07 Cryptographic Protocols

Question 1

What else do we need for Properties of Cryptographic Protocols

Answer 1

• Perfect Forward Secrecy and key agreement
• Scalability
• Avoidance of Single Points of Failures
• Selection of algorighms used for authentication and key establishment
• Generic authentication methods for the cryptographic protocol
• simplicity

Question 2

What does Protocol Try 6 do?

Answer 2

Removing the Authentication Server
- add Long-term key – changes once a day
– KA,B,Longterm is cached and trusted by A and B for sometime.
– KA,B,Longterm authenticates Diffie Hellman key exchange

Question 3

What does Protocol Try 7 do?

Answer 3

Generic AUTH payload and Selection of Algorithms

Question 4

What does Protocol Try 8 do?

Answer 4

AUTH Payload Rework
– AUTH payloads are different and contain information provided by both principals
– Proposed/ chosen crypto algs are now authenticated

Question 5

How does a FINAL Authentication and Key establishment looks like? – 4 Goals

Answer 5

1. Alice and Bob have a shared session key for a secure channel.
2. Alice and Bob have agreed on the cryptographic algorithms to be used for the secure channel
3. Mutual authentication and freshness Alice(Bob) must be able to verify that Bob participated in the protocol run and that they is alive
4. Alice and Bob must know that K (A,B) is newly generated.

Question 6

What is the general purpose of a X509 certificate

Question 7

How to prevent Replay Attack

Answer 7

add Nonces - only accept unknown nonce
set Timestamp - have well synchronized clocks, only recently used timestamps must be memorized

Question 8

Forward Secrecy - what do we want to avoid

Answer 8

Long-term key leaks
If attacker has recorded previous communication, they can decrypt key transport message and data exchanged over secure channel
store now, decrypt later