EE5612/EE5653: Communication Network Security - Modification of the Diffie-Hellman Protocol - IT/Computer Science Assignment Help

Assignment Task:

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1. a) Passwords are the most widely used means of authenticating human 

users to computer networks. However, the user ID and password only system may require a secondary method to improve the security of the system. Explain the “2-step authentication” method used in Microsoft Office 365 Outlook. 

[10 marks] 

b) The National Cyber Security Centre (UK) recently advised against a 

regular change of password. Explain why a regular change of password may increase the risks. 

[10 Marks] 

2. 

a) With the help of a diagram, briefly explain the five IEEE 802.11i phases 

of operation for a robust security network (RSN). 

[13 marks] 

b) Consider the following encryption mode for applying AES-128 with a 

key K to a message M that consists of l 128-bit blocks, M1, ... ,Ml. The sender first picks a random 128-bit string, C0, which is the first block of ciphertext. Then for i > 0, the ith ciphertext block is given by 

Ci = Ci-1 ⊕ AES-128K(Mi). The ciphertext is the concatenation of these individual blocks: 

C = C0 || C1 || C2 ...|| Cl where || denotes concatenation. 

Discuss why C0 needs to be random. Is this mode of encryption se- cure? If so, state what desirable properties it has that make it secure. If not, sketch a weakness and propose a scheme to improve security. 

[7 marks] 

3. 

a) Anna and Elsa want to communicate confidentially using a symmetric 

key cryptosystem. Thus, they need to establish a shared secret key over a public communication channel first. To do so, they decide to use the following modification of the Diffie-Hellman protocol, referred to as the Frozen Diffie-Hellman. 

i) Anna and Elsa are both assumed to possess a (public key, private key) pair, (PKA; SKA) for Anna, and (PKE; SKE) for Elsa. ii) Anna and Elsa now publicly agree on a prime number p and an integer α, with 1 ≤ α ≤ (p - 1), such that α is a primitive root of the set Z ∗ = 1,2, ... , − 1 , 

iii) Anna generates an integer μA with 1 ≤ μA ≤ (p - 1), and computes 

= αμ (mod ). Next, Anna encrypts using Elsa's public key PKE. She then transmits encrypted message PKE( ) to Elsa. iv) Elsa computes an integer μE with 1≤μE ≤ (p -1). She then com- 

putes = αμ (mod ), and encrypts it using Anna's public key PKA. She then transmits encrypted message PKA( ) to Anna. v) Elsa decrypts the received message using her secret key 

SKE(PKE( )), and computes the shared secret key 

= ( )μ = μ μ ( ) 

vi) Similarly, Anna decrypts the received encrypted message using 

her secret key SKA(PKA( )), and computes shared key 

= ( )μ = μ μ ( ) 

Anna and Elsa communicate using key . 

Draw a diagram of the Frozen Diffie-Hellman protocol. Explain the concept of man-in-the-middle (MitM) attack and critically discuss whether the Frozen Diffie-Hellman protocol is resistant to MitM attack. 

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b) A generalization of Caesar cipher, known as the affine Caesar cipher has the following form. For each plaintext p, substitute it with the ci- phertext letter C: C=E([a,b],p)=(ap+b) mod 26; where a and b are two integers. A basic requirement of any encryp- tion algorithm is that it needs to be one to one, i.e., if p≠q, E(k,p) ≠ E(k,q). Critically analyse the restrictions for a and b. 

c) Bob and Alice want to use hash function for data integrity in their com- 

munications. Alice proposes that she and Bob use a hash function h(m): Z → Z , defined as 

h( ) = ( || ) + 7( || ) + 735 mod 2 where || denotes concatenation. Discuss why the proposed hash func- tion would or would not be secure to use. 

4. 

a) Access security terminates in difference parts of the 3GPP core network for Universal Terrestrial Access Network (UTRAN) and GSM/EDGE Radio Access Network (GERAN) services. With a help of a diagram, explain the reason for this mentioning where in each case the access security terminates and its significance. 

b) In the context of telecommunications explain what Signalling System 

No. 7 (SS7) refers to. Outline the major security vulnerabilities that the use of SS7 gives rise to. 

5. a) Draw a lattice of security diagram for the labels: 

(Top Secret), (Top Secret, Chongqing), (Top Secret, Chongqing, Diplomatic), (Top Secret, Bahrain), (Secret, Chongqing), (Secret, Chongqing, Diplomatic), (Secret, Bahrain), (Secret), (Unclassified). 

b) Critique Multilevel Secure (MLS) Systems. 

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