Cryptology I (Autumn 2007)

Take-home exercises, 3rd set

Deadline: September 24th

Exercise 3a In the file below (under your name) you'll find two ciphertexts that are encrypted with the substitution cipher in the CBC-mode (the length of a block is one letter; instead of XOR we're using addition modulo 26). You also find the plaintext corresponding to the first ciphertext. Find the plaintext corresponding to the second ciphertext.
Silver Aabjõe Madis Abel Lidija Agen Erkki Alamäe Alexander Andrusenko Mart Aomets Kalmer Apinis Liisi Haav Roman Jagomägis Andres Kalle Diana Korsmik Magnus Leppik Jelena Makarova Avo Muromägi Jaan Neljandik Long Ngo Long Nguyen Hoang Margus Niitsoo Indrek Priks Kairi Schotter Ivo Seeba Andres Tiko Aleksandr Tkatšenko Ain Uljas Merli Uus Yanjun Yao

Exercise 3b All modern computers are based on binary logic. The least unit of information in such computers is a bit which may have two possible values. However, computers based on ternary logic have been developed as well (and it is not much more difficult to do). The least unit of information in these computers (called trit) has three possible values. Were a ternary computer used to implement a linear keystream generator, the output would presumably be a sequence of trits, not bits.

In the file below (under your name) you find a prefix of such a sequence of trits (the three possible values are represented by 0, 1 and 2). Find the coefficients of the minimal-size linear keystream generator that generates a keystream beginning with such prefix.

Silver Aabjõe Madis Abel Lidija Agen Erkki Alamäe Alexander Andrusenko Mart Aomets Kalmer Apinis Liisi Haav Roman Jagomägis Andres Kalle Diana Korsmik Magnus Leppik Jelena Makarova Avo Muromägi Jaan Neljandik Long Ngo Long Nguyen Hoang Margus Niitsoo Indrek Priks Kairi Schotter Ivo Seeba Andres Tiko Aleksandr Tkatšenko Ain Uljas Merli Uus Yanjun Yao

Please submit the whole solution, including