IT IS 6200/8200.

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Presentation transcript:

IT IS 6200/8200

Authentication & key exchange Try to merge the previous two steps: Establish the session key and persuade Alice that you are Bob Tens of protocols have been developed: some good, some bad Lesson we learn: Do not try to be smart Do not make messages too similar to each other

Protocol 1: Wide-Mouth Frog Two-step protocol through TTP Problems: Why we need the timestamp: resend attack With timestamp: still not safe Message 1 and 2 too similar to each other M keeps the timestamp fresh until it crashes the key Is the key generated by Alice secure enough

Protocol 2: Needham-Schroeder 5 step protocol The freshness is protected by random numbers and it seems that A has assured it. What about B: K is for A and B, is it fresh? Is the other side A? (we can only verify that the other side knows K) Problem: old key matters (B is not involved until step 3) Solution: add a timestamp

Protocol 3: Otway-Rees ( a very good protocol) 4 steps The freshness is protected by the random numbers Both parties get involved from the very beginning

The Protocol n || Alice || Bob || { r1 || n || Alice || Bob } kA Alice { r2 || n || Alice || Bob } kB Cathy Bob n || { r1 || ks } kA || { r2 || ks } kB Cathy Bob n || { r1 || ks } kA Alice Bob

Protocol 4: Neuman-Stubblebine 4 step protocol Good point: Only timestamp from one party is involved, so avoid synchronization Even M cracks K, he cannot conduct the attack and impersonate A since the timestamp would have expired. Problem: Type-flaw attack: an attacker can pretend to be Alice (it needs to control the path between B and T)

Protocol 5: Denning-Sacco (an unsafe protocol) 3 step protocol Using public-private key Still not safe: B can impersonate A to talk to anyone else Fix for this: Identify communication parties that use the key

Lessons we learn: Identify the communication parties Protect the freshness of the key Get both parties involved early Do not make packets similar to each other Do not try to outsmart other researchers or hackers

Secret splitting: Secret sharing: All parties are required to recover the secret Solution: XOR Secret sharing: Only a threshold number of parties are required to recover the secret The example using polynomial