﻿ CS233

## Homework 2 (due 1/22) CSC 233

We talked about basic terminology in cryptography (including a discussion of Kerkhoff's principles), the Caesar cipher, and how to break it by hand, and automatically (using frequency analsyis); we then moved on to general monoalphabetic substitution ciphers, a subject we'll discuss in more depth next week. We saw some of the early, Arabic history, of this type of cipher.

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 sections 2.1-2.6 of the textbook. We'll continue with chapter 2 next time, so if you want to read ahead, check out 2.7-2.17 (though we won't cover all of those topics).

2. (Shift Cipher, 24pt) You'll be breaking several shift ciphers in this problem. You'll be allowed different tools for the different texts, ranging from a by hand solution, to an automated solution. For each problem, determine the original plaintext, and the shift used to encrypt it.

a) [8pt] Solve this problem by hand (you can write the solution in a texteditor, but don't use alphabet-strips to shift around, that'll be part b). Include a screenshot of your solution.

TACPQ PCHLW VTYR

b) [8] For this problem, use strips each consisting of two standard alphabets (horizontally or vertically). You can print them, or work with them in a text-editor (as we did), or even use the powerpoint implementation I showed in class (available on d2l). Text-editor version is probably easiest. Include a screenshot showing the strips.

OJCJK ZRVNO JZSKZ XO

Note: you don't need a strip for every letter, once you have the shift you can decrypt by group. So about 5 strips should be enough.

c) [8pt] Use the Caesar cipher spreadsheet we saw in class (available on d2l) to find the correct solution. Fill in the ciphertext, hit "analyze ciphertext", and then try all different shifts, looking for the shift that's closest to English. Include a screenshot (just the relevant parts showing the shift and the sum).

YFNHL ZTBTF DVKYV IVRJF EJWFI RGGIF MZEXN YRKNV CZB

Note: You need to enable macros for the spreadsheet to work. If you cannot run the spreadsheet on your computer or on a lab computer, use the method from b) instead.

3. (Transposition or Substitution, 16) You intercepted the four ciphertexts a) through d) below; you know that the original plaintexts are in English and that either a substitution cipher (such as the shift cipher, or the monoalphabetic substitution cipher) or a transposition cipher (such as the skytale) was used. For each ciphertext, determine whether it is the result of a substitution cipher or a transposition cipher and explain how you reached your decision. You  do not have to find out which particular cryptosystem was used in each case (we haven't seen most the four systems yet); in particular, you do not have to break the ciphers.

a) NLNNJ PUPRN UDCCX JPFEF JVKPP FUXUR PLNUJ PVCEU VFRVT UIRVI EFUJV TCVSU VJUVT UJPCD EUVTC VPJIU NROXU

b) SNTOH OSICH TEOUD EEEPL REIIN LIPRS NHESR AOCAA HIBFO NSTSL TNCUH EEEVE RISNE DENAC NAIGR OINSS ENEDN TF