The Signature class manipulates digital signatures using a key produced by the KeyPairGenerator class. The following methods are used in the example below:

• KeyPairGenerator.getInstance("RSA"), .initialize(1024), and .generateKeyPair(): Generates the keys.
  
  
• Cipher.getInstance("MD5WithRSA"): Creates the Signature object.
  
  
• .initSign(key.getPrivate()): Initializes the Signature object.
  
  
• .update(plainText) and .sign(): Calculates the signature with a plaintext string.
  
  
• .initVerify(key.getPublic()) and .verify(signature): Verifies the signature.

 
import java.security.*;
import javax.crypto.*;
//
// This example uses the digital signature features to generate and
// verify a signature much more easily than the previous example
public class DigitalSignature2Example {

  public static void main (String[] args) throws Exception {
    //
    // check args and get plaintext
    if (args.length !=1) {
      System.err.println("Usage: java DigitalSignature1Example text");
      System.exit(1);
    }
    byte[] plainText = args[0].getBytes("UTF8");
    //
    // generate an RSA keypair
    System.out.println( "\nStart generating RSA key" );
    KeyPairGenerator keyGen = KeyPairGenerator.getInstance("RSA");
    keyGen.initialize(1024);

    KeyPair key = keyGen.generateKeyPair();
    System.out.println( "Finish generating RSA key" );
    //
    // get a signature object using the MD5 and RSA combo
    // and sign the plaintext with the private key,
    // listing the provider along the way

    Signature sig = Signature.getInstance("MD5WithRSA");
    sig.initSign(key.getPrivate());
    sig.update(plainText);
    byte[] signature = sig.sign();
    System.out.println( sig.getProvider().getInfo() );
    System.out.println( "\nSignature:" );
    System.out.println( new String(signature, "UTF8") );
    //
    // verify the signature with the public key
    System.out.println( "\nStart signature verification" );
    sig.initVerify(key.getPublic());
    sig.update(plainText);
    try {
      if (sig.verify(signature)) {
        System.out.println( "Signature verified" );
      } else System.out.println( "Signature failed" );
    } catch (SignatureException se) {
      System.out.println( "Signature failed" );
    }
  }
}