testEncodesWithOffset(base64(), "foobar", 3, 1, "Yg==");
    testEncodesWithOffset(base64(), "foobar", 4, 0, "");
  }

  public void testBase64Url() {
    testDecodesByBytes(base64Url(), "_zzz", new byte[] {-1, 60, -13});
    testDecodesByBytes(base64Url(), "-zzz", new byte[] {-5, 60, -13});
  }

  public void testBase64UrlInvalidDecodings() {
    assertFailsToDecode(base64Url(), "+zzz", "Unrecognized character: +");