Week 5

Question 1

What is a Nonce and how it is used?

Answer 1

Number that is used only once.

Nonce is decrypted, one is added, encrypted again.

Prevents against replays.

Question 2

What is a Key Establishment Protocol?

Answer 2

Principals set up a session key.

To be sure they are communicating with the correct principal, they must either know each others public keys or use a TTP.

Question 3

Describe the Needham-Schroeder public key protocol?

Answer 3

Assume A and B know eachothers public keys:

A -> B: Eb(Na, A)
B -> A: Ea(Na, Nb)
A -> B: Eb(Nb)

Na and Nb can then be used to generate a symmetric key.

Question 4

What is an attack against NH protocol, and how do we prevent it happening?

Answer 4

Man-in-the-middle.

2. B -> A : Ea(Na, Nb, B)

Question 5

What is Forward Secrecy?

Answer 5

When a message is kept secret from an attacker who has:

• Recording of the protocol run.
• Long term keys of the principals.

Question 6

Describe Station-to-Station Protocol?

Answer 6

A -> B: gx
B -> A: gy, {Sb(gy, gx)}gxy
A -> B: {Sa(gy, gx)}gxy
B -> A: {M}gxy

x, y, gxy not stored.
Ensures forward secrecy.

Question 7

What is a Certificate?

Answer 7

TTP signs their identity and public key.

Question 8

Describe NS key establishment protocol?

Answer 8

A and B use trusted third party S to establish Kab.

Question 9

What are some key establishment goals?

Answer 9

Key freshness - either from trusted third party or using a new nonce.

Key exclusivity.

Question 10

What are some authentication goals?

Answer 10

Far-end operative: A knows that B is currently active.

Once authentication: A knows that B wishes to communicate with A.

These together form Entity Authentication.

Question 11

When does a protocol provide Mutual Belief?

Answer 11

When its key is a good key and it has entity authentication.