Here's how it would work:
- The server has a long term secret key K, which it never gives out - When the server wants to give a token to a client, it picks a random value R, and securely gives the client the values R and E_K(R) - When the client wants to use the token, it picks a value i, and computes the key Hash( R || i). It uses that key to protect the message, and also sends the server the values E(R) and i - The server decrypts the value E(R) to recover R, it computes Hash( R || i) to recover the message key, and then decrypts the message That way, the server doesn't have to send the client N different tokens...
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