## Homework 1 (due 4/12) CSC 233

We are in chapter 1, getting closer to the death of Mary, Queen of Scots; we saw an excerpt from Elizabeth: The Golden Years giving a highly inaccurate if stylish account of The Babington Plot. There are historically more accurate accounts of how Walsingham trapped Mary. We took a detour through early cryptography, and talked about basic terminology in cryptography. See the notes linked off the main page.

There've been a couple of spectacular security failures in the news recently, check out:

If you want to stay on top of security news, be sure to subscribe to Bruce Schneier's CyrptoGram newsletter or read his blog.

We will talk about Arab cryptography and substitution ciphers next, after which we'll move on to Renaissance cryptography (Chapter 2), bringing us back to Mary, Queen of Scots.

Submission: The homework is due at the beginning of class Tuesday. You can either submit it by hardcopy or email it to me directly.

1. (Reading Assignment) Read Chapter 1 of Simon Singh's The Code Book. (We have covered about half the material already, and will cover the rest next week).

3. (Skytale, 15pt) The following text has been encrypted using a skytale (and removing resulting spaces); to make the ciphertext easier to read I have split the letters into blocks of 5. Recover the plaintext, and include a description of how you found the plaintext (including failed attempts). You know that the number of columns is between 5 and 10.

"TOMAN HFILC EFSVE PREIR ETGIE EICDR LEONA"

4. (Caesar Cipher, 10pt) The following ciphertext results from a Caesar cipher (and removing all spaces and splitting the letters into blocks of 5):

"OZYEW PEDNS ZZWTY RTYEP CQPCP HTESJ ZFCPO FNLET ZY"

Determine the shift, and recover the plaintext by hand (briefly describe how you did it, you don't have to use brute force!).

5. (Polybius Cipher, 15pt) You've intercepted the ciphertext

12 42 42 21 43 44 25 33 11 21 34 13 25 34 23

Based on the ciphertext you suspect a Polybius cipher, but when you decrypt you get nonsense, so the sender must have used a keyed Polybius cipher. You know that the sender is lazy and suspect that the keyword is short (length 3). By bribing the secretary of the sender you find out that the keyword contains the letter P and the other two letters of the keyword precede P in the alphabet.

Decrypt the message, and describe how you solved the problem.

6. (Extra Credit, 10pt) Find a historical cryptosystem (in use before 1000) that we have not covered in class and that is not taken from the book; describe what we know about it, including a sample encryption (and decryption) done by yourself. Include information about your sources, and make sure you write up your description in your own words.

7. (Extra Credit - Programming, 10pt): Implement the skytale cipher (encryption, and, for extra extra credit, decryption, both given a number of columns) in the programming language of your choice.

Marcus Schaefer
Last updated: April 5th, 2011.