Securely store a password in program code?

Question

My application makes use of the RijndaelManaged class to encrypt data. As a part of this encryption, I use a SecureString object loaded with a password which get's get converted to a byte array and loaded into the RajindaelManaged object's Key at runtime.

The question I have is the storage of this SecureString. A user entered password can be entered at run-time, and that can be "securely" loaded into a SecureString object, but if no user entered password is given, then I need to default to something.

So ultimately the quesiton comes down to:

If I have to have some known string or byte array to load into a SecureString object each time my application runs, how do I do that? The "encrypted" data ultimately gets decrypted by another application, so even if no user entered password is specified, I still need the data to be encrypted while it goes from one app to another. This means I can't have the default password be random, because the other app wouldn't be able to properly decrypt it.

One possible solution I'm thinking is to create a dll which only spits out a single passphrase, then I use that passphrase and run it through a couple of different hashing/reorganizing functions at runtime before I ultimately feed it into the secureString object. Would this be secure enough?

Edit For clarity*: The encrypted data is being passed via files between machines. Think of it as a Zip file which always has a password, a default one is assumed if nothing is directly entered by the user.

Answer 1

There is no point in symmetrically encrypting with a string that's hard-coded into your executable. It will only give a false sense of security. No amount of hashing fixes this scheme.

See this Pidgin FAQ for the same point in a different context.

I am unclear why you think you need the inter-app communication to be encrypted. If this communication is local to the machine, then I don't see the need for encryption, particularly encryption that isn't user-specific. Is this a DRM scheme?

EDIT: If it's being passed to a different machine, perhaps you can hard-code a public key, and then have the other machine decrypt with the matching private key.

Answer 2

Let me tackle your final question first.

"Would this be secure enough?"

The only one that can answer that is you. Nobody here knows what "secure enough" means in the context of your application.

Are you building an application to keep the diary of teenage girls? Sure, it would be "secure enough".

Are you building an application to encrypt information or authentication for military grade secure systems? Nope, not even close.

You can only rely on one type of security if you intend to store the password in your source code and thus executable, and that is security by obscurity.

If your problem is that you can't, or won't, store the password in the source code, then moving it into a separate dll solves nothing, you've just moved the problem to a different project.

However, I'm wondering about something. You say "I have to default to something". Is that it? You're trying to store a default value for the secure password string in the source code? How about "THISISNOTAPASSWORD"?