7COM1027 : Distributed Systems Security - IT/Computer Science Assignment Help

Assignment Task:

Task:

Question1 "A problem with most symmetric key servers is that they know the client keys,” explains a representative of the ‘Happy Go Lucky’ Enterprise. “Fortunately, the Enterprise have developed a solution for their trusted employees to use. Instead of using just one key server, they are going to use two. Suppose that Alice based in London and wants to agree a fresh session key with Bob who is based in Geneva. The proposed Enterprise protocol runs as follows: (1) S1 à A : Ea1 [ X ] (2) S1 à B : Eb1 [ X ] (3) S2 à A : Ea2 [ Y ] (4) S2 à B : Eb2 [ Y ] (5) A à B : Ez [ message ] ] Here a1 is the key that S1 shares with Alice, and so on, and X and Y are random sequences of bits chosen independently by S1 and S2 respectively. The session key z is generated from X xor Y. Bob knows that only he and Alice can read the message. The servers themselves can’t, because neither of them can learn z.” The representatives colleague looks intrigued. “What’s the strength of the cryptosystem?” they ask. “Another problem with conventional symmetric key distribution is that it’s vulnerable to a meet in the middle attack, for example on messages (1) and (2)”. “The Enterprise have thought of that,” reports the representative happily, “so the two key servers are implemented in tamper-evident hardware. S1 is physically connected to Alice’s computer, and S2 to Bob’s. Consequently, an attacker can only ever actually intercept messages (2), (3), and (5). Even if they manage somehow to get hold of one of the session keys, they still can’t use it to attack the server keys b1 and a2. The enterprise plan to use DES as the crypto-algorithm: the first four messages use ECB mode, and the fifth uses OFB.” The representatives colleague shuddered.

a) Explain what is meant by the following:

i) brute force search ii) session key iii) man in the middle attack

iv) meet in the middle attack

v) tamper evident and tamper proof

vi) modes ECB and OFB (6 Marks)

b) What would be the expected number of attempts to find a symmetric key for DES using a brute force search. Justify your answer. Describe one other problem in using a brute force attack on ciphertext to obtain the original plaintext. (5 Marks)

c) Explain why the system described in the Enterprise protocol above must prevent an attacker from obtaining the first message, by showing, in detail, how an attacker could mount a meet in the middle attack against a standard key server protocol to obtain the keys a1 and b1 from messages (1) and (2). (7 Marks)

d) Could an attacker modify a meet in the middle attack to work against the Enterprise protocol if they somehow obtained a single session key z? Justify your answer with details. How might they obtain z? \

Question 2 

a) State the Diffie Hellman key agreement protocol. Given that Alice and Bob agree to work in mod 13 derive an appropriate primitive ? for the Diffie Hellman Protocol. If Alice uses a random number a = 3 and Bob uses a random number b = 7 derive the shared key that each would have at the end of the protocol. (8 marks)

b) Using two rounds of the six-stage modelling process create a model for a quantum based key agreement protocol. Using the model that you have created describe three significant differences between a quantum based key agreement protocol and a classical one. Include at least one disadvantages that both key agreement protocols share. How could the disadvantage be overcome?

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