RsaKeyValidator.java for RSA Key Validation

Cryptography Tutorials - Herong's Tutorial Examples - Version 5.36, by Dr. Herong Yang

Cryptography Tutorials - Herong's Tutorial Examples

∟RSA Implementation using java.math.BigInteger Class

∟RsaKeyValidator.java for RSA Key Validation

This section provides a tutorial example on how to validate RSA keys by encrypting and decrypting some random sample messages.

Before writing the RSA encryption and decryption programs, I want to validate RSA keys generated from by RsaKeyGenerator.java program.

Here is my RSA key validation program:

/* RsaKeyValidator.java
#- Copyright (c) 2013, HerongYang.com, All Rights Reserved.
 */
import java.math.BigInteger;
import java.util.Random;
import java.io.*;
class RsaKeyValidator {
   private BigInteger n, e, d;
   public static void main(String[] a) {
      if (a.length<1) {
         System.out.println("Usage:");
         System.out.println("java RsaKeyValidator input");
         return;
      }
      String input = a[0];

      RsaKeyValidator validator = new RsaKeyValidator(input);
      validator.test();
      validator.test();
      validator.test();
   }

// Reading in RSA public key and private key
   RsaKeyValidator(String input) {
      try {
         BufferedReader in = new BufferedReader(new FileReader(input));
         String line = in.readLine();
         while (line!=null) {
            if (line.indexOf("Modulus: ")>=0) {
               n = new BigInteger(line.substring(9));
            }
            if (line.indexOf("Public key: ")>=0) {
               e = new BigInteger(line.substring(12));
            }
            if (line.indexOf("Private key: ")>=0) {
               d = new BigInteger(line.substring(13));
            }
            line = in.readLine();
         }
      } catch (Exception ex) {
         ex.printStackTrace();
      }
      System.out.println("--- Reading public key and private key ---");
      System.out.println("Modulus: "+n);
      System.out.println("Key size: "+n.bitLength());
      System.out.println("Public key: "+e);
      System.out.println("Private key: "+d);
   }

// Testing encryption and description
   public void test() {
      Random rnd = new Random();
      int size = rnd.nextInt(n.bitLength()-1);
      BigInteger text = new BigInteger(size,rnd);
      BigInteger cipher = text.modPow(e,n);
      BigInteger decrypted = cipher.modPow(d,n);
      boolean isPassed = text.equals(decrypted);
      System.out.println("--- RSA encryption test ---");
      System.out.println("Is test passed: "+isPassed);
      System.out.println("Original text: "+text);
      System.out.println("Cipher text: "+cipher);
      System.out.println("Decrypted text: "+decrypted);
   }      
}

Some notes on RsaKeyValidator.java:

Reading RSA key back from a file is easy, looping through each line, then parsing the modulus, the public key and the private key back.
The test() method is designed to generate a random positive integer less that the modulus, n. Then random integer is encrypted and decrypted back to see if it changed or not.

Testing result is presented in the next tutorial.

Last update: 2013.

Table of Contents

About This Book

Cryptography Terminology

Cryptography Basic Concepts

Introduction to AES (Advanced Encryption Standard)

Introduction to DES Algorithm

DES Algorithm - Illustrated with Java Programs

DES Algorithm Java Implementation

DES Algorithm - Java Implementation in JDK JCE

DES Encryption Operation Modes