Crypto Samples in java part two - Message Digest Streams

SendStream.java writes data along with message digest to a file "test".

SendStream.java

import java.io.*;
import java.security.*;

public class SendStream {
 public static void main(String args[]) {
  try {

   // We need to write data + message digest to this file.
   // Basically this is the file we need to send to the receiver
   FileOutputStream fos = new FileOutputStream("test");

   // We are going to calculate the message digest with SHA
   MessageDigest md = MessageDigest.getInstance("SHA");

   // Updates the associated message digest
   // using the bits going through the stream.
   DigestOutputStream dos = new DigestOutputStream(fos, md);

   ObjectOutputStream oos = new ObjectOutputStream(dos);

   String data = "This have I thought good to deliver thee, "
     + "that thou mightst not lose the dues of rejoicing "
     + "by being ignorant of what greatness is promised thee.";

   // Write the specified object to the ObjectOutputStream
   // Now the message being written to the FileOutputStream
   // Also - this will update the message digest of the written data as well..
   oos.writeObject(data);

   // urns the digest function on or off. The default is on. When it is on, a call to one
   // of the write methods results in an update on the message digest. But when it is off,
   // the message digest is not updated.
   dos.on(false);

   // Now let's also write the message digest to the file,
   oos.writeObject(md.digest());

  } catch (Exception e) {
   System.out.println(e);
  }
 }
}

RecieveStream.java takes file "test" as an input reads the data and calculate digest of that data and compare it with the original message digest read from the file "test".

RecieveStream.java

import java.io.FileInputStream;
import java.io.ObjectInputStream;
import java.security.DigestInputStream;
import java.security.MessageDigest;

public class RecieveStream {
 public static void main(String args[]) {
  try {

   // Here we get a file from the sender - and we need to verify it's message digest
   // The file contains both the message and the digest.
   FileInputStream fis = new FileInputStream("test");

   // We are going to calculate the message digest with SHA
   MessageDigest md = MessageDigest.getInstance("SHA");

   // Updates the associated message digest
   // using the bits going through the stream.
   DigestInputStream dis = new DigestInputStream(fis, md);

   ObjectInputStream oos = new ObjectInputStream(dis);

   // Read the message from the file
   String data = (String) oos.readObject();
   
   dis.on(false);

   // Original message digest from the input file.
   byte[] originalDigest = (byte[]) oos.readObject();

   if (MessageDigest.isEqual(originalDigest, dis.getMessageDigest().digest())) {
    System.out.println("VALID");
   } else {
    System.out.println("INVALID");
   }

  } catch (Exception e) {
   System.out.println(e);
  }
 }
}