COMP 133 - Assignment 3

Due in class November 15

Problem 1. The following Enigma ciphertext contains the crib "HEILHITLER":

SRUPH TEPPQ LZREF MNIJT EPAZL SDHPQ WCVTP IQASY UFLIW CGFPO MULCD

State all of the positions in the ciphertext that it could occur.

Problem 2. The following two ciphertexts were created using the same (not so random) one-time pad. Can you find the two plaintexts? Hint: The first ciphertext contains the word "licorice".

bmcdm okfsu mhila rsfhl iyami aulrz mzxlfr
mzhbx rtlxh jtsim wubvj wuara ccxpn kyptqf

Problem 3. In the following sentences, all vowels and spaces have been removed. See if you can reconstruct the original sentence. (Note: it is possible that the answer is not unique.)

  1. MYFVRTCLSSSCRYPTGRPHY
  2. THRNNSPNFLLSMNLYNTHPLN
  3. CNTGTNSTSFCTN
  4. LLTHTHRKDSWTHTHPMPDPKCKSYDBTTRN

Problem 4. Let E-DES-K(x) denote the DES encryption of string x using key K and let D-DES-K(x) denote the DES decryption of string x using key k. In particular, note that D-DES-K(E-DES-K(x)) = x. Triple DES (usually written 3DES) uses three DES keys K1, K2, and K3 and encrypts a string x as follows: 3DES(x) = E-DES-K3(D-DES-K2(E-DES-K1(x))).

  1. What would be the corresponding decryption function for 3DES?
  2. Why do you think triple DES does a decryption as its second step instead of an encryption?

Problem 5. Assume Bob's public key is n = 18721 and e = 25 . Further assume that Alice sends a message to Bob by representing each alphabetic character of the message as an integer between 0 and 25 with a = 0, b = 1,..., z =25, and then encrypting and sending each letter separately. Describe how Eve might attack such a system without factoring n . Illustrate your attack on the following message: 365, 0, 4845, 14930, 2608, 2608, 0. Note: there are a number of sites on the web that compute powers using modular arithmetic. Here is the one I used to create this problem: http://computerscience.jbpub.com/cryptography/TestPowApplet.cfm
Report problems to dkrizanc at wesleyan dot edu Top of Page