CMSC 414   

			       Fall 2002


Homework # 1   				Due September 19, 2002

						
In order to receive credit, solutions still must be electronically 
SUBMITted by 12:00 Noon on the above date.  Directions use of the SUBMIT 
program will be discussed in class on on Tuesday, September 17 and posted
to the webpage before then.


Written Exercises:

(Doing the programming problems first will probably help you to solve
the written problems.  It's OK to use your programs to solve the written
problems, but all steps must be explained in order to receive credit.
"Computer programs says XXXX" is NOT an explanation)



1.  	(Problem # 6 from Chapter 9 of the textbook) Deciphering 
	ciphertext generated by a Caesar cipher involves enciphering
	with a key which is different from the enciphering key.
	Why then is a Caesar cipher considered to be a classical
	cryptosystem instead of a public key cryptosystem.  Be
	specific.  


2.  	As discussed in class, one-time pads are provably secure but 
	there are some logistical problems associated with their use.  
	Give an example of a situation where a one-time pad can easily  
	be used and an example where use of a one-time pad would be 
	difficult to implement.  (The example given in class is 
	acceptable, but expand and fully explain why)

3.  	The following ciphertext is suspected to be the output of a
	Vigenere cipher.  Solve for the plaintext, explaining all 
	assumptions and showing each step.  (Obtaining the correct
	plaintext without explaining the steps is NOT a correct solution,
	and will not receive any credit)  Also keep in mind the class
	discussion on transcription errors when working these problems:

	fycpa lgvqq gqmfc odzjr joert tpdne jrlla njixo alfeq
	qvyxz wqbsy tphry mfofo lsrak iniva lglfs cllbn eozrg
	yvivo lsvbe ajfek haahf cqtqg lqody brtos cyrjr nchfu
	iopyk phdwn eowai jfzge pxxqy emwum usatf xhryy vifty
	qfvhs mqmmh qsfkq rnqfe ioyxj rvhwn wfjoe kuzbf hkfec
	uenwf ouakf twpps yvzfo mprbd srddm dldnx pwedk zklel
	ycggi clrto alycg vpchz ioixf kqrng xwwui lxvkw s

	(Robert Heinlein)
	
4.	Please decipher the following ciphertext.  Explain each step.
	(correct plaintext with no explanation is not an valid answer):

	Uiptf xip xpvme hjwf vq fttfoujbm mjcfsuz up qvsdibtf b mjuumf 

	ufnqpsbsz tbgfuz eftfswf ofjuifs mjcfsuz ops tbgfuz

	(Ben Franklin)



5.	Please decrypt the following cipher text.  Show and explain all
	steps (correct plaintext with no explanation is not an valid 
	answer):

        A HGWSIHLK C SSCIQE WPVU E KRKAR WCXTVDGD LRAY QEH ZKNQDX

        FAANVCT YEC D IPVUTC ICMEUZIT

        PANVC T VDG PVFTC Q WTR UIGCXY HTG WTCVYROS G

        SEN S OVTEPY EDVUP DSWZ OWBGV SEX OPMLRK P MTHTK YRFYS EBYKP

        NAGMC S PTY XVJT A YYWY OO EWZUG WXX IBSV OHTVD GSRU LMQX WKY

        AVSRX AHGZSJ ZID SPZ A YQPX YPSMSR UVFCDXR JKK ZMTR E FYRKOEEMS

        DWINO VZJOWVHY H VGZ MT REFYR RUYRB SXYAO REEK LKXKGYOIBED


Programming Exercises:

(You are encouraged to use Java, if you use Java you must use JDK 1.3 or
above.  In future assignments you may be required to use Java, and they
may build on this assignment.)   Submit the source code as well as an
executable version using the "submit" procedure which will be detailed
later.


1.	Write a program which counts the instances of each letter in
	an input file (the default input file should be ciphertxt.txt) 
	and performs a frequency analysis.  You may ignore non-alphabetic 
	characters and need not distinguish between upper- and lower-case
	letters.  Name the source file tally.java (or .c, .cpp, etc if 
	you are ignoring the above suggestions).  Give total number of 
	occurrences of each letter as well as expressing the frequency 
	as a percent of the characters.  Also output the total number of 
	characters.

	Usage:   java tally
		 java tally input.file

2.	Write a program to calculate the index of coincidence of
	provided ciphertext.  By default the ciphertext will be in a file 
	called ciphertxt.txt, but an option should be provided for 
	specifying a different file from the command line .   Name your 
	source file ic.java (.c, .cpp, etc).

	Usage:   java ic
		 java ic input.file
 
3.	Write a program that uses kasiski's method to determine the
	probable key period of a Vigenere cipher, stored by default in
	ciphertxt.txt.  Name your source file kasi.java (.c, .cpp, etc).

	Usage	 java kasi
		 java kasi input.file

4.	Write a program to sort the ciphertext stream into individual
	alphabet streams (for use when applying Kasiski's method to
	a Vigenere cipher).  Name your source file sort.java (.c, 
	.cpp, etc).  The number of streams should be a command line
	input, the source file shall be handled as in the above cases.

	Usage 	 java sort NumStream
		 java sort NumStream input.file