HTW Berlin Fachbereich 4 Internationaler Studiengang Internationale Medieninformatik (Bachelor) Aktuelle Themen: Cryptography Winter Term 2013/14

Lab 4: Vigenère Cipher

1. Implement a small application in your favorite programming language to encode and decode using a Vigenère cipher! The program will take a keyword and a plaintext and return a cipher text in blocks of 5 characters, or it takes a keyword and a cipher text and returns the plaintext.
2. Encode a message with your algorithm and publish it to the secret message forum together with your key. Can you decode all of the other messages? What problems did you have? Discuss this in your report!
3. (For the bored) Can you write a program to look for the plaintext without having the key available? Look for repeated bits. The distance between the two is probably a multiple of the key length. If you can find a number of these - the longer the better - and have a number of distances, you might use a gcd to find the length :) Or you can use the Friedman test and calculate the coincidence index. With the length you can then find an easy way to determine the key so that you can recover the plaintext.

The report is due the evening before your next lecture at 23.00! You need to describe how you designed and implemented your code. If you were unable to implement anything, please record what you tried. You may work in groups of 2, but each of you must hand in their own copy of the report