How to AES-256 (CBC/CFB mode) encrypt and decrypt in Dart/Flutter with Pointy Castle

aes_encryption_helper.dart

import 'dart:convert';
import 'dart:typed_data';
import "package:pointycastle/export.dart";

import "./convert_helper.dart";

// AES key size
const KEY_SIZE = 32; // 32 byte key for AES-256
const ITERATION_COUNT = 1000;

class AesHelper {
  static const CBC_MODE = 'CBC';
  static const CFB_MODE = 'CFB';

  static Uint8List deriveKey(dynamic password,
      {String salt = '',
      int iterationCount = ITERATION_COUNT,
      int derivedKeyLength = KEY_SIZE}) {
    if (password == null || password.isEmpty) {
      throw new ArgumentError('password must not be empty');
    }

    if (password is String) {
      password = createUint8ListFromString(password);
    }

    Uint8List saltBytes = createUint8ListFromString(salt);
    Pbkdf2Parameters params =
        new Pbkdf2Parameters(saltBytes, iterationCount, derivedKeyLength);
    KeyDerivator keyDerivator =
        new PBKDF2KeyDerivator(new HMac(new SHA256Digest(), 64));
    keyDerivator.init(params);

    return keyDerivator.process(password);
  }

  static Uint8List pad(Uint8List src, int blockSize) {
    var pad = new PKCS7Padding();
    pad.init(null);

    int padLength = blockSize - (src.length % blockSize);
    var out = new Uint8List(src.length + padLength)..setAll(0, src);
    pad.addPadding(out, src.length);

    return out;
  }

  static Uint8List unpad(Uint8List src) {
    var pad = new PKCS7Padding();
    pad.init(null);

    int padLength = pad.padCount(src);
    int len = src.length - padLength;

    return new Uint8List(len)..setRange(0, len, src);
  }

  static String encrypt(String password, String plaintext,
      {String mode = CBC_MODE}) {
    Uint8List derivedKey = deriveKey(password);
    KeyParameter keyParam = new KeyParameter(derivedKey);
    BlockCipher aes = new AESFastEngine();

    var rnd = FortunaRandom();
    rnd.seed(keyParam);
    Uint8List iv = rnd.nextBytes(aes.blockSize);

    BlockCipher cipher;
    ParametersWithIV params = new ParametersWithIV(keyParam, iv);
    switch (mode) {
      case CBC_MODE:
        cipher = new CBCBlockCipher(aes);
        break;
      case CFB_MODE:
        cipher = new CFBBlockCipher(aes, aes.blockSize);
        break;
      default:
        throw new ArgumentError('incorrect value of the "mode" parameter');
        break;
    }
    cipher.init(true, params);

    Uint8List textBytes = createUint8ListFromString(plaintext);
    Uint8List paddedText = pad(textBytes, aes.blockSize);
    Uint8List cipherBytes = _processBlocks(cipher, paddedText);
    Uint8List cipherIvBytes = new Uint8List(cipherBytes.length + iv.length)
      ..setAll(0, iv)
      ..setAll(iv.length, cipherBytes);

    return base64.encode(cipherIvBytes);
  }

  static String decrypt(String password, String ciphertext,
      {String mode = CBC_MODE}) {
    Uint8List derivedKey = deriveKey(password);
    KeyParameter keyParam = new KeyParameter(derivedKey);
    BlockCipher aes = new AESFastEngine();

    Uint8List cipherIvBytes = base64.decode(ciphertext);
    Uint8List iv = new Uint8List(aes.blockSize)
      ..setRange(0, aes.blockSize, cipherIvBytes);

    BlockCipher cipher;
    ParametersWithIV params = new ParametersWithIV(keyParam, iv);
    switch (mode) {
      case CBC_MODE:
        cipher = new CBCBlockCipher(aes);
        break;
      case CFB_MODE:
        cipher = new CFBBlockCipher(aes, aes.blockSize);
        break;
      default:
        throw new ArgumentError('incorrect value of the "mode" parameter');
        break;
    }
    cipher.init(false, params);

    int cipherLen = cipherIvBytes.length - aes.blockSize;
    Uint8List cipherBytes = new Uint8List(cipherLen)
      ..setRange(0, cipherLen, cipherIvBytes, aes.blockSize);
    Uint8List paddedText = _processBlocks(cipher, cipherBytes);
    Uint8List textBytes = unpad(paddedText);

    return new String.fromCharCodes(textBytes);
  }

  static Uint8List _processBlocks(BlockCipher cipher, Uint8List inp) {
    var out = new Uint8List(inp.lengthInBytes);

    for (var offset = 0; offset < inp.lengthInBytes;) {
      var len = cipher.processBlock(inp, offset, out, offset);
      offset += len;
    }

    return out;
  }
}

convert_helper.dart

import "dart:typed_data";
import 'dart:convert';
import 'package:convert/convert.dart' as convert;

Uint8List createUint8ListFromString(String s) {
  var ret = new Uint8List(s.length);
  for (var i = 0; i < s.length; i++) {
    ret[i] = s.codeUnitAt(i);
  }
  return ret;
}

Uint8List createUint8ListFromHexString(String hex) {
  var result = new Uint8List(hex.length ~/ 2);
  for (var i = 0; i < hex.length; i += 2) {
    var num = hex.substring(i, i + 2);
    var byte = int.parse(num, radix: 16);
    result[i ~/ 2] = byte;
  }
  return result;
}

Uint8List createUint8ListFromSequentialNumbers(int len) {
  var ret = new Uint8List(len);
  for (var i = 0; i < len; i++) {
    ret[i] = i;
  }
  return ret;
}

String formatBytesAsHexString(Uint8List bytes) {
  var result = new StringBuffer();
  for (var i = 0; i < bytes.lengthInBytes; i++) {
    var part = bytes[i];
    result.write('${part < 16 ? '0' : ''}${part.toRadixString(16)}');
  }
  return result.toString();
}

List<int> decodePEM(String pem) {
  var startsWith = [
    "-----BEGIN PUBLIC KEY-----",
    "-----BEGIN PRIVATE KEY-----",
    "-----BEGIN ENCRYPTED MESSAGE-----",
    "-----BEGIN PGP PUBLIC KEY BLOCK-----\r\nVersion: React-Native-OpenPGP.js 0.1\r\nComment: http://openpgpjs.org\r\n\r\n",
    "-----BEGIN PGP PRIVATE KEY BLOCK-----\r\nVersion: React-Native-OpenPGP.js 0.1\r\nComment: http://openpgpjs.org\r\n\r\n",
  ];
  var endsWith = [
    "-----END PUBLIC KEY-----",
    "-----END PRIVATE KEY-----",
    "-----END ENCRYPTED MESSAGE-----",
    "-----END PGP PUBLIC KEY BLOCK-----",
    "-----END PGP PRIVATE KEY BLOCK-----",
  ];
  bool isOpenPgp = pem.indexOf('BEGIN PGP') != -1;

  for (var s in startsWith) {
    if (pem.startsWith(s)) {
      pem = pem.substring(s.length);
    }
  }

  for (var s in endsWith) {
    if (pem.endsWith(s)) {
      pem = pem.substring(0, pem.length - s.length);
    }
  }

  if (isOpenPgp) {
    var index = pem.indexOf('\r\n');
    pem = pem.substring(0, index);
  }

  pem = pem.replaceAll('\n', '');
  pem = pem.replaceAll('\r', '');

  return base64.decode(pem);
}

List<int> decodeHex(String hex) {
  hex = hex
      .replaceAll(':', '')
      .replaceAll('\n', '')
      .replaceAll('\r', '')
      .replaceAll('\t', '');

  return convert.hex.decode(hex);
}

Hi,
I wanted to use this for my own mobile project and It found that the ITERATION_COUNT is too large in the mobile operation and seems to block execution time(for like at least like1-2 sec) even when used with future async. So, I reduced it to 250 which didn't block the execution that long.
My question is, is it alright to reduce the ITERATION_COUNT?

Hellow
Why use it
KeyDerivator keyDerivator =
new PBKDF2KeyDerivator(new HMac(new SHA1Digest(), 64));
The decrypted data is preceded by IV