COM4501/6501 DEPARTMENT OF COMPUTER SCIENCE Spring Semester 2020-2021 Computer Security and Forensics 2 Hours Answer ALL THREE QUESTIONS. Be concise and to the point: most questions can be answered in a few sentences. Figures in square brackets indicate the marks allocated to each part of a question. COM4501/6501 1 TURN OVER Question 1 - Total 35% Part A - Authentication a) Authentication techniques are often categorised as: something you know, something you have, or someone you are. i) For each category identify an example technique [3%] ii) For each example technique you provide in i) above give a significant perceived strength of that example technique [3%] iii) For each example technique you provide in i) above give a significant perceived disadvantage or concern associated with that example technique [3%] iv) Explain briefly what is meant by 2-Factor Authentication (2-FA). Briefly give an example of a good 2-factor authentication approach, indicating why it is a good example. [3%] Part B - Encryption a) Block cipher modes of use. Let E(x) represent a block encryption algorithm E applied directly to data block x, i.e. data block x is encrypted in Electronic Code Book (ECB). Similarly, let D(y) be the corresponding decryption algorithm D applied directly to data block y. Let I be an “initial block”. i) Using E, D and I explain briefly how encryption and decryption operate when the algorithm is used in Cipher Block Chaining (CBC) mode to encrypt a 2-block message M1 M2. [4%] ii) Explain why might you prefer to use CBC mode rather than ECB mode for securing communications messages. That is, explain what problem associated with ECB mode CBC mode fixes. [2%] iii) What “error propagation” properties does CBC mode have? That is, if a received ciphertext block has an error, what plaintext blocks are affected? Explain your reasoning. [2%] b) Classical Stream Ciphers. The Truth Table for a classical combining cipher on three linear feedback shift registers is given below. Explain how you would attack this cipher to recover the key (i.e. the initial states of the three registers L1, L2 and L3). You should identify which register you would attack first (and why) and then indicate how any remaining register state values may be recovered. You should assume that you have access to a significant quantity of stream bits from the whole cipher, i.e. a stream of bits produced by f(x1,x2,x3) when the original encryption took place. [7%] CONTINUED COM4501/6501 x1x2x3 f(x1,x2,x3) x1x2x3 f(x1,x2,x3) 000 0 100 0 001 1 101 1 010 1 110 0 011 0 111 1 Truth Table for the Combining Function c) Linear Cryptanalysis. A 4-bit 3-round encryption algorithm is shown below. The algorithm uses 4 keys: K1, K2, K3, and K4, and also 3 identical S-boxes. The Linear Approximation Table (LAT) for the S-boxes is also given. Use the table to develop an approximation over the first two rounds, i.e. between the P, U3 and various K (key) bits. You should state the overall approximation. Using Matsui’s Lemma, calculate and present the probability with which your approximation holds. Show you working. [8%] The 3-round 4-Bit Encryption Algorithm and the Linear Approximation for the S-Boxes. All S-boxes are identical. Each key has 4 bits. Thus: K1= K1,1 K1,2 K1,3 K1,4 K2= K2,1 K2,2 K2,3 K2,4 and so on. Also U3= U3,1 U3,2 U3,3 U3,4 COM4501/6501 3 TURN OVER Question 2 - Total 30% Forensics Case study RYcommerce is an ecommerce company based in London providing an e-commerce marketplace for independent sculptors, artists and graphic designers. The company employs 50 individuals and has over the last year has seen a recent increase in sales. On their e-commerce website, they processed about 60,000 transactions worth over £ 8 million at the end of the financial year 2020. The e-commerce store makes use of magento open source and is hosted at their head office on a HP ML30 Gen9 CTO Tower server which runs CentOS, a linux based operating system and has 2 x 2TB hard drives. On 10th Feb, 2021, they notice that the e-commerce store has been defaced. To ensure this incident has minimal impact on sales, they revert to an old backup and the website is back online within a few minutes. On 12th Feb, 2021, they notice once again that the e-commerce store has been defaced. RYcommerce deploys the site on a backup server and employs a Magento e-commerce specialist to help resolve the issue while also exploring an in-depth forensics analysis. Assume, you work for ST forensics, an organisation providing forensics, first respondents and incident response services to various private companies, regional Police units and INTERPOL. Your colleagues at RYcommerce have provided you the server in question and provided you with a Live capture of the memory of the device. a) Using your knowledge of the Digital Forensics process/methodology and Linux forensics, describe how you would approach this case. Your answer should be verbose and cover what should be done from when the server was handed to you until you present a report [20%] b) Briefly describe two anti-forensics approaches that you believe could be employed in the above case and explain why [5%] c) Briefly explain why a chain of custody is important in a digital forensics investigation. [5%] CONTINUED COM4501/6501 Question 3 - Total 35% Part A - SDLC, Threat Modelling, Web security a) Consider a secure IoT-based healthcare environment (as shown below), where a local processing unit (LPU) collects sensor data and sends the data to the server. Then, the server analyses the data and makes a decision (such as call a doctor, or emergency services (like 999)). Outline four potential threats and possible countermeasures to this IoT-based healthcare system [5%] Fig. IoT-based healthcare system b) (i) Outline the role of the “Abuse Cases” in Secure Software Development Lifecycle (Secure-SDLC). [1%] (ii) Consider an automobile example, where an actor is the driver of the car, and this actor has a use case ’drive the car’. Then, outline a possible abuse case that would threaten the use case [2 %] (iii) Suggest a new use case which can be added to mitigate the abuse case and fortify the system. [2 %] c) Consider the below client-server-based authentication protocol, where both the client and server have shared a key K , E(M. K) denotes the message M has been encrypted using K and RA, RB denotes the random number generated by Alice and server, respectively. Is this protocol secure? Justify your answer. [5%] COM4501/6501 5 TURN OVER Fig. Client-Server-based authentication scheme d) Suppose you have captured a TLS packet in Wireshark with the following cipher suites specification: SSL_ECDHRSA_With_RC4_128_SHA256. What does it mean? [2%] e) What kind of XSS attack is depicted in the figure below? How to deal with such attacks? [3%] CONTINUED COM4501/6501 Part B - Secure Programming and Software Testing a) Consider the following Java program (i) What vulnerability does this program have? [2%] (ii) Rewrite this program to fix the vulnerability. [3%] b) Suppose you know that a particular web site uses a backend database to implement Authentication. Given a login page with username and password fields, what would you type into these fields to try to perform SQL injection to bypass proper authentication. Briefly explain why your approach would work. [2%] c) Let us assume you have two security testing tools with the following False Positive and False Negative rates: i) Assume you want to minimise the risk of delivering insecure software to customers. Would you use Tool A or Tool B? Briefly explain your choice. [4%] ii) Assume you want to minimise the impact (effort) of introducing a security-testing tool. As a developer, would you prefer Tool A or Tool B? Briefly explain your choice. COM4501/6501 7 TURN OVER [4%] End of Question Paper CONTINUED






















































































































































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