U4.1 Secure Protocols

U4.1 Secure Protocols

1. What is a protocol?

   • Agreed sequence of actions or operations on the part of 1 or more parties
     • Set of rules defining how messages should be exchanged between 2 or more participants over a network
       • Protocols tell participants how to interpret messages
   • Many protocols are excecuted without human intervention
     • They run automatically as in Mobile phone or banking networks

2. What is a secure protocol?

   • 1. When acting honestly and following protocol rules the participants achieve the stated objective of the protocol
   • Neither a passive eavesdropper or malicious active adversary can defeat this objective
     • Passive = observing traffic
     • Active = impersonating a participant.

3. Modelling the participants

   • Alice and Bob = 2 way communication
     • Alice = A
     • Bob = B
     • Mostly not human, more likely hosts on a network or applications
   • In more complex protocols there will be 3rd Participant
     • A Trusted 3rd Party = Trent
       • TTP = Trusted 3rd Party
         • trusted not to eavesdrop, disclose shared keys or use keys to impersonate A or B
       • CA = Certificate Authority
       • KDC = Key distribution Centre
       • Helps produce session keys
   • Eve
     • Passive eavesdropper, just observes traffic, does no modification or impersonation
       • Easy to use a sniffer and be a passive observer
   • Mallory
     • Active attacker, very powerful
       • Can view, alter, delete, redirect, delay, replay messages
       • Can inject messages, initiate protocols, hijack protocol runs
       • Secure protocols limits what M can do
         • Cannot invert 1 way hash, sign messages without the private key, compute a MAC without the key
           • Preventing these M actions relies on the use of strong crypto mechanisms
     • Aims to defeat the stated objective of the protocol