DsaSignatureGenerator.java - Generating DSA Digital Signature

Cryptography Tutorials - Herong's Tutorial Examples

∟DsaSignatureGenerator.java - Generating DSA Digital Signature

This section provides tutorial example on how to generate a digital signature for a message file with a DSA private key using the SHA1withDSA algorithm.

Here my DSA signature generation program, DsaSignatureGenerator.java, using the java.security.Signature class:

/* DsaSignatureGenerator.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
import java.io.*;
import java.security.*;
import java.security.spec.*;
class DsaSignatureGenerator {
   public static void main(String[] a) {
      if (a.length<3) {
         System.out.println("Usage:");
         System.out.println("java DsaSignatureGenerator keyFile"
            + " msgFile sigFile");
         return;
      }
      String keyFile = a[0];
      String msgFile = a[1];
      String sigFile = a[2];
      String keyAlgo = "DSA";
      String sigAlgo = "SHA1withDSA";
      
      try {
         PrivateKey priKey = readPrivateKey(keyFile,keyAlgo);
         sign(msgFile,sigFile,sigAlgo,priKey);
      } catch (Exception e) {
         System.out.println("Exception: "+e);
         return;
      }
   }
   private static PrivateKey readPrivateKey(String input,
         String algorithm) throws Exception {
      KeyFactory keyFactory = KeyFactory.getInstance(algorithm);
      FileInputStream priKeyStream = new FileInputStream(input);
      int priKeyLength = priKeyStream.available();
      byte[] priKeyBytes = new byte[priKeyLength];
      priKeyStream.read(priKeyBytes);
      priKeyStream.close();
      PKCS8EncodedKeySpec priKeySpec 
         = new PKCS8EncodedKeySpec(priKeyBytes);
      PrivateKey priKey = keyFactory.generatePrivate(priKeySpec);
      System.out.println();
      System.out.println("Private Key Info: ");
      System.out.println("Algorithm = "+priKey.getAlgorithm());
      return priKey;
   }
   private static byte[] sign(String input, String output, 
      String algorithm, PrivateKey priKey) throws Exception {
      Signature sg = Signature.getInstance(algorithm);
      sg.initSign(priKey);
      System.out.println();
      System.out.println("Signature Object Info: ");
      System.out.println("Algorithm = "+sg.getAlgorithm());
      System.out.println("Provider = "+sg.getProvider());
      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;
         sg.update(buffer,0,n);
         n = in.read(buffer,0,bufSize);
      }
      in.close();
      FileOutputStream out = new FileOutputStream(output);
      byte[] sign = sg.sign();
      out.write(sign);
      out.close();
      System.out.println();
      System.out.println("Sign Processing Info: ");
      System.out.println("Number of input bytes = "+count);
      System.out.println("Number of output bytes = "+sign.length);
      return sign;
   }
}

Some notes on DsaSignatureGenerator.java:

The private key is converted from the private key file encoded in PKCS#8 format. The public key is not needed for generating the signature file.
The java.security.Signature.getInstance(algorithm) method is used to create the signature object with the specified algorithm: "SHA1withDSA".
The initSign(privateKey) method is used at the beginning to initialize the signature object with the private key.
The update() method is used repeatedly to hash the message file in chunks.
The sign() method is used at the end to generate the signature as a byte array from the final hash value.