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: novel, newspaper, reference manual. Find on the Internet a similar single text (≥ 10,000 letters) written in an arbitrary foreign language. 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. Do the frequency distributions depend significantly on the type of text in English? 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. Determine how good is the match between frequency distributions for short texts and long texts of the same type, written in the same language. 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. 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?
