Computer Security & Forensics-Solution 1, Exercises of Computer Security

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3/13/25, 9:56 PM Solutions 01 Computer Security and Forensics Practical 1 Feb 16, 2024 Solutions Dr. B. Dowling Group Discussion (Basics). Take some time to think about and discuss: (a) What, for you, is the most important application of cryptography? Solution. There is obviously no definitive answer to this question, but positions may range from securing communication, to protecting privacy, to enabling online business, to shielding military operations, to .. — what you consider important or valuable applica- tions depends on your personal metrics and values. We will revisit this question towards the end of this course. (b) What is, in brief, the difference between the Vigenére cipher and the one-time pad? Solution. The one-time pad is essentially a Vigenére cipher with a key as long as a message. As a result, the Vigenére cipher can be easily broken by modern means, while the one-time pad is perfectly secure/tnbreakable. (The idea of encrypting using a long key (stream) XORed to a message is, in cryptography, called a stream cipher, and can be built efficiently with provable (though not perfect) security.) (0) Informally, what would you say integrity means for a symmetric eneryption scheme? Why does the IND-CPA security definition for symmetric encryption does not capture integrity? Solution. Integrity means that an adversary should not be able to change a ciphertext without this being noticeable (upon decryption). ‘The game defining IND-CPA only provides a (“left-or-right”) encryption oracle to the adversary. Therefore, it does not say anything about what happens when the scheme decrypts ciphertexts of the adversary’s choosing. Problem 1 (Basic Security Properties). A database at the University of Sheffield contains research data, The data itself is collected from users, and as such is anonymised and encrypted to align with GDPR regulations. A group of hackers break into the database and install ransomware, preventing the researchers from using the data, (a) What security property from the CIA triad does this most directly impact? Solution. ‘The security property most impacted by the hacker's ransomware attack is avail- ability. ‘The hackers manage to decrypt the data and upload a small percentage to a paste bin site, selling the rest of the data over the dark web. (b) This specific attack targets which property from the CIA triad? about:blank