It is a cardinal sin to ever store a user's password in plain text. Stored in plain text, a user's password is available to anyone from hackers sniffing network traffic to internal, disgruntled employees. Once these bad folks have clear text passwords they can easily get into associated accounts.
Modern applications today need to use cryptographically secure hashed passwords. Let's take a look at the abstract definition of how this works, then we'll look at the very few lines of AVR for .NET code required to implement it (thanks to the .NET Framework).
This article shows how to create cryptographically secure hashed passwords.