Cryptanalysis of Classic Ciphers | Custom Essay Papers
Assessment Item 1: Problem solving task Module 1 Task: Cryptanalysis of Classic Ciphers This is an individual assessment task: you may discuss this task with your classmates, but the work you submit must be your own individual effort. Your results for this task form 10% of your final result in IFN642. Problem solving task: The problems you will solve in this task are related to the cryptanalysis of classic ciphers. Individual ciphertexts have been created for you. These are available on the IFN642 Blackboard site: under the Assessment link, select Assessment Item 1: Problem solving task, and then Source material for Module 1 questions. • Download the Ciphers.zip file and extract to find the text file corresponding to your student number. o The files are labelled based on student numbers; the last five digits in your student number will be the prefix, and the suffix will be the file type: txt. o For example: for student number 01234567 the appropriate file is 34567.txt. • In your file you will find four ciphertexts. You need to apply cryptanalytic techniques to recover the corresponding four plaintexts. The tasks can be completed using CrypTool 2. Submission: You will submit a report documenting your cryptanalytic process in recovering the plaintexts. • Submission is electronic, via Blackboard. Look for the Turnitin link under the Assessment Item 1: Problem solving task. • It is important that your report is written in your own words. You may also include screenshots of CrypTool outputs. Do not ‘cut and paste’ or copy information from any source into your report without acknowledgement: that is considered plagiarism (a breach of academic integrity) and is not acceptable in Australian universities. If this is detected, the Unit Coordinator is obliged to notify the Faculty Academic Integrity Committee, and the penalties imposed may be severe (See the QUT MOPP for details). Marking Criteria: The marks assigned for cryptanalysis of your four ciphertexts are indicated below. Structure your report so that it is clear to the marker which ciphertext you are discussing, and be sure to include the specific items requested for each of the ciphers, as follows: Ciphertext 1 – Caesar cipher (4 marks) 26 character alphabet A-Z, non-alphabet characters such as spaces have not been encrypted. 1. Create a frequency analysis chart for your ciphertext (include a screenshot of this chart in your report). 2. Refer to your frequency chart and identify the ciphertext character that most likely represents the plaintext character ‘e’. Justify your answer. 3. Determine the key used with the Caesar cipher. Explain how you obtained this key. 4. Provide the plaintext corresponding to the ciphertext you were given.
Ciphertext 2 – Row/column Transposition Cipher (8 marks) 26 character alphabet, non-alphabet characters such as spaces and punctuation have been encrypted. Text begins at the start of a sentence, but may end mid-word. 1. Create a frequency analysis chart for your ciphertext. 2. Refer to the frequency chart and identify the ciphertext character that represents the plaintext character ‘e’. Justify your answer. 3. Determine the key used with the transposition cipher. Explain how you obtained this key. 4. Provide the plaintext corresponding to the ciphertext you were given. Ciphertext 3 – Vigenere Cipher (8 marks) 26 character alphabet, non-alphabet characters such as spaces and punctuation have not been encrypted. Text begins at the start of a sentence, but may end mid-word. 1. Remove the spaces and punctuation from the ciphertext string. 2. For the remaining characters, provide the output of Kasiski’s Test for factors up to 20. 3. Refer to the output of Kasiski’s Test and identify a probable key length. 4. Describe the process involved in breaking the Vigenere cipher once the key length is determined. 5. Determine the key used with the Vigenere cipher. Explain how you obtained the key. 5. Apply the key to your shortened string. 6. Provide the plaintext corresponding to the ciphertext you were given. Ciphertext 4 – Hill Cipher (10 marks) 26 character alphabet, non-alphabet characters such as spaces and punctuation have been removed. Text begins at the start of a sentence. 1. The Hill cipher is polygraphic, with block size m. For m = 2, explain how a known-plaintext attack is performed, and how much known plaintext is required for the attack to succeed. 2. For a ciphertext-alone attack on a Hill cipher, some plaintext must be guessed. The position of a plaintext word may be unknown, so many positions may be tested in a process called ‘crib dragging’. Assuming the plaintext is in English, list some common cribs or digrams. 3. Given the knowledge that your plaintext message contained the word that, apply the crib dragging process to attempt to obtain the key1. 4. Compute the decryption matrix.
5. Provide the plaintext corresponding to the ciphertext you were given.