Programming Assignment 07

Due: Week of Oct. 24 in lab

1. Write a library for encrypting and decrypting a message with a shift cipher (Caesar cipher). A shift cipher encodes a message by replacing each letter with a corresponding letter up or down by a specific amount in the alphebet. Write two clients for the library to aid a user in (1) encrypting a specific message and (2) decrypting an encoded message. Remember - BREAK THE PROBLEM DOWN INTO PIECES TO MAKE IT EASIER. Worth 60 pts. Messages are assumed to be upper case!
   Details for the library:
   • Name the file and program ShiftCipher.java
   • The API is as follows:
     • void encrypt(char[] msg, int shift)
     • void decrypt(char[] msg, int shift)
   • Encode a letter c with the following: (char)((c - 65 + shift) % 26 + 65)
   • Decode a letter c with the following: (char)((c - 65 - shift + 26) % 26 + 65)
   • Space characters should be ignored by encrypt and decrypt.
   • The main function for ShiftCipher.java should contain a well thought out unit test for encrypt and decrypt.
   
   Details for the encrypting client:
   • Name the file and program Encrypt.java
   • Request a one line message from the user from Standard Input. Use Scanner's nextLine() function, not any loops.
   • Request a shift size from the user from Standard Input.
   • Call the encrypt function, remember you have to convert the message to a char[] (toCharArray() in String).
   • Output the encoded message to a file. All of the message is on one line. The file name is given as a command-line argument to the program.
   
   Details for the decrypting client:
   • Name the file and program Decrypt.java
   • Read a one line encodes message from a file. The file's name is provided by a command-line argument to the program.
   • Request a shift size from the user from Standard Input.
   • Call the decrypt function, remember you have to convert the message to a char[] (toCharArray() in String).
   • Output the encoded message to standard output. All of the message is on one line.
   
   Examples:

   • Input:	$ java ShiftCipher
     Output:	(this is specific to your unit test)

   • Input:	$ java Encrypt msg.txt
     Output:	(user enters information when prompted)
     	Please enter a message: HELLO THERE Please enter a shift: 3 msg.txt contains: KHOOR WKHUH

   • Input:	$ java Decrypt msg.txt (msg.txt contains KHOOR WKHUH)
     Output:	(user enters information when prompted)
     	Please enter a shift: 3 HELLO THERE

2. Write a program using a recursive function that reverses a String. The String is input as a command-line argument and output to the terminal in the main function. Name the file and program ch2sec3prob00.java. Worth 20pts. Use only length(), substring(), and charAt() from String. The recursive function is ~3 lines of code and main is ~1 line of code.
   Examples:

   • Input:	$ java ch2sec3prob00 Hello
     Output:	olleH

3. Write a program using a recursive function that outputs the binary representation of an integer. The integer is input as a command-line argument. Name the file and program ch2sec3prob01.java. Worth 20pts. The recursive function is ~6 lines of code and main is ~2 line of code.
   Examples:

   • Input:	$ java ch2sec3prob01 10
     Output:	1010