Homework 4 (due 1/31)
CSC 233

We finished talking about the Vigenere cipher (covering the kappa test, counting coincidences), and saw a similar security failing in master-key systems (see links page for a link to the article). Next we covered Playfair ciphers and how to break them using a known plaintext attack. Next week we will talk about transposition ciphers and get started on chapter 3.

Midterm will be in sixth week (week after next).

1. (Vigenere Cipher) The following text has been encrypted using a Vigenere cipher.

xsazssewuv amxswhvzdi vzlmiwsfpf nboualbqht oueyscmmpk ighonwiork
jgtapigauh hxcssmbfrt oueffbtppi lwcstabhgx pfgxbbdbtv onmrsvvzlt

a) Determine the key length using the kappa test (counting coincidences for various shifts).  You can assume that the keyword has at most 8 letters.

b) Decrypt the text.

2. (Playfair Cipher) Decrypt the following ciphertext which was obtained using a Playfair cipher with key "beale". What do you think the original plaintext was? Why was it changed, before encryption?


3. (Playfair Cipher) You have intercepted the following plaintext/ciphertext pair:

You suspect a playfair cipher. Try to reconstruct the key matrix used, and make a 
guess at the password used to construct the key matrix
Note: Document and explain your approach step by step.

Marcus Schaefer
Last updated: January 25th, 2006.