Chapter 7: Diffie-Hellman Key Exchange

Question 1

Diffie-Hellman Key Exchange Mathematical Background

What is this?

Answer 1

Question 2

Diffie-Hellman Key Exchange Mathematical Background

What is this?

Answer 2

Question 3

Diffie-Hellman Key Exchange

What are the next step? Remarks?

Answer 3

• The Diffie-Hellman construction contains weak values, e.g. a = 0, b = 0
• Certain combinations of g and p
• There is also Diffie-Hellman based on Elliptic Curves, called ECDH

Question 4

Diffie-Hellman Key Exchange: key issue

• Alice and Bob are unaware of this problem!

Answer 4

• The protocol protects against passive attacks like eavesdropping
• However, an active Machine/Man in the Middle attacker (Eve), might intercept communication between
  Alice and Bob
• Eve performs two DH key exchanges: one with Alice, one with Bob.
• Result: Eve established K 1 between herself and Alice and K 2 between herself and Bob
• Eve is now able to decrypt, re-encrypt and forward messages between Alice and Bob

• Important: integrity and authenticity of DH key exchange messages must be protected with digital signatures → This is called Authenticated DH!

Question 5

Diffie-Hellman Key Exchange: Perfect Forward Secrecy

Alice sends messages to Bob, messages are encrypted with a session key, attacker eavesdrops all messages (including key exchange-related ones) and saves them for later (ab)use…

Explain two possible scenarios.

Answer 5