import java.security.*;
import javax.crypto.*;
//
// This program demonstrates the digital signature technique at the
// primative level by generating a message digest of the plaintext

// and signing it with an RSA private key, to create the signature.
// To verify the signature, the message digest is again generated from
// the plaintext and compared with the decryption of the signature
// using the public key.  If they match, the signature is verified.
public class DigitalSignature1Example {

  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");
    //
    // get an MD5 message digest object and compute the plaintext digest
    MessageDigest messageDigest = MessageDigest.getInstance("MD5");
    System.out.println( "\n" + messageDigest.getProvider().getInfo() );
    messageDigest.update( plainText );
    byte[] md = messageDigest.digest();
    System.out.println( "\nDigest: " );
    System.out.println( new String( md, "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 an RSA cipher and list the provider
    Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
    System.out.println( "\n" + cipher.getProvider().getInfo() );
    //
    // encrypt the message digest with the RSA private key
    // to create the signature
    System.out.println( "\nStart encryption" );
    cipher.init(Cipher.ENCRYPT_MODE, key.getPrivate());
    byte[] cipherText = cipher.doFinal(md);
    System.out.println( "Finish encryption: " );
    System.out.println( new String(cipherText, "UTF8") );
    //
    // to verify, start by decrypting the signature with the
    // RSA private key
    System.out.println( "\nStart decryption" );
    cipher.init(Cipher.DECRYPT_MODE, key.getPublic());

    byte[] newMD = cipher.doFinal(cipherText);
    System.out.println( "Finish decryption: " );
    System.out.println( new String(newMD, "UTF8") );
    //
    // then, recreate the message digest from the plaintext
    // to simulate what a recipient must do
    System.out.println( "\nStart signature verification" );
    messageDigest.reset();
    messageDigest.update(plainText);
    byte[] oldMD = messageDigest.digest();
    //
    // verify that the two message digests match
    int len = newMD.length;
    if (len > oldMD.length) {
      System.out.println( "Signature failed, length error");
      System.exit(1);
    }
    for (int i = 0; i < len; ++i)
      if (oldMD[i] != newMD[i]) {
        System.out.println( "Signature failed, element error" );
        System.exit(1);
      }
    System.out.println( "Signature verified" );
  }
}