      key,
      cipherBuf,
    );
    return new TextDecoder().decode(plainBuf);
  }

  /**
   * Export a CryptoKey as a base64-encoded raw byte string.
   * Used only for the AI transit flow (sent in POST body over HTTPS,
   * never stored by the server).
   *
   * @param {CryptoKey} key
   * @returns {Promise<string>}  base64
   */
  async function exportKeyBase64(key) {
    const raw = await crypto.subtle.exportKey("raw", key);
    return bufToBase64(raw);
  }

  /**
   * Import a base64-encoded raw AES-GCM key (e.g. server-provided room key).
   *
   * @param {string} b64  base64-encoded 256-bit key
   * @returns {Promise<CryptoKey>}
   */
  async function importKeyBase64(b64) {
    const buf = base64ToBuf(b64);
    return crypto.subtle.importKey(
      "raw",
      buf,
      { name: "AES-GCM", length: KEY_LENGTH_BITS },
      true,
      ["encrypt", "decrypt"],
    );
  }

  // ── Public API ─────────────────────────────────────────────────────────────
  return { deriveKey, encrypt, decrypt, exportKeyBase64, importKeyBase64 };
})();