Question

Problem 5: Key Distribution and Authentication Recall the following X.509 authentication and key distribution protocols discussed in class 12.1,".B,other..[Kablpxsx, Two-msg A B PK, SK Three-msg Where ti is the time-stamp from party i, ri Is a nonce generated by party i, Прк, denotes encryption using party i's public key, and s, denotes a message signed using i's private key. A and B represent Alice's and Bob's identifiers, respectively. For each of the following changes made to these protocols, comment on the security (or insecurity) of the resulting protocol as compared to the original. (Each answer in at most three sentences) I. Removing rb from Bob's response to Alice in the TWO-message protocol 2. Removing tb from Bob's response to Alice in the THREE-message protocol

Answer 1

Answer

1. r_b is used for nonce generated by Bob. Nonce is a special character added in encrypted message to avoid replay attack which can be caused by sending the same message from different machine using same IP address. Nonce used only once not repeatedly.Removing  r_b from Bob's response to Alice will create possibility of replay attack.

2. t_b represents the timestamp required for transfer of message. t_b gives the message arrival info from Bob to Alice.As we are using unique key in message known as nonce,it is important to know which messsage received first to discard later massages for avoiding replay attack.Removing timestamp (t_b) from message will create possibility of replay attack