MainPage.xaml.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Navigation;
using Microsoft.Phone.Controls;
using Microsoft.Phone.Shell;
using input_password.Resources;
using System.Windows.Input;
using System.Text.RegularExpressions;

namespace input_password
{
    public partial class MainPage : PhoneApplicationPage
    {
        string _enteredPasscode = "";
        string _passwordChar = "*";
        // Constructor
        public MainPage()
        {
            InitializeComponent();
        }

        private void PasswordTextBox_KeyUp(object sender, KeyEventArgs e)
        {
            //modify new passcode according to entered key
            _enteredPasscode = GetNewPasscode(_enteredPasscode, e);
            ActualPasscode.Text = _enteredPasscode;
            //replace text by *
            PasswordTextBox.Text = Regex.Replace(_enteredPasscode, @".", _passwordChar);

            //take cursor to end of string
            PasswordTextBox.SelectionStart = PasswordTextBox.Text.Length;
        }


        private string GetNewPasscode(string oldPasscode, KeyEventArgs keyEventArgs)
        {
            string newPasscode = string.Empty;
            switch (keyEventArgs.Key)
            {
                case Key.D0:
                case Key.D1:
                case Key.D2:
                case Key.D3:
                case Key.D4:
                case Key.D5:
                case Key.D6:
                case Key.D7:
                case Key.D8:
                case Key.D9:
                    newPasscode = oldPasscode + (keyEventArgs.PlatformKeyCode - 48);
                    break;
                case Key.Back:
                    if (oldPasscode.Length > 0)
                        newPasscode = oldPasscode.Substring(0, oldPasscode.Length - 1);
                    break;
                default:
                    //others
                    newPasscode = oldPasscode;
                    break;
            }
            return newPasscode;
        }
    }
}