Network File System authentication process

When a client wants to talk to a server, it randomly generates a key used for encrypting the time stamps. This key is known as the conversation key (CK).

The client encrypts the conversation key using the DES common key (described in Authentication Requirements) and sends it to the server in the first RPC transaction. This process is illustrated in the following figure.

This figure shows client A connecting to server B. The term K(CK) means CK is encrypted with the DES common key K. In its first request, the client RPC credential contains the client name (A), the conversation key (CK), and the variable called win (window) encrypted with CK. (The default window size is 30 minutes.) The client verifier in the first request contains the encrypted time stamp and an encrypted verifier of the specified window, win + 1. The window verifier makes guessing the right credential much more difficult, and increases security.

After authenticating the client, the server stores the following items in a credential table:

• Client name, A
• Conversation key, CK
• Window
• Time stamp

The server only accepts time stamps that are chronologically greater than the last one seen, so any replayed transactions are guaranteed to be rejected. The server returns to the client in the verifier an index ID into the credential table, plus the client time stamp minus 1, encrypted by CK. The client knows that only the server could have sent such a verifier, because only the server knows what time stamp the client sent. The reason for subtracting 1 from the time stamp is to ensure that it is not valid and cannot be reused as a client verifier. After the first RPC transaction, the client sends just its ID and an encrypted time stamp to the server, and the server sends back the client time stamp minus 1, encrypted by CK.