symKey = await this.importSymKey(symKeyStr, operation);
    let cryptMethod = (
      operation === OPERATIONS.ENCRYPT
        ? crypto.subtle.encrypt
        : crypto.subtle.decrypt
    ).bind(crypto.subtle);
    let algo = { name: CRYPT_ALGO, iv };

    let keyBytes = await cryptMethod.call(crypto.subtle, algo, symKey, data);
    return new Uint8Array(keyBytes);
  },

  async generateRandomKey() {
    this.log("generateRandomKey() called");
    let algo = {
      name: CRYPT_ALGO,
      length: CRYPT_ALGO_LENGTH,
    };
    let key = await crypto.subtle.generateKey(algo, true, CRYPT_ALGO_USAGES);
    let keyBytes = await crypto.subtle.exportKey("raw", key);
    return this.encodeBase64(new Uint8Array(keyBytes));
  },

  generateRandomIV() {
    return this.generateRandomBytes(AES_CBC_IV_SIZE);
  },

  generateRandomBytes(byteCount) {
    this.log("generateRandomBytes() called");

    let randBytes = new Uint8Array(byteCount);
    crypto.getRandomValues(randBytes);

    return this.encodeBase64(randBytes);
  },

  //
  // SymKey CryptoKey memoization.
  //

  // Memoize the import of symmetric keys. We do this by using the base64
  // string itself as a key.
  _encryptionSymKeyMemo: {},
  _decryptionSymKeyMemo: {},
  async importSymKey(encodedKeyString, operation) {
    let memo;

    // We use two separate memos for thoroughness: operation is an input to
    // key import.
    switch (operation) {
      case OPERATIONS.ENCRYPT:
        memo = this._encryptionSymKeyMemo;
        break;
      case OPERATIONS.DECRYPT:
        memo = this._decryptionSymKeyMemo;
        break;
      default:
        throw new Error("Unsupported operation in importSymKey.");
    }

    if (encodedKeyString in memo) {
      return memo[encodedKeyString];
    }

    let symmetricKeyBuffer = this.makeUint8Array(encodedKeyString, true);
    let algo = { name: CRYPT_ALGO };
    let usages = [operation === OPERATIONS.ENCRYPT ? "encrypt" : "decrypt"];
    let symKey = await crypto.subtle.importKey(
      "raw",
      symmetricKeyBuffer,
      algo,
      false,
      usages
    );
    memo[encodedKeyString] = symKey;
    return symKey;
  },

  //
  // Utility functions
  //

  /**
   * Returns an Uint8Array filled with a JS string,
   * which means we only keep utf-16 characters from 0x00 to 0xFF.
   */
  byteCompressInts(str) {
    let arrayBuffer = new Uint8Array(str.length);
    for (let i = 0; i < str.length; i++) {
      arrayBuffer[i] = str.charCodeAt(i) & 0xff;
    }
    return arrayBuffer;
  },

  expandData(data) {
    let expanded = "";
    for (let i = 0; i < data.length; i++) {
      expanded += String.fromCharCode(data[i]);
    }
    return expanded;
  },

  encodeBase64(data) {
    return btoa(this.expandData(data));
  },

  makeUint8Array(input, isEncoded) {
    if (isEncoded) {
      input = atob(input);
    }
    return this.byteCompressInts(input);
  },
};
PK