MainWindow.xaml

<Window x:Class="WpfApplication7.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:c="clr-namespace:WpfApplication7"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <c:BindingData x:Key="bd" PreviewType="password"/>
    </Window.Resources>
    <StackPanel>
        <StackPanel DataContext="{StaticResource bd}">
            <TextBlock Text="{Binding PreviewType}"></TextBlock>
            <TextBox Text="{Binding InputString, Mode=OneWayToSource, UpdateSourceTrigger=PropertyChanged}"></TextBox>
            <TextBlock Text="{Binding InputString, Mode=OneWay}"></TextBlock>
            <TextBlock Text="{Binding PreviewString, Mode=OneWay}"></TextBlock>
        </StackPanel>
        <StackPanel>
            <StackPanel.DataContext>
                <c:BindingData PreviewType="raw"/>
            </StackPanel.DataContext>
            <TextBlock Text="{Binding PreviewType}"></TextBlock>
            <TextBox Text="{Binding InputString, Mode=OneWayToSource, UpdateSourceTrigger=PropertyChanged}"></TextBox>
            <TextBlock Text="{Binding InputString, Mode=OneWay}"></TextBlock>
            <TextBlock Text="{Binding PreviewString, Mode=OneWay}"></TextBlock>
        </StackPanel>
    </StackPanel>
</Window>

MainWindow.xaml.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApplication7
{
    public enum PreviewType {
        raw,
        password
    }

    public class BindingData : DependencyObject
    {
        public static readonly DependencyProperty PreviewTypeProperty = DependencyProperty.Register(
            "PreviewType", typeof(PreviewType), typeof(BindingData), new PropertyMetadata(PreviewType.raw));

        public PreviewType PreviewType {
            get { return (PreviewType)this.GetValue(PreviewTypeProperty); }
            set { this.SetValue(PreviewTypeProperty, value); }
        }

        public static readonly DependencyProperty InputStringProperty = DependencyProperty.Register(
        "InputString", typeof(string), typeof(BindingData), new FrameworkPropertyMetadata(new PropertyChangedCallback(OnInputStringPropertyChanged)));
        private static void OnInputStringPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var s = e.NewValue as string;
            switch ((PreviewType)d.GetValue(PreviewTypeProperty))
            {
                case PreviewType.password:
                    d.SetValue(PreviewStringProperty, new String((s.Select(c => '*').ToArray<char>())));
                    break;
                case PreviewType.raw:
                    d.SetValue(PreviewStringProperty, s);
                    break;
            }
        }
        public string InputString
        {
            get { return (string)this.GetValue(InputStringProperty); }
            set { this.SetValue(InputStringProperty, value); }
        }

        public static readonly DependencyProperty PreviewStringProperty = DependencyProperty.Register(
"PreviewString", typeof(string), typeof(BindingData), new PropertyMetadata(""));

        public string PreviewString
        {
            get { return (string)this.GetValue(PreviewStringProperty); }
            set { this.SetValue(PreviewStringProperty, value); }
        }
    }

    /// <summary>
    /// MainWindow.xaml の相互作用ロジック
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
    }
}