JcePublicCipher.java - Public Key Encryption Sample Program

Cryptography Tutorials - Herong's Tutorial Examples

∟Cipher - Public Key Encryption and Decryption

∟JcePublicCipher.java - Public Key Encryption Sample Program

This section provides a tutorial example on how to write a public key encryption and private key decryption program using the javax.crypto.Cipher class.

The following sample program shows you how to do encryption and decryption with a private and public key pair:

/* JcePublicCipher.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
import java.io.*;
import java.security.*;
import java.security.spec.*;
import javax.crypto.*;
import javax.crypto.spec.*;
class JcePublicCipher {
   public static void main(String[] a) {
      if (a.length<5) {
         System.out.println("Usage:");
         System.out.println("java JcePublicCipher algorithm mode"
            +" keyFile input output");
         return;
      }
      String algorithm = a[0];
      String mode = a[1];
      String keyFile = a[2];
      String input = a[3];
      String output = a[4];
      try {
         Key ky = readKey(algorithm, mode, keyFile);
         publicCipher(algorithm, mode, ky, input, output);
      } catch (Exception e) {
         e.printStackTrace();
         return;
      }
   }
   private static Key readKey(String algorithm, String mode,
      String input) throws Exception {
      KeyFactory keyFactory = KeyFactory.getInstance(algorithm);
      System.out.println();
      System.out.println("KeyFactory Object Info: ");
      System.out.println("Algorithm = "+keyFactory.getAlgorithm());
      System.out.println("Provider = "+keyFactory.getProvider());
      System.out.println("toString = "+keyFactory.toString());

      FileInputStream fis = new FileInputStream(input);
      int kl = fis.available();
      byte[] kb = new byte[kl];
      fis.read(kb);
      fis.close();
      Key ky = null;
      if (mode.equalsIgnoreCase("encrypt")) {
         X509EncodedKeySpec pubKeySpec 
            = new X509EncodedKeySpec(kb);
         ky = keyFactory.generatePublic(pubKeySpec);
      } else if (mode.equalsIgnoreCase("decrypt")) {
         PKCS8EncodedKeySpec priKeySpec 
            = new PKCS8EncodedKeySpec(kb);
         ky = keyFactory.generatePrivate(priKeySpec);
      } else
         throw new Exception("Invalid mode: "+mode);
      System.out.println();
      System.out.println("Key Object Info: ");
      System.out.println("Algorithm = "+ky.getAlgorithm());
      System.out.println("Saved File = "+input);
      System.out.println("Length = "+kl);
      System.out.println("Format = "+ky.getFormat());
      System.out.println("toString = "+ky.toString());
      return ky;
   }
   private static void publicCipher(String algorithm, String mode,
      Key ky, String input, String output) throws Exception {
      Cipher cf = Cipher.getInstance(algorithm);
      if (mode.equalsIgnoreCase("encrypt")) 
         cf.init(Cipher.ENCRYPT_MODE,ky);
      else if (mode.equalsIgnoreCase("decrypt"))
         cf.init(Cipher.DECRYPT_MODE,ky);
      else
         throw new Exception("Invalid mode: "+mode);
      System.out.println();
      System.out.println("Cipher Object Info: ");
      System.out.println("Block Size = "+cf.getBlockSize());
      System.out.println("Algorithm = "+cf.getAlgorithm());
      System.out.println("Provider = "+cf.getProvider());
      System.out.println("toString = "+cf.toString());

      FileInputStream fis = new FileInputStream(input);
      FileOutputStream fos = new FileOutputStream(output);
      int bufSize = 1024;
      byte[] buf = new byte[bufSize];
      int n = fis.read(buf,0,bufSize);
      int fisSize = 0;
      int fosSize = 0;
      while (n!=-1) {
         fisSize += n;
         byte[] out = cf.update(buf,0,n);
         fosSize += out.length;
         fos.write(out);
         n = fis.read(buf,0,bufSize);
      }
      byte[] out = cf.doFinal();
      fosSize += out.length;
      fos.write(out);
      fis.close();
      fos.close();
      System.out.println();
      System.out.println("Cipher Process Info: ");
      System.out.println("Input Size = "+fisSize);
      System.out.println("Output Size = "+fosSize);
   }
}

Note that:

This sample program requires a public key file to encrypt plaintext. A public key can be generated by my JcaKeyPair.java program as described in the previous chapter.
This sample program requires a private key file to decrypt ciphertext. A private key can be generated by my JcaKeyPair.java program as described in the previous chapter.