## Homework 4 (due 5/10) 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). We then talked started talking about Playfair ciphers and saw hwo to mouont a known plaintext attack on Playfair. Next week, we will complete the attack and continue to talk about transposition ciphers.

Midterm will be on 5/3, in class (next week).

1. (Vigenere Cipher, 15pt) The following text has been encrypted using a Vigenere cipher

lknwrsunyj gnrsqkrohy gftxgzhajv kwqjpdrlpg zdcnfsoffl kzataydwig zdcpagzbng avfwbfjjsq yoxwvwvrsv ldwigzhajv kwqjpdrlpg zdcnfsoffl kurlulhghr hwfmrfbxze wohzcgqryn fgcmrflcnf erajjjrwlg zdwdbmzxzy vwqnacdlqb unlthdgkjv fdlniaorer vfxzaluh

a) [5pt] Find three long (at least four letters) repeating sequences of letters  by hand (i.e. don't use any programs), list their distance, and factor the distances.

b) [2pt] Based on your results from a), what are likely key lengths?

c) [8pt] Decrypt the text using the vigenere analysis tool we used in class. Warning: the ciphertext contains spaces and line breaks. For the program to work correctly, you need to remove all spaces and linebreaks before analyzing it. 2nd Warning: The keyword is not an English word.

2. (Vigenere Cipher, 10pt) The following text has been encrypted using a Vigenere cipher.

KHFNGNIIWV RMRGCFMMYS GXSCDBEGGU ARJWLSGNVF MZYTUKXIWX JAAJLVKTKD BCRRFWSETG RKGMRLXORR XKZEAJPPOT PJHYXRKBWH RQRVONJBHZ AVVBUNBZEE KPVRUOQWGK YEJGWRKMIA AMIISGUWRC IIWGFTHYIU LHJTEGRXWK JAAJAVJXJH  LUJKZXD

a) [5pt] Determine the key length using the kappa test (counting coincidences for various shifts).  You can assume that the keyword has between 6 and 15 characters. (You can compute coincidences using the vigenere analysis page). Write down the results and determine the most likely key length based on the results.

b) [5pt] Decrypt the text (you can use the vigenere analysis page or the tool we saw in class).

3. (Playfair Cipher, 20pt) You have intercepted the following plaintext/ciphertext pair (don't try to make sense of the plaintext ...):

```doomnoborgdimlackramtoox
EDNWBNMNBKIFQMRARBNLMEDW```
`You suspect a playfair cipher. `

a) [18pt] Reconstruct the key matrix used.

b) [2pt] Guess the keyword that was used to construct the key matrix.

`Note: Document and explain your approach step by step.`

Marcus Schaefer
Last updated: April 27th, 2011.