Shaz Software Solutions

Java Cryptography Encrypt Decrypt

Class: Cryptography
Author: Shahrzad Jahangirian
Language: Java (using Java 6) Download Java 6 and Download Java Runtime 6(JRE)
Date Added: Dec 11, 2013
Licence: All source codes are for showing my skills in Software Development to get me a Job as an Application Developer. You are not allowed to sell this Application or use my codes to develop an application for Sale or apply my codes to develop software for commercial use and profit. You can offer me a price to buy the Application for mass production or buy my codes to develop your application for profit. Also you may offer me a price to buy this Application for your Business and ask me to update the codes to match your Business Requirements.

Description: This is my java programming solution for an exercise from Deitel Book "Java how to program" Ninteh Edition page 150 Exercise 4.38 (Enforcing Privacy with Cryptography) and its description is as follows:
4.38 (Enforcing Privacy with Cryptography) The explosive growth of Internet communications and data storage on Internet connected computers has greatly increased privacy concerns. The field of cryptography is concerned with coding data to make it difficult (and hopefully with the most advanced schemes impossible) for unauthorized users to read. In this exercise you"ll investigate a simple scheme for encrypting and decrypting data. A company that wants to send data over the Internet has asked you to write a program that will encrypt it so that it may be transmitted more securely. All the data is transmitted as four digit integers. Your application should read a four digit integer entered by the user and encrypt it as follows: Replace each digit with the result of adding 7 to the digit and getting the remainder after dividing the new value by 10. Then swap the first digit with the third, and swap the second digit with the fourth. Then print the encrypted integer. Write a separate application that inputs an encrypted four digit integer and decrypts it (by reversing the encryption scheme) to form the original number. [Optional reading project: Research "public key cryptography" in general and the PGP (Pretty Good Privacy) specific public key scheme. You may also want to investigate the RSA scheme, which is widely used in industrial strength applications.]
How to Run: I tested all codes on Windows 7 Command Prompt with Java(TM) SE Runtime Environment 6 on my computer. ( you may download it by visiting this link )
You need to create all files and save it in a folder in your computer such as C:\javacodes and compile and run at command prompt as follows. I assumed you saved all files at C:\javacodes
C:\javacodes> javac Cryptography.java -d c:\javacodes "this statement compiles the class and creates cryptographyPKG package"
C:\javacodes> javac EncryptMain.java "compiles the Main class to test all methods including encrypt and decrypt methods of Cryptography class"
C:\javacodes> Java EncryptMain "and this statement runs the EncryptMain.java"

Cryptography Java class code

/*
 Class: Cryptography
 Author: Shahrzad Jahangirian
 Date: Dec 11, 2013
 Description: This is my programming solution for Exercise 4.38 (Enforcing Privacy with Cryptography)
 at page 150 of Java How To Program book from DEITEL Ninth Edition and the question is as follows:

4.38 (Enforcing Privacy with Cryptography) The explosive growth of Internet communications
 and data storage on Internet connected computers has greatly increased privacy concerns. The field
 of cryptography is concerned with coding data to make it difficult (and hopefully with the most
 advanced schemes impossible) for unauthorized users to read. In this exercise you'll investigate a
 simple scheme for encrypting and decrypting data. A company that wants to send data over the Internet
 has asked you to write a program that will encrypt it so that it may be transmitted more securely.
 All the data is transmitted as four digit integers. Your application should read a four digit
 integer entered by the user and encrypt it as follows: Replace each digit with the result of adding 7
 to the digit and getting the remainder after dividing the new value by 10. Then swap the first digit
 with the third, and swap the second digit with the fourth. Then print the encrypted integer. Write
 a separate application that inputs an encrypted four digit integer and decrypts it (by reversing the
 encryption scheme) to form the original number. [Optional reading project: Research "public key
 cryptography" in general and the PGP (Pretty Good Privacy) specific public key scheme. You may
 also want to investigate the RSA scheme, which is widely used in industrial-strength applications.]
*/

package cryptographyPKG;

import java.lang.*;

public class Cryptography {

public Cryptography() {

}

public boolean validateSize(String data) {

boolean valid = false;
    if (data.length() == 4) {
      valid = true;
    }
    return valid;
  }

/*
   * Method Name: isIntegr(String input)
   * Description: This method pass a String parameter and it validate the input to
   * see is it a valid Integer Number.
   */
  public boolean isInteger(String input) {

boolean valid = true;
    int number = 0;
    input = input.trim(); // no space in front and end

if (input.isEmpty() == false) {
      try {
        number = Integer.valueOf(input).intValue();
      }
      catch (NumberFormatException e) {
        valid = false;
      }
    }
    return valid;
  }

/*
   * Method Name: encrypt(String data)
   * Description: This method pass a String of four digits as a parameter & encrypt it
   * by Replacing each digit with the result of adding 7 to the digit and getting the
   * remainder after dividing the new value by 10. Then swap the first digit with the third,
   * and swap the second digit with the fourth and then return the encryptedCode as a String.
   */
  public String encrypt(String data) {
    String encryptedCode = new String();

encryptedCode += ( (Character.getNumericValue(data.charAt(2)) + 7) % 10);
    encryptedCode += ( (Character.getNumericValue(data.charAt(3)) + 7) % 10);
    encryptedCode += ( (Character.getNumericValue(data.charAt(0)) + 7) % 10);
    encryptedCode += ( (Character.getNumericValue(data.charAt(1)) + 7) % 10);

return encryptedCode;
  }

/*
   * Method Name: decript(String data)
   * Description: This method inputs an encrypted four-digit integer and decrypts it
   * (by reversing the encryption scheme) to form the original number.
   */
  public String decrypt(String data) {
    String decryptedCode = new String();

decryptedCode +=
        ( ( ( (Character.getNumericValue(data.charAt(2))) + 10) - 7) % 10);
    decryptedCode +=
        ( ( ( (Character.getNumericValue(data.charAt(3))) + 10) - 7) % 10);
    decryptedCode +=
        ( ( ( (Character.getNumericValue(data.charAt(0))) + 10) - 7) % 10);
    decryptedCode +=
        ( ( ( (Character.getNumericValue(data.charAt(1))) + 10) - 7) % 10);

return decryptedCode;
  }

}

This is the EncryptMain class to runs validateSize(String data), isInteger(String input), String encrypt(String data) , String decrypt(String data) methods of Cryptography class

/*
 * Class: EncryptMain
 * Author: Shahrzad Jahangirian
 * Date: Dec 10, 2013
 * Description: This is a main code to test Cryptography class. It asks for a four digits value from 
 user and validated for correct entry and encrypt it, then takes the encrypted value and decrypted to its 
 original number.
 */
import java.io.*;
import cryptographyPKG.Cryptography;

public class EncryptMain {

public static void main(String[] args) {
    Cryptography crypto = new Cryptography();
    String value = "";
    String encryptedValue = new String();
    String decryptedValue = new String();
    boolean intFlag = true;
    boolean sizeFlag = true;
   
    BufferedReader userinput = new BufferedReader(new InputStreamReader(System.
        in), 1);

try {
      System.out.println(
          "\nEnter a four digit Integer value to Encrypt. (E to Exit)");
      value = userinput.readLine();
      value = value.trim();
    }
    catch (IOException ex) {
      System.out.println(ex.getMessage());
    }
    while ( (value.equals("E") || value.equals("e")) == false) {
      intFlag = crypto.isInteger(value);
      if (intFlag == true) {
        sizeFlag = crypto.validateSize(value);
        if (sizeFlag == true){ 
           encryptedValue = crypto.encrypt(value);
           System.out.println("\n"+value+" encrypted to "+encryptedValue);
           decryptedValue = crypto.decrypt(encryptedValue);
           System.out.println("\n"+encryptedValue+" decrypted to "+decryptedValue);
        }
        else {
        System.out.println("Error in Size: you must enter a 4 digit Integer");
      }
      }
      else {
        System.out.println("Error in Type: you must enter a 4 digit Integer");
      }

try {
        encryptedValue = "";
        decryptedValue = "";
        System.out.println(
            "\nEnter a four digit Integer value to Encrypt. (E to Exit)");
        value = userinput.readLine();
        value = value.trim();
      }
      catch (IOException ex) {
        System.out.println(ex.getMessage());
      }
    }
  }
}