[WPF]PasswordBoxのセキュアなパスワードSecurePasswordにバインドしたい ref: http://qiita.com/pierusan2010/items/5d4ceb28ee18cd4e3853

file0.cs

public static class SecureStringEx
{
    public static bool Equals(this SecureString a, SecureString b)
    {
        if (a == null && b == null)
         { return true; }

        if (a == null || b == null)
        { return false; }

        if (a.Length != b.Length)
        { return false; }

        var aPtr = Marshal.SecureStringToBSTR(a);
        var bPtr = Marshal.SecureStringToBSTR(b);
        try
        {
            return Enumerable.Range(0, a.Length)
              .All(i => Marshal.ReadInt16(aPtr, i) == Marshal.ReadInt16(bPtr, i));
        }
        finally
        {
            Marshal.ZeroFreeBSTR(aPtr);
            Marshal.ZeroFreeBSTR(bPtr);
        }
    }
}

file1.cs

public static class SecureStringEx
{
    public static void CopyFromBSTR(this SecureString self, IntPtr bstr, int count)
    {
        self.Clear();
        var chars = Enumerable.Range(0, count)
            .Select(i => (char)Marshal.ReadInt16(bstr, i * 2));
        foreach (var c in chars)
        {
            self.AppendChar(c);
        }
    }
}

file2.cs

using System.Runtime.InteropServices;
using System.Security;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Interactivity;

public class SecurePasswordBindingBehavior : Behavior<PasswordBox>
{
    public SecureString SecurePassword
    {
        get { return (SecureString)GetValue(SecurePasswordProperty); }
        set { SetValue(SecurePasswordProperty, value); }
    }

    public static readonly DependencyProperty SecurePasswordProperty =
        DependencyProperty.Register("SecurePassword", 
        typeof(SecureString), 
        typeof(SecurePasswordBindingBehavior), 
        new PropertyMetadata(new SecureString(), SecurePasswordPropertyChanged));

    private static void SecurePasswordPropertyChanged(DependencyObject d, 
        DependencyPropertyChangedEventArgs e)
    {
        var behavior = d as SecurePasswordBindingBehavior;
        if (d == null)
        { return; }

        var passwordBox = behavior.AssociatedObject as PasswordBox;
        if (passwordBox == null)
        { return; }

        var newPassword = e.NewValue as SecureString;
        if (newPassword == null)
        { return; }

        var oldPassword = e.OldValue as SecureString;
        if (newPassword.Equals(oldPassword))
        { return; }

        var bstr = Marshal.SecureStringToBSTR(newPassword);
        try
        {
            passwordBox.SecurePassword.CopyFromBSTR(bstr, newPassword.Length);
        }
        finally
        {
            Marshal.ZeroFreeBSTR(bstr);
        }
    }

    protected override void OnAttached()
    {
        base.OnAttached();
        AssociatedObject.PasswordChanged += PasswordBox_PasswordChanged;
    }

    protected override void OnDetaching()
    {
        AssociatedObject.PasswordChanged -= PasswordBox_PasswordChanged;
        base.OnDetaching();
    }

    private void PasswordBox_PasswordChanged(object sender, RoutedEventArgs e)
    {
        SecurePassword = AssociatedObject.SecurePassword.Copy();
    }
}

file3.xml

<Window
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
        Title="Input Password" Height="420" Width="800">
    <Grid>
        <PasswordBox Width="120" PasswordChar="*">
            <i:Interaction.Behaviors>
                <my:SecurePasswordBindingBehavior 
                   SecurePassword="{Binding SecurePassword, Mode=TwoWay}"/>
             </i:Interaction.Behaviors>
        </PasswordBox>
    </Grid>
</Window>