Cryptographic Protocols

Question 1

What are Message Authentication Codes?

Answer 1

They are used to calculate a keyed hash of the message, then append it to the end of the message.
- They provide integrity and authenticity, but not confidentiality

Question 2

What are Length Extension attacks?

Answer 2

Assumes an attacker doesn’t know k, but they do know the keyed hash at the end of the message

Question 3

How do Length Extension attacks work?

Answer 3

• Observe message and keyed hash message
• Create a new attack message m|x
• Initialise hash function with an internal state of the keyed hash message
• Complete the hash with x to produce keyed hash attack message

Question 4

What is the Record Layer in TLS?

Answer 4

Uses established symmetric keys and other session info, will encrypt application packets

Question 5

What is the Handshake Layer in TLS?

Answer 5

Used to establish session keys, as well as authenticate either party - Usually the server using a public-key certificate

Question 6

What are the steps of the TLS handshake?

Answer 6

• Client says hello, with a random number and a list of supported cipher suites
• Server responds with a hello, random number, chosen cipher suite and its certificate
• Key exchange between the two parties is performed
• Client verifies the certificate that was originally sent by computing the digital signature with the public key and private key
• Sends conclusive finish messages before the connection is then deemed fully established