Question

What is the best method for saving passwords on a computer so that they can not be accessed? I would like to store them in the Registry encrypted. I would like you to be able to reset the password but this is not for the server. This is for storing them on a computer to remember them and sign in automatically.

IMPORTANT EDIT: I need to be able to retrieve the plain-text password from within the program, just not anywhere else.

Answer 1

CryptProtectData and CryptUnprotectData are your best bet on Windows. They encrypt the data using login credentials, so the passwords are safe from attacks to the disk. However, they can be accessed by any program running under the same user. I would recommend storing them in a file whose permissions prevent other programs from accessing them (such as a file that requires administrator privileges to access).

The managed class ProtectedData uses these function, so it can be used from C#.

You can also use these functions directly using P/Invoke. There is some example code that does exactly that here.

Expansion in response to additional requirements:

There is a way to ensure that your program is the only one able to access the password without needing your program to be launched with administrator privileges, though it will take a lot more work.

The basic idea is this: you create a Windows service that is installed when you install your application. It should be launched on demand from your application when it wants to store/retrieve the user’s password. The service will simply provide read/write access to a file with permissions set so that only administrators can read/write it. The additional security comes from the IPC connection to the process, which will use a Named Pipe. You can then use GetNamedPipeClientProcessId (sorry, you need P/Invoke) to authenticate the request by looking up the process ID of the client that connected to the pipe.

Depending on how worried you are about security, you can verify the process ID using code signing, if you have access to a valid certificate. Or, you can verify the checksum of the executable or something of that nature.

This is the only way I can think of to create the security you are looking for on Windows. Your application should also use ProtectedData to encrypt the data before handing it over to the Windows service to protect against hard disk attacks.