JCA - Message Digest

Part:   1   2 

This chapter describes some parts of the JCA (Java Cryptography Architecture) which has been included in JDK since 1.1:

• What is message digest?
• The MessageDigest class in JDK.
• A sample program on MessageDigest class.

What Is Message Digest

Message digest is a one-way hash function that takes arbitrary-sized data and outputs a fixed-length hash value.

Known message digest algorithms are:

• SHA - The Secure Hash Algorithm, as defined in Secure Hash Standard, NIST FIPS 180-1.
• MD2 - The MD2 message digest algorithm as defined in RFC 1319.
• MD5 - The MD5 message digest algorithm as defined in RFC 1321.

The MessageDigest Class

java.security.MessageDigest is an abstract class providing a link to implementation classes of message digest algorithms provided by various security package providers. Major methods in the MessageDigest class:

getInstance() - Returns a message digest object of the specified algorithm from the implementation of the specified provider. If provider is not specified, the default implementation is used. This is a static method.

update() - Adds more data to the current message digest object for processing.

digest() - Returns the digest data of the input data processed so far in the current message digest object. It also removes the input data received in the object.

getAlgorithm() - Returns the algorithm name of the current message digest object.

getProvider() - Returns the provider as a provider object of the current message digest object.

Message Digest Sample Program - JcaMessageDigest.java

The following sample program shows you how to invoke the message digest algorithms implemented by the default provider, Sun, and digest the data from the input file.

/**
 * JcaMessageDigest.java
 * Copyright (c) 2002 by Dr. Herong Yang
 */
import java.io.*;
import java.security.*;
class JcaMessageDigest {
   public static void main(String[] a) {
      if (a.length<3) {
         System.out.println("Usage:");
         System.out.println("java JcaMessageDigest input output"
            +" algorithm");
         return;
      }
      String input = a[0];
      String output = a[1];
      String algorithm = a[2]; // SHA, MD5
      try {
         digest(input,output,algorithm);
      } catch (Exception e) {
         System.out.println("Exception: "+e);
         return;
      }
   }
   private static void digest(String input, String output, 
      String algorithm) throws Exception {
      MessageDigest md = MessageDigest.getInstance(algorithm);
      System.out.println();
      System.out.println("MessageDigest Object Info: ");
      System.out.println("Algorithm = "+md.getAlgorithm());
      System.out.println("Provider = "+md.getProvider());
      System.out.println("Digest Length = "+md.getDigestLength());
      System.out.println("toString = "+md.toString());
      FileInputStream in = new FileInputStream(input);
      int bufSize = 1024;
      byte[] buffer = new byte[bufSize];
      int n = in.read(buffer,0,bufSize);
      int count = 0;
      while (n!=-1) {
         count += n;
         md.update(buffer,0,n);
         n = in.read(buffer,0,bufSize);
      }
      in.close();
      FileOutputStream out = new FileOutputStream(output);
      byte[] digest = md.digest();
      out.write(digest);
      out.close();
      System.out.println();
      System.out.println("Message Digest Processing Info: ");
      System.out.println("Number of input bytes = "+count);
      System.out.println("Number of output bytes = "+digest.length);
   }
}

(Continued on next part...)

Part:   1   2