Authentication for clients?

Hey

Is there any unique string or number or something I can take to give some form of authentication that a request has come from within NS2?

I can GetSteamId from Client .. and GetUserName .. but is there anything I can get from the Client that will allow me some way to validate that a request has come from a user while the game is running and not someone trying to spoof ? some form of pgp from within the engine I could use a public key to check its valid?

Cheers,

Bal