Password generator using dart programming

import 'dart:convert';
import 'dart:io';
import 'dart:math';

void main() {
  stdout.write("How strong a password do you want? Weak, Medium or Strong: ");
  String choice = stdin.readLineSync().toLowerCase();

  passwordGenerator(choice);
}

// Create a random sequence of characters
void shuffleGenerator(int strength) {
  final random = Random.secure();
  List<int> intList = List.generate(strength, (_) => random.nextInt(255));
  List charList = base64UrlEncode(intList).split('').toList();
  charList.shuffle();
  print("\nYour password is: ${charList.join('')}\n");
}

void passwordGenerator(String strength) {
  if (strength == "weak") {
    shuffleGenerator(5);
  } else if (strength == "medium") {
    shuffleGenerator(15);
  } else if (strength == "strong") {
    shuffleGenerator(25);
  } else {
    print("Incorrect word is given");
  }
}