Designing a Security Protocol

Question 1

What is the goal of designing a security protocol between principals and a key server?

Answer 1

The goal is to create a security protocol in which the key-server can exchange symmetric keys to parties
for secure communication.

Question 2

What are the 4 assumptions that need to be made to ensure a secure protocol is created?

Answer 2

Intruder is able to eavesdrop on all communications in the protocol; Intruder can intercept and re-route
messages on the network to prevent availability; Intruder may be an insider, outsider or both; Intruder can obtain
old session keys used in previous runs.

Question 3

Define attempt 1… What are the issues with this? What is the solution?

Answer 3

A makes request to KS, KS responds with key in the open and unencrypted. Intruder can access key, intercept
and compromise communications between parties. The solution is to have pre-shared keys between KS and principles, and encrypt the symmetric key with each pre-shared key.

Question 4

Define attempt 2… What are the issues with this? What is the solution?

Answer 4

A,B obtain pre-shared keys from KS. A requests KS for symmetric Key between A,B. KS responds with key encrypted with pre-shared keys for each user. However, initial request with users is unencrypted, thus, intruder can also obtain a pre-shared key and then sit between A and KS, and replace B with ‘i’ in initial request. Thus, KS responds with symmetric key for A and ‘i’. ‘i’ can sit between A and B, thus A shares key with i instead of B. 2 main attacks are Compromise availability and double interception. The solution to this is to cryptographically bind identities to the symmetric key.

Question 5

Define attempt 3… What are the issues with this? What is the solution?

Answer 5

Cryptographically bind the identities of the users to the symmetric keys being sent. This prevents intruders sitting
intercepting and inserting their identity into their protocol as an authentic user. Thus, this solution provides
authentication. However, an issue with this is that the intruder can site between A and KS, and replay an old key
as the valid session key. Thus, enabling i to view transmission between A and B and also enabling i to replay
old transmissions uses with replayed key. A solution to this is to add a nonce to the initial request, thus
ensuring the KS responds with a fresh key.

Question 6

Define attempt 4… What are the issues with this? What is the solution?

Answer 6

Add nonce to initial request to ensure key freshness from the KS. However, B can’t verify key freshness, thus ‘i’
can intercept A to B and send old session key to B. Solution is for B to sends ID and nonce to A first.

Question 7

Define attempt 5…

Answer 7

B sends nonce and ID to A, A appends their ID and nonce to B’s and sends to KS. KS encrypts session key for each agent.

Question 8

Define a protocol

Answer 8

Set of rules that explains how messages should be exchanged. It’s a distributed algorithm with emphasis on communication. In the below example, the distributed components are the users.

Question 9

Define a Security Protocol

Answer 9

Use cryptographic mechanisms to achieve any form of security objectives e.g Message authentication, key establishment, integrity etc.

Question 10

Explain the Needham Schroeder Protocol… What attack is it vulnerable to? What is the solution to this?

Answer 10

mutual authentication protocol that ensures parties involved in communication are in fact who they say they are.
Lowe’s attack.
Lowe’s fix.