U4.3 Fresheness

U4.3 Fresheness

1. Nonces

   • Number used only once. One time property
     • Sender creates a nonce and sends to receiver with message
       • Receiver sends response with originally sent nonce
     • Generated randomly to prevent attacker guessing

2. Time Stamps

   • Messages sent with time stamp
     • Sender sends the message which includes the time stamp concatenated with the receivers ID, encrypted with their shared key, all in a single message
   • Require synchronised clocks
   • Clock drift means there is a "window of acceptability" for the time to be out
     • window is decided by your level of risk acceptance

3. Sequence numbers

   • each time a message is sent a value is included
     • values increment by 1 with each message
       • If 1 participant receives a number which is lower than the value of the last received number the message is rejected