Deposit-case attack against secure roaming
A secure roaming protocol involves three parties: a roaming user, a visiting foreign server and the user's home server. The protocol allows the user and the foreign server to establish a session key and carry out mutual authentication with the help of the home server. In the mutual authentication, user authentication is generally done in two steps. First, the user claims that a particular server is his home server. Second, that particular server is called in by the foreign server for providing a 'credential' which testifies the user's claim. We present a new attacking technique which allows a malicious server to modify the user's claim in the first step without being detected and provide a fake credential to the foreign server in the second step in such a way that the foreign server believes that the malicious server is the user's home server. We give some examples to explain why it is undesirable in practice if a roaming protocol is vulnerable to this attack. We also show that there are three roaming protocols proposed previously which are vulnerable to this attack.