Homework 1 (due 1/15)
CSC 233

We've talked about some early cryptographic history, focussing on the Atbash, Skytale and Polybius ciphers. Next week, we'll continue with a more in-depth study of substitution ciphers (such as the Caesar cipher).

If you want to stay on top of security news, I'd strongly recommend the blogs by Brian Krebs and Bruce Schneier.

Submission: The homework is due by midnight (I will not accept late homeworks). You can submit your homework through d2l into the drop-box for this homework.

Please prepare your homework as a single file containing all answers (e.g. doc, docx, or pdf, not a zip file). For an example, see hwexample.docx . How to take screenshots? Check out screenshots for MAC, Windows, Linux.

1. (Reading Assignment) Read Chapter 1 of the textbook, it covers early history (including some history we didn't discuss in class). Also, read Section 4.1.2 on breaking rectangular transpositions. The book only covers the balanced case (where the number of letters is a multiple of the number of columns). In class we saw how to break the more general case. If you want to read ahead, check out Chapter 2 on monoalphabetic substitution ciphers.

2. (Academic Integrity) Read and sign the academic integrity statement (word, pdf). Upload the signed document (you can sign electronically, by typing in your name, or taking a picture of the signature page) as part of your homework submission.

3. (Skytale, 10pt) You know that the ciphertext


was encrypted using a Skytale transposition with 6 columns. Find the plaintext. Note: include the filled out row/column display, as well as the final plaintext (with word separations).

4. (Skytale, 15pt) The following text has been encrypted using a skytale transposition. Recover the plaintext, and include details of how you found the plaintext (including at least one failed attempt). You know that the number of columns is between 5 and 10.


Hint: You don't have to complete the decryptions for each failed attempt if you see early on that you are not getting English (as you look at the first letters in the decrypt). Include details for at least one failed attempt, including a brief explanation why it did not work. For the correct number of columns, include the full row/column display as well as the final plaintext (with word separations). 

5. (Polybius Cipher, 15pt)

a) [3pt] Create the Polybius square for the keyword "ATTACK". Note: we make the standard assumption that i = j so we have a 25 letter alphabet.

b) [3pt] Use the Polybius square from a) to encrypt the plaintext "secret".

c) [3pt] Use the Polybius square from a) to decrypt the ciphertext "13321144443123312221"

d) [6pt] You've intercepted the ciphertext

14 42 54 35 44 34 13 42 12 32

Based on the form of the ciphertext you suspect a Polybius cipher, but when you decrypt (with the standard square) you get nonsense. The sender must have used a Polybius cipher based on some keyword. 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 used to construct the Polybius square did not contain letters in the second half of the alphabet (N-Z).

Decrypt the message, reconstruct the Polybius square, and find the keyword used to create it. Hint: the fact that the keyword doesn't contain the letters N-Z tells you something about the square. Use that. Some trial/error, guessing may be involved. Include a brief description of how you managed to break the cipher.

Marcus Schaefer
Last updated: January 9th, 2020.