Crypto U8 (part 2), Freshness

Crypto U8 (part 2), Freshness

1. freshness mechanism

   • techniques used to provide assurance that a message is new
     • i.e. a message is not a replay of an old message
   • not assurance of "aliveness" of sender because message could be delayed

2. 3 kinds

   • sequence number
     • sender looks up a sequence number (counter) from a database & sends with message
       • receiver looks up sequence number in DB and compares with sent sequence number
     • challenges
       • sender and receiver must have databases
       • DBs must be synchronized
       • communication delays matter only if messages get mixed up
       • need a way to ensure integrity of sequence #
         • often a MAC is used to ensure that the sequence # has not been altered
   • clock based
     • message sent with a time stamp
       • receiver checks to see if time is within "window of acceptability"
     • challenges
       • sender and receiver must have clocks
       • clocks must be synchronized
         • problem with "clock drift"
         • need clock resync method
       • not useful for scenarios with big communication delays
       • needs a way to ensure the integrity of timestamp
         • often a MAC is used to ensure the timestamp has not been altered
   • nonce based
     • nonce = "number used only once"
       • sender creates nonce and sends to receiver with message
         • receiver sends response along with originally sent nonce
       • challenges
         • freshness requires 2 communication passes
         • does not required synchronization of clocks or DBs
         • sender must have access to a random number generator
         • does not provide message authentication
         • attacker may have guessed the nonce (unlikely)