CP2003 Assignment One

CP2003 Assignment 1 Cryptographic Program

Suppose a sender wants to send a message to a receiver securely. That is, the sender and receiver do not want an eavesdropper to read the message. Originally the message is in plain text or clear text form. The process of disguising a message in such a way as to hide its substance is encryption. An encrypted message is cipher text. The process of turning cipher text back into plain text is decryption. A cryptographic algorithm, also called a cipher, is the mathematical function used for encryption and decryption. Typically the algorithm is made public and each user is allocated a secret key which can be applied to the message using the algorithm. The success of the system lies in keeping the key a secret. The following figure illustrates this process.

A common and simple form of cipher is a substitution cipher where each character in the plain text is substituted for another character in the cipher text. e.g. 'A' becomes 'D', 'B' becomes 'E', etc. ROT13 is an encryption program commonly found on UNIX systems which utilizes such a substitution cipher rotating each character by 13.

Task: You are to write a program in Java which implements a substitution cipher where every character and symbol in a plain text message is rotated by 1.

e.g: plain text a, b, c, d, e, f, ..., etc.
cipher text b, c, d, e, f, g, ..., etc.

The program should be able to accept a plain text message either from the keyboard or file, encrypt it, and then write the output to either the screen or a file depending on the command line arguments given.

Several scenarios must be catered for:

Command line Task

java Cipher Displays main menu and only accepts keyboard I/O

java Cipher e Enters Encrypt mode and only accepts keyboard I/O

java Cipher d Enters Decrypt mode and only accepts keyboard I/O

java Cipher e infile Enters Encrypt mode using infile as a source for plain text, prompts user for outfile

java Cipher e infile outfile Enters Encrypt mode using infile as a source for plain text, writes cipher text to outfile

java Cipher d infile Enters Decrypt mode using infile as a source for plain text, prompts user for outfile

java Cipher d infile outfile Enters Decrypt mode using infile as a source for plain text, writes cipher text to outfile

If no command line arguments are given the program will present the user with a main menu providing the following options:

java Cipher

*** Main Menu ***

e Encrypt mode
d Decrypt mode
x Exit

Enter choice:

Depending on the choice selected the program will enter the corresponding mode and prompt the user to enter the plain text message.

Enter choice: e

*Encrypt Mode*

Enter message to encrypt: The fat cat sat on the mat.

Enter choice: d

*Decrypt Mode*

Enter message to decrypt: Uif gbu dbu tbu po uif nbu/

Enter choice: x

Exiting program..

Once entered the program will encrypt/decrypt the message, display the encrypted/decrypted message on the screen and then return to the main menu.

Encrypted message: Uif gbu dbu tbu po uif nbu/

Decrypted message: The fat cat sat on the mat.

Alternately encrypt or decrypt mode may be entered into directly at the command line bypassing the main menu by providing the command line arguments 'd' or 'e'. With all input being entered via keyboard. When the operation is complete the main menu will be displayed.

java Cipher e
*Encrypt Mode*
Enter message to encrypt:
java Cipher d
*Decrypt Mode*
Enter message to decrypt:

In addition the user may specify file input or output by providing file names at the command line.

java Cipher e infile.txt outfile.txt
*Encrypt Mode*
Encryption Complete
Output written to outfile.txt
Exiting program..
java Cipher d infile.txt outfile.txt
*Decrypt Mode*
Decryption Complete
Output written to outfile.txt
Exiting program..

If the outfile name is absent the program will prompt the user for a name before encyrption/decryption commences.

java Cipher e infile.txt
*Encrypt Mode*
Please enter name of output file:
java Cipher d infile.txt
*Decrypt Mode*
Please enter name of output file:

Once the desired operation has been performed the program terminates.

Due Date

Week 7

Submission Details

Email source code to your tutor

Marking Scheme

Execution
Program structure and design
Exception checking/handeling
Comments
Formatting

Command line	Task
java Cipher	Displays main menu and only accepts keyboard I/O
java Cipher e	Enters Encrypt mode and only accepts keyboard I/O
java Cipher d	Enters Decrypt mode and only accepts keyboard I/O
java Cipher e infile	Enters Encrypt mode using infile as a source for plain text, prompts user for outfile
java Cipher e infile outfile	Enters Encrypt mode using infile as a source for plain text, writes cipher text to outfile
java Cipher d infile	Enters Decrypt mode using infile as a source for plain text, prompts user for outfile
java Cipher d infile outfile	Enters Decrypt mode using infile as a source for plain text, writes cipher text to outfile