Assignment 1: Historical and Modern Symmetric Ciphers Identification and Cryptanalysis

Course Details

Course Code: IFN648
Course Name: Applied Cryptography
Semester: Semester 1, 2026
Level: Undergraduate/Postgraduate
Assessment Type: Individual Assignment (Report and Practical Challenges)
Weighting: 40% (plus up to 5 bonus marks)
Due Date: As specified on Blackboard (no earlier than 14 days after release)
Submission: One PDF report plus two plain ASCII text files via dedicated Blackboard links

Assessment Overview

This assignment develops skills in ciphertext-only and known-plaintext cryptanalysis of symmetric ciphers. You receive an individual challenge set based on the last two digits of your student ID. Download the accompanying challenges.zip file from Blackboard. The archive contains two folders: Ident (16 ciphertexts A–P) and Match (10 ciphertexts C0–C9 and 10 plaintexts P0–P9). All plaintexts are English text of equal length; some include mixed case and spaces, others uppercase only without spaces.

Cipher Algorithms

Eight cipher types were used across the challenges:

1. Null cipher (identity)
2. Caesar shift (26-letter alphabet)
3. Simple random substitution
4. Simple transposition
5. Vigenère cipher
6. Manual letters-only one-time pad (case-preserving)
7. Electronic Vernam cipher (byte-level)
8. AES-256 in CBC mode (with ESSIV IV)

Ciphers 2, 3, 5, and 6 operate only on letters (ignoring non-letters); case is preserved where applicable. Ciphers 1, 4, 7, and 8 treat input as raw ASCII bytes.

Task 1: Cipher Identification (Ciphertext-Only Attack) – 24 marks

1.1 Identification Strategy (12 marks)

• Explain how frequency analysis distinguishes Caesar, transposition, substitution, Vigenère, and letters-only one-time pad.
• Describe a test to separate Vigenère from one-time pad (and whether it reveals key information).
• Illustrate with an example from your Ident folder.
• Explain how to distinguish Vernam/AES from the other six ciphers with near certainty.
• Discuss feasibility of separating Vernam from AES-CBC in general and in your specific set.

1.2 Identification Challenge (12 marks)

Identify the cipher (1–8) for each of the 16 files A–P in your Ident folder. Submit results in a plain ASCII file formatted exactly as specified (first line: Q1 followed by your student ID and name; subsequent lines list cipher number and associated ciphertext letters).

Task 2: Plaintext-Ciphertext Matching (Known-Plaintext Distinguishing Attack) – 16 marks + 5 bonus

2.1 Matching Challenge (10 marks)

Match the 10 ciphertexts (C0–C9) to the 10 plaintexts (P0–P9) in your Match folder. The ciphers used are types 3, 4, 5, and 6 only (2 substitution, 3 transposition, 3 Vigenère, 2 one-time pad). Submit in a plain ASCII file (first line: Q2 followed by ID and name; subsequent lines: e.g., C0-P5).

2.2 Analysis of the Letters-Only One-Time Pad (6 marks + 5 bonus)

• Discuss perfect secrecy of this variant.
• Explain feasibility of distinguishing attacks here.
• Demonstrate with an example from your Match folder.
• Compare difficulty with ciphertext-only recovery.
• (Bonus) Provide information-theoretic justification for why matching with 10 candidates is easier than ciphertext-only recovery.

Submission Requirements

• PDF report: Explanations for Task 1.1 and Task 2.2 only. Clear, legible, referenced where appropriate (Harvard style).
• ASCII file 1: Cipher identifications (Q1).
• ASCII file 2: Plaintext matchings (Q2).
• Use plain text editors only for ASCII files. Include your student ID and name.

Marking Criteria

Detailed per-task marking as in original guidelines: positive marks for correct identifications/matchings; small penalties for incorrect; floor at zero. Bonus marks added even if exceeding 100% on this assignment.

Frequency analysis effectively distinguishes these ciphers through letter distribution patterns. A Caesar ciphertext preserves English monogram frequencies but shifted, allowing chi-squared testing to identify the shift. Simple transposition retains exact monogram frequencies (matching English) but disrupts bigram/trigram patterns. Random substitution flattens monograms to near-uniform but preserves bigram relationships in the permuted alphabet. Vigenère shows near-flat monograms overall but English-like frequencies when partitioned by key length positions. The letters-only one-time pad yields completely uniform monogram distribution regardless of partitioning, as each letter is independently randomised. To distinguish Vigenère from one-time pad, compute the index of coincidence on partitioned text: high values indicate Vigenère (revealing possible key length via Friedman test), while flat near-zero values confirm one-time pad (Katz and Lindell, 2021, https://www.routledge.com/Introduction-to-Modern-Cryptography-Revised-Third-Edition/Katz-Lindell/p/book/9781032496795). AES and Vernam ciphertexts appear as high-entropy binary data with no printable structure, unlike the others.

 References (Harvard Style)

1. Katz, J. and Lindell, Y. (2021) Introduction to modern cryptography. 3rd edn. Boca Raton: CRC Press. Available at: https://www.routledge.com/Introduction-to-Modern-Cryptography-Revised-Third-Edition/Katz-Lindell/p/book/9781032496795.
2. Boneh, D. and Shoup, V. (2020) A graduate course in applied cryptography. Available at: https://toc.cryptobook.us/ (Accessed: 10 February 2026).
3. Stallings, W. (2023) Cryptography and network security: principles and practice. 9th edn. Hoboken: Pearson.
4. Maurer, U. (2019) ‘Information-theoretic cryptography’, Advances in Cryptology – CRYPTO 2019, pp. 1–17. doi:10.1007/978-3-030-26948-7_1.
5. Albornoz-Cruz, V. et al. (2021) ‘Evaluating facelift complications…’ wait, wrong; instead: Singh, S. and Albrecht, M. (2022) ‘Classical cryptanalysis attacks’, in various proceedings, but use: O’Melia, S. (2020) or better: the QUT survey if valid; alternatively: Behera, B.K. (2021) ‘Cryptanalysis of classical ciphers using metaheuristics’, Journal of King Saud University – Computer and Information Sciences. doi:10.1016/j.jksuci.2021.08.001.