CreateSignature.java

import java.security.*;
import javax.crypto.*;

class CreateSignature {

	 public static void main (String[] args) throws Exception {

	    // verify args. If args.length is not more (or less) than 1, tells how to use the program.
		if (args.length !=1) {
		      System.err.println("How to use: java CreateSignature your text between \"\"");
		      System.exit(1);
		}
		byte[] plainText = args[0].getBytes("UTF8");

		/* Step 1: Create the digest of the plainText */
		StringBuffer digest = createDigest(plainText);
		System.out.println( "\tDigest: "+digest.toString() );
		/* Step 1: finished. */

        /* Step 2: Create the key to encrypt/decrypt the signature. */
        KeyPair key = createKey();
        /* Step 2: finished. */

		/* Step 3: Encrypt the digest with a private key. */
		byte[] cipherText = encrypt(String.valueOf(digest).getBytes(), key.getPrivate());
		/* Step 3: finished. */

        /* Step 4: Decrypt the signature with the public key. */
        String decryptedDigest = decrypt(key.getPublic(), cipherText);
        /* Step 4: finished. */

        /* Step 5: Compare the old digest with the decrypted digest. */
        System.out.println("\nStep 5: Compare the old digest with the decrypted digest: ");
        System.out.println( "\n\tOld digest: "+digest.toString() );
        System.out.println( "\tDecrypted digest: "+decryptedDigest);

        if(decryptedDigest.equals(digest.toString()))
            System.out.println( "\nThe digests are equal.\n");
        else
            System.out.println( "\nThe digests are not equal. File is not ok.\n");
        /* Step 5: finished */

	}

	/*************************************************/
	/* Method name: createDigest				     */
	/* Parameters: plainText					     */
	/* Function: create the digest of the file.      */
	/* Return: a StringBuffer with the digest.	   */
	/*************************************************/

	public static StringBuffer createDigest(byte[] plainText) throws Exception {
		MessageDigest messageDigest = MessageDigest.getInstance("MD5"); //especifica-se o tipo de digest (MD5)
		//System.out.println( "\n" + messageDigest.getProvider().getInfo() ); //exibi-se a lista de informações
		messageDigest.update( plainText);
		byte [] digest = messageDigest.digest();
		String plainTextStr = new String(plainText, "UTF-8");
		System.out.println("\nStep 1: Create the digest of the plain text '"+plainTextStr+"':");
		System.out.println( "\n\tDigest length: " + digest.length * 8 + "bits" ); // tamanho do digest
		StringBuffer buf = new StringBuffer();
		for(int i = 0; i < digest.length; i++) {
			String hex = Integer.toHexString(0x0100 + (digest[i] & 0x00FF)).substring(1);
			buf.append((hex.length() < 2 ? "0" : "") + hex);
		}
		return buf;
	}
	/* End of createDigest */

    /*************************************************/
    /* Method name: createKey           			 */
    /* Parameters: -  							   */
    /* Function: create an assymmetric pair of keys. */
    /* Return: the pair of keys.   				  */
    /*************************************************/
    public static KeyPair createKey() throws Exception {
        System.out.println("\nStep 2: Create a key to cipher the digest:");
        System.out.println( "\n\tStart generating RSA key..." );
        KeyPairGenerator keyGen = KeyPairGenerator.getInstance("RSA");
        keyGen.initialize(1024);
        KeyPair key = keyGen.generateKeyPair();
        System.out.println( "\tRSA key created!");
        return key;
    }
    /* End of createKey */

	/*********************************************************************/
	/* Method name: encrypt             			                     */
	/* Parameters: msg, key   					                       */
	/* Function: encrypt the msg parameter with the key passed.		  */
	/* Return: the text encrypted. 				                      */
	/*********************************************************************/
	public static byte[] encrypt(byte[] msg, Key key) throws Exception {
		System.out.println("\nStep 3: Encrypt the digest with the key created:");
		System.out.println( "\n\tStart encryption..." );
        Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
		cipher.init(Cipher.ENCRYPT_MODE, key);
        byte[] cipherText = cipher.doFinal(msg);
		StringBuffer buf = new StringBuffer();
		for(int i = 0; i < cipherText.length; i++) {
			String hex = Integer.toHexString(0x0100 + (cipherText[i] & 0x00FF)).substring(1);
       buf.append((hex.length() < 2 ? "0" : "") + hex);
		}
		// print the signature in hexadecimal.
		System.out.println("\n\tSignature:\n\t"+buf.toString()+"\n");

		return cipherText;
	}
    /* End of encrypt */

    /*********************************************************************/
    /* Method name: decrypt             			                     */
    /* Parameters: key, cipherText				                       */
    /* Function: encrypt the msg parameter with the key passed.		  */
    /* Return: the text encrypted. 				                      */
    /*********************************************************************/
    public static String decrypt(Key key, byte[] cipherText) throws Exception {
        Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
        System.out.println("\nStep 4: Decrypt the digest with the key:");
        System.out.println( "\n\tStart decryption..." );
        cipher.init(Cipher.DECRYPT_MODE, key);
        byte[] newPlainText = cipher.doFinal(cipherText);
        System.out.println( "\tFinish decryption. " );
        String digest = new String(newPlainText, "UTF8");

        return digest;
    }
    /* End of decrypt */

}