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 diagrams and trigrams.

1 Installation In order to complete this lab you might want to install CrypTool on your personal computer/laptop. You can download CrypTool from http://www.cryptool.org. The recommended version of software is CrypTool 1 (CT1) ver. 1.4.42 – English. 2 Frequency Analysis Task 1 (10 points) Prepare three relatively long English texts (each of the size of 10,000 or more letters) taken from: ˆ novel, ˆ newspaper, ˆ reference manual. Find on the Internet a similar single text (≥ 10,000 letters) written in an arbitrary foreign language. ˆ 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 diagrams and trigrams. ˆ Do the frequency distributions depend significantly on the type of text in English? ˆ Do these distributions depend significantly on the language in which the message was written? Task 2 (10 points) Take a small subset of all four texts prepared in Task 1, e.g., their first 200 letters, and recompute all frequency distributions. ˆ Determine how good is the match between frequency distributions for short texts and long texts of the same type, written in the same language. ˆ Summarize your observations. Task 3 (25 points) Encrypt a single long and a single short English text taken from a novel, prepared in Tasks 1 and 2, using the following 6 classical ciphers available in CrypTool: Caesar, Vigenere, Hill, Substitution, Playfair, and Permutation. Compute the frequency distribution of single letters, diagrams, and trigrams for all 12 obtained ciphertexts. ˆ What are the characteristic features of the obtained distributions? ˆ How you could use them to determine which cipher was used to obtain the given ciphertext?

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