EE5612, EE5653 - Predictable Frequency of Occurrence - Calculation of Unicity distance - Engineering Assignment Help

Assignment Task :

Aims

The aims of this practical work are to: 

• Provide an understanding of the concept of entropy in the context of written language.

• Give a better understanding of how unicity distance is calculated.

• Illustrate the weakness of classical ciphers

• Reinforce understanding of the calculation of unicity distance 

• Demonstrate the utility of knowledge of known plaintext to the cryptanalyst.

1. Learning Outcomes

After completing this practical work you will

• Appreciate that for large amounts of written text the most commonly used letters have a predictable frequency of occurrence.

• Understand how frequency analysis can be used to break classical ciphers.

• Know how to perform a ciphertext only attack on a monoalphabetic cipher.

• Know how to perform a known plaintext attack on a classical cipher.

Question :

1. Prepare three relatively long English texts (each of the size of 15,000 or more letters) taken from a

• novel or story

• news report

• technical report

, respectively, and save each as a word document.

Find on the Internet a similar single text (≥ 15,000 letters) written in an arbitrary foreign language, copy it and save it as a word document.

Determine and provide a histogram showing in the graphical form the relative frequency of letters in all four prepared long texts, as well, as the textual listings of 26 most frequent digrams and trigrams. This can be done using CrypTool. In Cryptool from the menu File>Open open the word document containing the text you want to analyse. The histogram can be obtained by selecting Analysis>Tools for Analysis >Histogram and the the digrams and trigrams by selecting Analysis>Tools for Analysis >N-Gram and selecting the appropriate option. 

Q2. For the English language novel/story text of Q1, using the N-Gram tool in Cryptool, note down the five most frequent letters in the text. For first 500, 1K, 2K, 5K, 10K, and 15K letters in the text, record the estimated probabilities of each of the five letters you noted down; The estimated probability of a letter is its frequency in a sample of text divided by the total number of letters in the sample of text.

Q3. For the English text taken from a novel of Q1, for the first 1,000 letters and then the entire text give the frequency distribution of the 26 most frequent N-Grams of single letters, diagrams, and trigrams. Now encrypt the first 1,000 letters and then the entire text using the following 3 classical ciphers available in CrypTool: Vigenere, Hill, and Substitution. Give the frequency distribution of the 10 most frequent N-Grams of single letters, diagrams, and trigrams for all 6 obtained ciphertexts. 

What are the characteristic features of the obtained distributions? How you could use them to determine which cipher was used to obtain the given ciphertext?

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