PasswordBox and MVVM

I have solved this problem by creating a UserControl that exposes a SecureString dependency property that can be bound to. This method keeps the password in a SecureString at all times, and doesn't "break" MVVM.

UserControl

XAML

<UserControl x:Class="Example.PasswordUserControl"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
         mc:Ignorable="d"
         d:DesignHeight="300" d:DesignWidth="300">
    <Grid>       
        <PasswordBox Name="PasswordBox" />
    </Grid>
</UserControl>

CS

public partial class PasswordUserControl : UserControl
{
    public SecureString Password
    {
        get { return (SecureString) GetValue(PasswordProperty); }
        set { SetValue(PasswordProperty, value); }
    }
    public static readonly DependencyProperty PasswordProperty =
        DependencyProperty.Register("Password", typeof(SecureString), typeof(UserCredentialsInputControl),
            new PropertyMetadata(default(SecureString)));


    public PasswordUserControl()
    {
        InitializeComponent();

        // Update DependencyProperty whenever the password changes
        PasswordBox.PasswordChanged += (sender, args) => {
            Password = ((PasswordBox) sender).SecurePassword;
        };
    }
}

Example usage

Using the control is very straightforward, just bind the password DependencyProperty on the control to a Password property on your ViewModel. The ViewModel's Password property should be a SecureString.

<controls:PasswordUserControl Password="{Binding Password, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>

Change the Mode and UpdateSource trigger on the binding to whatever is best for you.

If you need the password in plain text, the following page describes the proper way to convert between SecureString and string: http://blogs.msdn.com/b/fpintos/archive/2009/06/12/how-to-properly-convert-securestring-to-string.aspx. Of course you shouldn't store the plain text string...

Personally I just pass the entire PasswordBox control to my LoginCommand

I know it breaks MVVM because the ViewModel layer now references a View-specific object, but I think in this specific case it's OK.

So I might have XAML that looks like this:

<Button Content="Login" 
        Command="{Binding LoginCommand}" 
        CommandParameter="{Binding ElementName=MyPasswordBox}" />

And a LoginCommand that does something like this:

private void Login(object obj)
{
    PasswordBox pwBox = obj as PasswordBox;

    SomeBlackBoxClass.ValidatePassword(UserName, pwBox.Password);
}

I suppose you could also run some kind of encryption algorithm on the value and compare the hash of that value to the hash of the user's password too

private void Login(object obj)
{
    PasswordBox pwBox = obj as PasswordBox;
    var encryptedPassword = SomeLibrary.EncryptValue(pwBox.Password, someKey);

    if (encryptedPassword == User.EncryptedPassword)
        // Success
}

I'm no expert on the PasswordBox control or security, but I do know that you don't want to be storing the user's password in plain text anywhere in memory within your application

(Technically, it's stored as plain text in PasswordBox.Password - you can use something like Snoop to verify this if you want - however typically the PasswordBox doesn't exist for longer than it takes the user to login, and the actual "password" is just text entered by the user, which may or may not be correct. A keylogger could get you the same information.)