DsaKeyChecker.java - Reading and Checking DSA Keys

Cryptography Tutorials - Herong's Tutorial Examples

∟DsaKeyChecker.java - Reading and Checking DSA Keys

This section provides a tutorial example on how to read DSA public key and private key back from the key files, assuming they are using PKCS#8 and X.509 encoding formats.

In my DsaKeyGenerator.java program, DSA public key and private key are saved in 2 separate files. If you want to learn how read them back into Java programs, you can read my Java program, DsaKeyChecker.java:

/* DsaKeyChecker.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
import java.io.*;
import java.math.*;
import java.security.*;
import java.security.interfaces.*;
import java.security.spec.*;
class DsaKeyChecker {
   public static void main(String[] a) {
      if (a.length<1) {
         System.out.println("Usage:");
         System.out.println("java DsaKeyChecker input");
         return;
      }
      String input = a[0];
      String algorithm = "DSA";
      try {
         KeyPair pair = readKeyPair(input,algorithm);
         checkDsaKeyPair(pair);
      } catch (Exception e) {
         System.out.println("Exception: "+e);
         return;
      }
   }
   private static KeyPair readKeyPair(String input,
         String algorithm) throws Exception {
      KeyFactory keyFactory = KeyFactory.getInstance(algorithm);

      String priKeyFile = input+".pri";
      FileInputStream priKeyStream = new FileInputStream(priKeyFile);
      int priKeyLength = priKeyStream.available();
      byte[] priKeyBytes = new byte[priKeyLength];
      priKeyStream.read(priKeyBytes);
      priKeyStream.close();
      PKCS8EncodedKeySpec priKeySpec 
         = new PKCS8EncodedKeySpec(priKeyBytes);
      PrivateKey priKey = keyFactory.generatePrivate(priKeySpec);

      String pubKeyFile = input+".pub";
      FileInputStream pubKeyStream = new FileInputStream(pubKeyFile);
      int pubKeyLength = pubKeyStream.available();
      byte[] pubKeyBytes = new byte[pubKeyLength];
      pubKeyStream.read(pubKeyBytes);
      pubKeyStream.close();
      X509EncodedKeySpec pubKeySpec 
         = new X509EncodedKeySpec(pubKeyBytes);
      PublicKey pubKey = keyFactory.generatePublic(pubKeySpec);

      return new KeyPair(pubKey, priKey);
   }
   private static void checkDsaKeyPair(KeyPair pair) {
      DSAPublicKey pubKey = (DSAPublicKey) pair.getPublic();
      DSAPrivateKey priKey = (DSAPrivateKey) pair.getPrivate();
      DSAParams params = priKey.getParams();
      BigInteger p = params.getP();
      BigInteger q = params.getQ();
      BigInteger g = params.getG();
      BigInteger x = priKey.getX();
      BigInteger y = pubKey.getY();

      System.out.println();
      System.out.println("DSA Key Parameters: ");
      System.out.println("p = "+p);
      System.out.println("q = "+q);
      System.out.println("g = "+g);
      System.out.println("x = "+x);
      System.out.println("y = "+y);

      System.out.println();
      System.out.println("DSA Key Verification: ");
      System.out.println("What's key size? "+p.bitLength());
      System.out.println("Is p a prime? "+p.isProbablePrime(200));
      System.out.println("Is q a prime? "+q.isProbablePrime(200));
      System.out.println("Is p-1 mod q == 0? "
         +p.subtract(BigInteger.ONE).mod(q));
      System.out.println("Is g**q mod p == 1? "+g.modPow(q,p));
      System.out.println("Is q > x? "+(q.compareTo(x)==1));
      System.out.println("Is g**x mod p == y? "+g.modPow(x,p).equals(y));
   }
}

Some notes on DsaKeyChecker.java:

The java.security.KeyFactory.getInstance(algorithm) method is used to initiate a key factory object for the specified algorithm, "DSA".
The java.security.spec.PKCS8EncodedKeySpec constructor is used to load the private key back from the byte array, because it is encoded in PKCS#8 format.
The java.security.KeyFactory.keyFactory.generatePrivate(priKeySpec) method is used to convert the private key back from private key spec.
The java.security.spec.X509EncodedKeySpec constructor is used to load the public key back from the byte array, because it is encoded in X.509 format.
The java.security.KeyFactory.generatePublic(pubKeySpec) method is used to convert the public key back from public key spec.
The checkDsaKeyPair() method is used to retrieve DSA key parameters from the DSA key pair and check their properties.