﻿ CS233

## Homework 6 (due 2/26) CSC 233

We completed talking about the Playfair cipher, and how to break it in a known-plaintext attack. We saw several ciphers used during early American history and then focussed on columnar transposition ciphers. Next week we'll see how to break irregular transposition ciphers (with crib).

After completing transposition ciphers, we will move on to the mechanization of cryptology up to the time of WWI.

From class:

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 Section 4.1-4.4 of the textbook on transposition ciphers.

2. (An American Friend, 10pt) An American friend of yours, JL for short, sent you the following message.

I can only say that  22. 21. 24. 21. 9. 3. 6. 9. 27. 2. 23.

Hint: Find the right system. Any guesses for the key? You may want to reread the initial part of this problem.

3. (Simple regular Transposition Cipher, 20pt)

You suspect the following text to have been the result of a regular transposition cipher.

```TIEUT FLTST ADTAA ETIEH OUOHT HSEUO EEINS XRTRC ISVAS
LFOAH EBIER TMMGE HSNSR MUHTI LSUDT ASOAN TAFO
```

a) [3pt] Determine all possible key lengths (based on the fact that it is a regular cipher) in the range 5 - 13.

b) [6pt] Test all possible key lengths from a) using the vowel test: at least half the rows should have at least 40% vowels and the other rows shouldn't be too far off, count Y as a vowel. Show results of your counts for all key-lengths. Based on the test, order the key-lengths in the most likely order. (State that order explicitly.)

c) [11pt] Try key lengths in the order suggested by b) and decrypt the ciphertext by anagramming. Include details of how you broke the cipher, including failed attempts.

4. (Simple irregular Transposition Cipher, 10pt)

You have intercepted the following ciphertext, and you suspect an irregular transposition cipher (so the number of letters need not be a multiple of the keylength).

`EUONI TSTEE TAMTE IRSSN HQFIV EORSW IGIYP IRGNH ARTIH OG`

The text was encrypted by your friend Charles Babbage, and you suspect he was lazy and used either his first or last name as a keyword.

a) [3pt] Based on the number of letters and the suspected keys, how many long and how many short columns are there?

b) [3pt] For both possible keys, determine the order in which the columns of the pt occur in the ciphertext. (E.g. if the key were pear, the order would be 3, 2, 1, 4)

c) [4pt] Based on your findings in a) and b) decrypt the ciphertext.

Marcus Schaefer
Last updated: February 20th, 2020.