Question

Hello! I'm trying to write a code that will either encrypt/decrypt a message from an inputed text. I'm having the absolute hardest time getting stared and figuring out how to identify in the code if the input needs to be encrypted/decrypted and identifying the string to encrypt/decrypt with. These are the instructions:

Objectives

• Command line input
• File input and output
• Rethrowing exceptions

Program Description

Gaius Julius Caesar encoded his battle messages so that the opponent could not read them should they intercept them. The cipher required that both Caesar and the recipient knew the key to the cipher.

You are to write a program that reads in a message from a text file, encodes the message with a Caesar cipher, and then outputs the message to another file. Your program should also be able to read in an encoded message and decode it.

1. Define What this program must do. We did this in class. Check your class notes!

2. Outline How this program will accomplish the task. Your solution should be modularized.

Specifications

Input

The program should read, in order, command line arguments.
- The filename of the text file to open
- The cipher key for encryption/decryption
- A digit: 1 to encode or 2 to decode

*Using an IDE, look for "Run configurations" to set arguments. The IDE arguments needed are only the filename, cipher key, and encode/decode digit. You don't need the "java Cipher" portion that would be required if truly running from the command line.

Example of full command line input to encrypt:

 java Cipher messageFile secretword 1    
 The result is a new encoded file called "messageFile.coded"  

Example of full command line input to decrypt:

 java Cipher messageFile.coded secretword 2  
 The result is a new decoded file called "messageFile.decoded"

Note that the decoded message in "messgeFile.decoded" should be identical to the original file "messageFile"

Errors and Exception handling

• If the file does not exist, or the user somehow provides bad input data, the program should "crash", that is, rethrow the error to allow the default handler take care of it. (In the second version, this program will handle the errors with try/catch constructs.)
• Only the digit 1 or 2 is allowed. Any other integer value will result in an error message and the program will terminate. For example, if the arguments are messageFile secretword 3, the error message will be:
  Option 3 is not valid
• You may assume the cipher key is a single word of alphabet characters only.
• You may assume the text in the message file has alphabetic characters and punctuation characters (spaces, exclamation points, periods) only.

Encryption and Decryption

• To encrypt a message, each letter in the message is shifted right by a number corresponding to the distance of the cipher key letter from the first letter in the alphabet. The cipher key is reused over and over until the message is encrypted. Punctuation and spaces are not encrypted but still appear in the encoded message. Uppercase should be converted to lowercase. Letters "wrap" around from 'z' to 'a'.

• Message: the ships sail
  Cipher key: swiftly
  Output: ldm lsgho xttj wb wluf

Message letter	Alphabet value	Cipher letter	Alphabet Value	Add Values / Wrap for Offset	'a' + Offset = Result
t	19	s	18	37 % 26 = 11	l
h	7	w	22	29 % 26 = 3	d
e	4	i	8	12 % 26 = 12	m
		f	none
s	18	t	19	37 % 26 = 11	l
h	7	l	11	18 % 26 = 18	s
i	8	y	24	32 % 26 = 6	g
p	15	s	18	33 % 26 = 7	h
s	18	w	22	40 % 26 = 14	o
		i	none
s	18	f	5	23 % 26 = 23	x
a	0	t	19	19 % 26 = 19	t
i	8	l	11	19 % 26 = 19	t
l	11	y	24	9 % 26 = 9	j

Other examples:

• Message: the ships sail at dawn
  Cipher key: secretword
  Output: llg waedj kekc tp udor

• Message: the ships sail at dawn
  Cipher key: aaa
  Output: the ships sail at dawn

Output

• Output is written to a text file which is named same as the input file plus the extension:
  .coded if being encrypted or
  .decoded if being decrypted .

Helper Methods

You will want to add methods to shift up and down.

And this is the template provided:

/*
* Fix me
*/
import java.util.Scanner;
import java.io.PrintWriter;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;

public class Cipher {

   public static final int NUM_LETTERS = 26;
   public static final int ENCODE = 1;
   public static final int DECODE = 2;

   public static void main(String[] args) /* FIX ME */ {

// letters
String alphabet = "abcdefghijklmnopqrstuvwxyz";
  
// Check args length, if error, print usage message and exit
if (args.length...

// Extract input args to variables
       String inputFilename =
       String key =
       int action =
       String outputFilename = getOutputFilename(inputFilename, action);
       Scanner input = openInput(inputFilename);
       PrintWriter output = openOutput(outputFilename);

// Read in data and output to file
// Convert all letters to lowercase for output

       // Close streams

   }

   /**
   * Open input for reading
   *
   * @param filename
   * @return Scanner
   * @throws FileNotFoundException
   */

  
   /**
   * Open output for writing
   *
   * @param filename
   * @return PrintWriter
   * @throws FileNotFoundException
   */

/**
* Encode letter by some offset d
*
* @param c input character
* @param offset amount to shift character value
* @return char encoded character
*/
   public static char shiftUpByK(char c, int distance) {
       if ('a' <= c && c <= 'z')
           return (char) ('a' + (c - 'a' + distance) % NUM_LETTERS);
       if ('A' <= c && c <= 'Z')
           return (char) ('A' + (c - 'A' + distance) % NUM_LETTERS);
       return c; // don't encrypt if not an ic character
   }
  
   /**
* Decode letter by some offset d
*
* @param c input character
* @param offset amount to shift character value
* @return char decoded character
*/

   /**
   * Changes file extension to ".coded" or ".decoded"
   *
   * @param filename
   * @return String new filename or null if action is illegal
   */

   public String getInfo() {
       return "Program 3, Student's name here";
   }

}

If someone could please help and use the template to code this in JAVA in the next couple of days it would be greatly appreciated, thank you!!!

Answer 1

Hi, I have written an implementation for the above scenario with appropriate comments wherever necessary for better understanding.

Note : in the below implementation input and output are as follows:

INPUT :   java Cipher messageFile.decoded secretword 1 ( to encode )

OUTPUT :   java Cipher messageFile.coded secretword 2 ( to dencode )

In the below implementation

key.charAt(i % key.length()) - 'a') gives us the next character of the key that is to be used to shift the character.

basically, we have to use mod operator that limits the integer values till key length.

Other functions should be straightforward to understand. If you find difficulty understanding anything, please drop a comment.

Following is the implementation:

import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintWriter;
import java.util.Scanner;

public class Cipher {

    public static final int NUM_LETTERS = 26;
    public static final int ENCODE = 1;
    public static final int DECODE = 2;

    public static void main(String[] args) {

        // Check args length, if error, print usage message and exit
        if (args.length < 3) {
            System.out.println("Invalid arguments");
            System.exit(0);
        }
        // Extract input args to variables
        String inputFilename = args[0];
        String key = args[1];
        int action = Integer.parseInt(args[2]);

        // check if action is valid
        if(action!=1 || action!=2) {
            System.out.println("Option #"+action+" is not valid");
            System.exit(0);
        }

        // get output filename
        String outputFilename = getOutputFilename(inputFilename, action);

        // set input and output
        Scanner input = openInput(inputFilename);
        PrintWriter output = openOutput(outputFilename);

        // Read in data and output to file
        while (input.hasNextLine()) {
            // read next line
            String line = input.nextLine();
            String result = "";
            // if action is encode
            if (action == ENCODE) {
                result = encodeInput(line, key);
            } else {
                result = decodeInput(line, key);
            }
            System.out.println(result);
            // write to file
            output.write(result);
        }

        // Close streams
        input.close();
        output.close();

    }

    /**
     * Encode letter by some offset d
     *
     * @param c      input character
     * @param offset amount to shift character value
     * @return char encoded character
     */
    public static char shiftUpByK(char c, int distance) {
        if ('a' <= c && c <= 'z')
            return (char) ('a' + (c - 'a' + distance) % NUM_LETTERS);
        if ('A' <= c && c <= 'Z')
            return (char) ('A' + (c - 'A' + distance) % NUM_LETTERS);
        return c; // don't encrypt if not an ic character
    }

    /**
     * Decode letter by some offset d
     *
     * @param c      input character
     * @param offset amount to shift character value
     * @return char decoded character
     */
    public static char shiftDownByK(char c, int distance) {
        if ('a' <= c && c <= 'z')
            return (char) ('a' + (c - 'a' - distance + 26) % NUM_LETTERS);
        if ('A' <= c && c <= 'Z')
            return (char) ('A' + (c - 'A' - distance + 26) % NUM_LETTERS);
        return c; // don't encrypt if not an ic character
    }

    /**
     * Open input for reading
     *
     * @param filename
     * @return scanner object of input file to read the file
     * @throws FileNotFoundException
     */
    public static Scanner openInput(String inputFile) {
        File file = new File(inputFile);
        // Creating Scanner instnace to read File in Java
        try {
            return new Scanner(file);
        } catch (FileNotFoundException e) {
            System.out.println("invalid input file");
            e.printStackTrace();
        }
        return null;

    }

    /**
     * Open output for writing
     *
     * @param filename
     * @return PrintWriter
     * @throws FileNotFoundException
     */
    public static PrintWriter openOutput(String outputFile) {
        File file = new File(outputFile);
        try {
            return new PrintWriter(file);
        } catch (FileNotFoundException e) {
            System.out.println("invalid output filename");
            e.printStackTrace();
        }
        return null;
    }

    /**
     * Changes file extension to ".coded" or ".decoded"
     * 
     * @param filename
     * @return String new filename or null if action is illegal
     */
    public static String getOutputFilename(String input, int action) {
        // if file extension is not provided
        if (!input.contains(".")) {
            System.out.println("invalid input file passed");
            return null;
        } else {
            // get the file name
            String fileName = input.split("\\.")[0];
            if (action == ENCODE)
                return fileName + ".coded";
            else
                return fileName + ".decoded";
        }
    }

    // encode the text input using a key
    public static String encodeInput(String input, String key) {
        // initial result
        String result = "";
        for (int i = 0; i < input.length(); i++) {
            // to encode shift up by k
            char encodedLetter = shiftUpByK(input.charAt(i), key.charAt(i % key.length()) - 'a');
            // append the encoded character to the result
            result += encodedLetter;
        }
        // return result
        return result;
    }

    // decode the encoded input using a key
    public static String decodeInput(String input, String key) {
        // initial result
        String result = "";
        // loop throuh the input
        for (int i = 0; i < input.length(); i++) {
            // to decode shift down by k
            char encodedLetter = shiftDownByK(input.charAt(i), key.charAt(i % key.length()) - 'a');
            // append the decoded character to the result
            result += encodedLetter;
        }
        // return result
        return result;
    }

}