Question

Write a simple program in Java or C, that will help you to figure out the key used for Vigenère encrypted file.

For this exercise, assume the key length is less than five characters long and only English upper case letters from A-Z are used. You may also assume the plaintext contains only the upper case English letters from A-Z, ignore the space characters.

Your program should take a ciphertext file (encrypted using the Vigenère encryption algorithm) as input and produce an output file where at the first line print the “key” used for the encryption, followed by a decrypted plaintext in it.

Answer 1

/*
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package vingenere;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileWriter;
import java.io.IOException;
import java.util.Scanner;

/**
*
* @author Gajendra
*/
public class Vingenere {

/**
* @param args the command line arguments
*/
public static void main(String[] args) throws FileNotFoundException, IOException {
// TODO code application logic here
  
String inputFileName = "input.txt";;
  
if(args.length>0) {
inputFileName =args[0];
}
  
File file = new File(inputFileName);
FileInputStream fis = new FileInputStream(file);
byte[] data = new byte[(int) file.length()];
fis.read(data);
fis.close();

String inputText = new String(data, "UTF-8");
inputText =inputText.toUpperCase();
  
String key="";
System.out.println("Enter key (A-Z) max 5 characters :");
Scanner s = new Scanner(System.in);
key = s.nextLine();
key = key.toUpperCase();
String encryptedText =encrypt(inputText,key);
  
String outputFile ="output.txt";
file = new File(outputFile);
file.createNewFile();
  
//Write Content
FileWriter writer = new FileWriter(file);
writer.write("Key :"+key);
writer.write(System.lineSeparator());
writer.write("Encrypted Text :"+encryptedText);
writer.close();

  
  
}
  
private static String encrypt(String text, final String key)
{
String res = "";
text = text.toUpperCase();
for (int i = 0, j = 0; i < text.length(); i++)
{
char c = text.charAt(i);
if (c < 'A' || c > 'Z')
continue;
res += (char) ((c + key.charAt(j) - 2 * 'A') % 26 + 'A');
j = ++j % key.length();
}
return res;
}

  
}

output:

input file text:

ASDSGSDAG SAGASA GDSAAG AGSGDSGDSAGSDGADSGAHFDNFDNARA
ASGDASEGDGAFHAHAHA

output.txt

Key :WAWEWT
Encrypted Text :WSZWCLZACWWZWSWKZLWACECLCDOKZLWGOHCTZSCEDYZNBHJTNAWWCWWSAKZZWFDEDTDA