CPSC 120 -- Assignment #3: A Cryptic Assignment
Due Monday, November 10, 2008 by 4 p.m.

Your task is to write a program that can encipher and decipher text files using a Vigenère cipher. A Vigenère cipher works by shifting each character in a message by an amount determined by a keyword. Characters can be shifted quite easily in Java because characters are represented by numbers. Recall that (int)'a' in Java would evaluate to 97 because each character has a number associated with it. In this case the character 'a' is associated with the number 97. Also, because the letters 'a' through 'z' have sequential numbers it is possible to shift a character to the right by first converting it to an integer, then adding the amount to shift it by, and finally converting it back to a character. For example, in order to shift the character 'a' three letters to the right to 'd' the following code (char)(((int)'a')+3) could be used. This could be simplified to just (char)('a'+3) by taking advantage of automatic promotion in Java. In order to shift the character 'y' to the right by five to the character 'd' it is not sufficient to just add 5 to the character 'y'. Instead you must have the shift wrap back around to the beginning of the alphabet if it exceeds the last character, 'z'. (hint: this can be accomplished using modulo)

In a Vigenère cipher each character in the message to be enciphered is shifted to the right by an amount based on a character in a keyword. For example, to shift the character 's' to the right by the character 'd' the character 'd' is converted into an integer based on how many characters it is from the character 'a'. The character 'd' is 3 characters from the letter 'a'. This can be computed in by subtracting 'a' from 'd'. Therefore, in order to shift the character 's' to the right by 'd', shift the character 's' by 3. In order to decipher messages the same procedure is followed, except, characters are shifted to the left instead of the right.

However, in a Vigeneger cipher not every character in a message is shifted by the same character. Each character is shifted by the corresponding character in the keyword. That is, if the message being enciphered is "something fowl", the keyword is "duck", and the first letter of the message, 's', is shifted by the first letter of the keyword, 'd', then the second letter of the message, 'o' is shifted by the second letter of the keyword, 'u'. If there are more letters in the message than there are in the keyword, which there usually are, then the amount to shift by wraps back around to the first letter in the keyword after a character in the message has been shifted by the last letter. Characters that are not in the alphabet, like the space character, should not be shifted. Using the above example the message "something fowl" enciphered using the keyword "duck" is:

plain text

s

o

m

e

t

h

i

n

g

f

o

w

l

shift ammounts

d

u

c

k

d

u

c

k

d

u

c

k

d

u

enciphered text

v

i

o

o

w

b

k

x

j

h

y

z

f

Your program should prompt the user to enter the name of a text file that contains a message, the keyword to use with the message, and whether to encipher or decipher the message. It should then print the result of enciphering or deciphering the text file to the console. In the Java Character class there is a method isLetter that returns true if a character is a letter and false if it is not. Capital letters can be converted to lowercase letters by using the toLowerCase method in the String class. You can test your program on the following files:

• message1.txt - the message "something fowl"
• message2.txt - message 1 enciphered with the keyword "duck"
• message3.txt - a message enciphered with the keyword "ManchesterBluff"
• message4.txt - a message enciphered with the keyword "liberty"
• message5.txt - a message enciphered with the keyword "paradox"