How to mask all the characters after the N-th in an ASP.NET textbox? [duplicate]

I’m using a TextBox in WebForms to get user input but at the same time one of my requirements dictates that all the characters after the 3rd one should be masked, thereby appearing as ‘*’ (ex. Nor**). Is there a way to achieve this and keep the text intact for server-side processing?

I tried replacing those characters in client-side Javascript and keeping a ‘clean’ version in a hidden-field but it becomes a problem once user tries to add a character in the middle or delete.