## Homework 6 (due 2/21) CSC 233

We saw a known plaintext attack on a single rotor cipher in detail, and talked more about the Enigma (taking material from Chapter 4). Next week, we will talk about Rejewski's and Turing's work on breaking the enigma, and watch (excerpts from) the movie Enigma.

Note: your more detailed proposal is also due by next week Tuesday.

1. (Wheel Cipher) Using the virtual M94 simulator, decrypt the message

HWQCURJBRKMBESXXHBEQMLWFE

which was encrypted using the keyword "parsimonious".

2. (Wheel Cipher) You have intercepted the following ciphertext, for which you also know the corresponding plaintext:

```badreasoningaswellasgoodr
FVPJGDWUCWANPDAHVXKMUPXFO
```

The encryption was performed with the cylinders listed in the M94 (virtual M94 simulator). You also suspect that the encrypter was lazy, and advanced the wheels (i.e. moved them up) by only five positions, to go from plaintext to ciphertext. Can you figure out, which is the first wheel used in the encryption (only the first wheel)? Why?

3. (ADFGX) Encrypt the plaintext "Turing" using the key matrix we saw in class:

 A D F G X A c o x f m D k a z n w F l ij d s y G h u p v b X r e q t g

and the keyword "enigma" in the ADFGX system.

4. (Vernam Cipher) You want to encrypt the text "time" using the Vernam Cipher.

a) Using the Baudot code, translate "time" into binary (see Baudot table for the code).

b) You have two tapes, containing "110" and "01011". Use Morehouse's (cryptographically flawed) method to construct a long key, and use that key to encrypt your result from a).

5. (Rotor Cipher) Next time in class we will do problem P3.2 on the handout. For that, prepare two sheets: first, as described on the pages numbered 25/26 on the handout, fill in the double rows of 26 columns with plaintext and ciphertext. The rotor starting position is C, so the first column you will fill out is the D column (the rotor advances before encryption) with plaintext T and ciphertext S. Secondly, transcribe this result into the Friedman tableau (in the handout example, the result is on page 28). This is what we will start with next week.

6. (Extra Credit) Returning to the problem in Exercise 2: the operator did not advance by five positions, as claimed, but by a different number.

a) Give a short sketch, how you would go about finding the order of the wheels.

b) Find the order of the wheels that was used for this example.

Marcus Schaefer
Last updated: February 19th, 2004.