In this key establishment protocol, - A interacts with a trusted server T and a party B. - A and T share a symmetric key KAT, and B and T share a symmetric key KBT. - T will choose a key k to be shared by A and B. - NA and NB are nonces chosen by A and B respectively. - We will write EK(x) to mean encrypting a message x using a symmetric key encryption algorithm E with key K. The protocol runs as follows: 1. AT:A,B,NA. 2. TA:EKAT(NA,B,k,EKBT(k,A)). 3. AB:EKBT(k,A). 4. BA:Ek(NB). 5. AB:Ek(NB1). What guarantees the freshness of the message A receives from T ? NA in message 1. KBT in message 2. B in message 1. A in message 1. NA in message 2. KAT in message 2