Computer Networks · free preview
Authentication Protocols
Authentication proves a message is fresh and from whom it claims, and freshness is the subtle half, timestamps need synchronized clocks that are themselves attack targets, nonces need memory of every value ever used, and challenge-response protocols combine both by having the verifier pick the nonce live, public-key protocols then build mutual authentication from signed timestamps and nonces with session keys delivered under the other's public key, while secret-key protocols run through the KDC, Needham-Schroeder using a nonce to assure freshness and a ticket as the KDC's signed introduction, the design Kerberos productized with master keys derived from passwords, every message in all of these protocols load-bearing, since history's authentication bugs were omissions, missing names, reused nonces, that opened replay and impersonation.
1. Freshness First
Freshness First
2. Proving It Both Ways With Public Keys
Proving It Both Ways With Public Keys
3. The KDC in the Middle
The KDC in the Middle
4. Reading a Protocol Like an Auditor
Reading a Protocol Like an Auditor
5. The recorded login
An attacker records a correctly signed login message and replays it the next day. What property did the protocol fail to enforce?
6. Two weaknesses, one fix
Why do practical protocols combine timestamps and nonces?
7. The live proof
In challenge-response, why does the verifier generate the nonce itself?
8. The load-bearing field
A signed authentication message omits the intended recipient's identity. What attack opens?
9. The ticket as signature
In Needham-Schroeder, why does the ticket function as a signed statement from the KDC?
10. Kerberos's root
What does Kerberos derive each user's master key from, and what weakness follows?
11. The used-once value
What is a random value included in a message to guarantee it has never been used before called?
12. The live exchange
What protocol shape has the verifier mint a fresh value and demand the prover sign or encrypt it, proving the reply was made now?
13. The canonical trio
Which classic secret-key authentication protocol runs Alice, Bob, and a KDC through nonces and a ticket, and what system productized it?
14. The recorded attack
What class of attack re-sends a previously recorded genuine message to gain entry?
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