How to make a WPF TextBox use password characters?

You have to use PasswordBox instead of TextBox:

<PasswordBox Height="42" Width="200"  Margin="22,28,28,0"          Name="passwordBox1" VerticalAlignment="Top"          Background="LightBlue" Foreground="DarkBlue"          MaxLength="25" PasswordChar="*"          />

There's a new control in WPF designed for passwords, it's called PasswordBox. You should use that instead of a TextBox if you need to mask the input.

Here's a brief article about it. To retrieve the value that was entered, use the Password property.

EDIT: You've pretty much asked a new question - how can you unmask the text in a WPF PasswordBox? To the best of my knowledge you can't, though you could of course display it in a regular TextBox on demand by getting the value of the password from PasswordBox.Password

Databinding to a PasswordBox isn't possible without a custom helper class - though this would reduce the increased security offered by the new PasswordBox control (as described here). With that considered, this article includes a section on creating a helper class that allows you to databind to a PasswordBox.

Or use 'password font'. A font that shows all characters as dots. Remember to disable cut and copy clipboard functions.