CS 538 - Homework 1

                                                                                                                               CS 538
                                                                                                                               Spring 1999

                              Assignment 1


Date Due:  Tuesday, February 2

Reading:  Chapter 1, pages 1-39.


Problems:


1. Decode the coded paragraphs in the syllabus. 
Find the key(s) for all the codes and write the coding
algorithm for the third code. Note that I have not 
changed the capitalization of the letter but have encoded 
capital and small letters
in the same way in each case. I have only encoded letters and not any other symbols.

2. Page 40, #1.1 (a) and (b)

3. Page 40, # 1.2, all

4. Page 41, #1.4

5. Page 41, #1.6

6. Decipher ycxkhld knowing that an affine cipher with additive constant
7 was used and that it is an English word.

7. Cryptanalyze the following ciphertext and find the keys. 
A key phrase monoalphabetic
cipher was used. This is a substitution cipher where the key is a word 
or phrase P together with a letter L. Starting at L the substitution
is made with the corresponding letter of P. At the end of P all the other
letters, those not in P, are placed alphabetically. SO for example if
the phrase is computer and the letter is d, then the substitution is:

a b c d e f g h i j k l m n o p ...

x y z c o m p u t e r a b d f g ...

-------------------------------------------------------------------

uc zitks, siojtr ucamtikaudm euxx xibcks euclreo km
dmtourco rn rxl nidrtuamo zrcryrxg icl oktijjxm, io emxx io
atuduix ybtobua icl oxucqr. asm qizmo euxx kroa ijrba bo$30
uc tmaiux oartmo icl euxx ixor jm orxl rc asm krzyicg'o emj
ouam.

asm jmdmtxg, ziooiksbomaao-jioml krzyicg euxx xibcks
astmm mziux dmtourco rn qizmo cmfa zrcas icl 11 asuo gmit.
i kl-trz euas rcm qizm euxx kroa $15 uc oartmo, icl rcxg
rcm yxigmt sio ar sidm ua. jba jras yxigmto sidm ar sidm
euclreo-jioml mziux asia kic omcl iaaikszmcao, ikkrtlucq
ar i siojtr oyrwmoerzic. asm yxigmt euas asm mziux dmtourc
rn oktijjxm, nrt mfizyxm, ucduamo i ntumcl ar yxig dui mziux.
asm ntumcl zboa lrecxril asm oktijjxm iyyxukiaurc, esuks
aiwmo ijrba 20 zucbamo rc i 28.8 zrlmz, icl asmc mziux
jikw ar yxig asm qizm. miks mziux qrmo astrbqs omtdmto
ziucaiucml jg siojtr.

8. Devise your own classical cipher system. 

It should be reasonably hard to decode but not mathematically sophisticated.
So no public key encryption or hard math past modular 
arithmetic should be used. Try to make it harder to break and a
permutation cipher, but still easy to code and decode. 
Give both an encoding the decoding algorithm for your system.
And give an example as well. 

Now criticise your own system. Hoe hard do you think it is to break ?
What kinds of attacks is it susceptible to ?