Username and password storage location

Question

I am writing a program in vb.net that requires a user to log in before he can use the application. The main user is created when the program is installed, similar to how windows works when it is installed.

The main user can add additional users to the program. I already know that I should store the passwords encrypted. My question is, where should I store the usernames and passwords? The registry, Isolated storage or .config file. I don't want any user to be able to modify/delete that file as the other user would obviously not be able to log in. Also, this file should be accessible for any user that logs into the computer.

The computer is not guaranteed to be connected to the internet, so it must be stored locally.

Thanks

Answer 1

If you are not able to use windows users/credentials in any way on the machine, then there really is no absolute way to prevent the file from being removed/changed, Since anyone on the computer has the same access as the main user, who needs rights to modify the file in order for him to add users through the program.

The only way to do it for sure is to have the main user logon with a different user name, and set the file permissions on that file/folder to make sure that only the main user has modify permission to the file (and the other user account does not have the right to modify permissions). I know you said it wouldn't work in your environment(which is?) but you might be able to create users and run stuff under different credentials through your code without having the users log on any different.

The only crazy way I can think of is to create a service on the computer that once it starts running, it opens and holds a handle to that file with sharing set such that no other process can open the file for writing. You'd of course have to workout some way for the main user to be able to add users.

Answer 2

To tell you the truth if someone has the will power to look for the file they will find it, so storage can help up security but I would focus on the contents of the file itself.

You could try to store the data of the application as a encrypted file which could stop the amateur attempts but as you are using the .net framework your program could could be decompiled and any symmetric encryption algorithms could be rendered useless.

I think your best bet would be to either generate a seed according to the computer the program is on, and if decryption fails call home or go into Lock Down.

Another option would be to store the encrypted (encrypted with your symmetric key) file and a hash file (in different locations probably). If the hash of the loaded file then does not match the hash file your program could then call home (If you have a home to call).

This is just a idea, haven't actually tried anything like this.