# DigitalSignature.java

    /*
     ** Module   : DigitalSignature.java
     ** Abstract : A tool to sign messages and verify their signature for certain private / public key pair.
     **
     ** Copyright (c) 2024, Golden Code Development Corporation.
     **
     ** -#- -I- --Date-- ---------------------------------Description---------------------------------
     ** 001 GBB 20240213 Created initial version
     */
    /*
     ** This program is free software: you can redistribute it and/or modify
     ** it under the terms of the GNU Affero General Public License as
     ** published by the Free Software Foundation, either version 3 of the
     ** License, or (at your option) any later version.
     **
     ** This program is distributed in the hope that it will be useful,
     ** but WITHOUT ANY WARRANTY; without even the implied warranty of
     ** MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
     ** GNU Affero General Public License for more details.
     **
     ** You may find a copy of the GNU Affero GPL version 3 at the following
     ** location: https://www.gnu.org/licenses/agpl-3.0.en.html
     **
     ** Additional terms under GNU Affero GPL version 3 section 7:
     **
     **   Under Section 7 of the GNU Affero GPL version 3, the following additional
     **   terms apply to the works covered under the License.  These additional terms
     **   are non-permissive additional terms allowed under Section 7 of the GNU
     **   Affero GPL version 3 and may not be removed by you.
     **
     **   0. Attribution Requirement.
     **
     **     You must preserve all legal notices or author attributions in the covered
     **     work or Appropriate Legal Notices displayed by works containing the covered
     **     work.  You may not remove from the covered work any author or developer
     **     credit already included within the covered work.
     **
     **   1. No License To Use Trademarks.
     **
     **     This license does not grant any license or rights to use the trademarks
     **     Golden Code, FWD, any Golden Code or FWD logo, or any other trademarks
     **     of Golden Code Development Corporation. You are not authorized to use the
     **     name Golden Code, FWD, or the names of any author or contributor, for
     **     publicity purposes without written authorization.
     **
     **   2. No Misrepresentation of Affiliation.
     **
     **     You may not represent yourself as Golden Code Development Corporation or FWD.
     **
     **     You may not represent yourself for publicity purposes as associated with
     **     Golden Code Development Corporation, FWD, or any author or contributor to
     **     the covered work, without written authorization.
     **
     **   3. No Misrepresentation of Source or Origin.
     **
     **     You may not represent the covered work as solely your work.  All modified
     **     versions of the covered work must be marked in a reasonable way to make it
     **     clear that the modified work is not originating from Golden Code Development
     **     Corporation or FWD.  All modified versions must contain the notices of
     **     attribution required in this license.
     */

    package com.goldencode.p2j.security;

    import com.goldencode.p2j.main.*;
    import com.goldencode.p2j.util.logging.*;

    import java.io.*;
    import java.math.*;
    import java.security.*;
    import java.security.cert.*;
    import java.security.cert.Certificate;

    /** A tool to sign messages and verify their signature for certain private : public key pair. */
    public class DigitalSignature
    {
       /** The algorithm used for signing the values, corresponds to the server certificate algo. */
       private static final String ALGORITHM = "SHA256withRSA";

       /** Private key */
       private final PrivateKey privateKey;

       /** Public key */
       private final PublicKey publicKey;

       /**
        * Public constructor accepting private : public key pair.
        *
        * @param    privateKey
        *           Private key.
        * @param    publicKey
        *           Public key.
        */
       public DigitalSignature(PrivateKey privateKey, PublicKey publicKey)
       {
          this.privateKey = privateKey;
          this.publicKey = publicKey;
       }

       /**
        * Reads the server key store, retrieves the public and private key pair and returns an instance of
        * {@link DigitalSignature}.
        *
        * @return   An instance of {@link DigitalSignature} with the public and private key pair for the server.
        *
        * @throws   KeyStoreException
        * @throws   UnrecoverableKeyException
        * @throws   NoSuchAlgorithmException
        * @throws   IOException
        * @throws   CertificateException
        */
       public static DigitalSignature fromServerKeyStore()
       throws KeyStoreException,
              UnrecoverableKeyException,
              NoSuchAlgorithmException,
              IOException,
              CertificateException
       {
          ServerKeyStore serverKeyStore = ServerKeyStore.getStore();
          byte[] keyStoreContent = serverKeyStore.getKeyStore();
          char[] keyStorePassword = serverKeyStore.getKeyStorePassword().toCharArray();
          char[] managerPassword = serverKeyStore.getKeyManagerPassword().toCharArray();
          String alias = serverKeyStore.getTrustServerAlias();

          KeyStore ks = KeyStore.getInstance("JKS");
          ks.load(new ByteArrayInputStream(keyStoreContent), keyStorePassword);

          PrivateKey privateKey = (PrivateKey) ks.getKey(alias, managerPassword);
          Certificate certificate = ks.getCertificate(alias);
          PublicKey publicKey = certificate.getPublicKey();

          return new DigitalSignature(privateKey, publicKey);
       }

       /**
        * Converts a byte array into a hex format.
        *
        * @param   bytes
        *          The byte array to be converted.
        *
        * @return  The converted string.
        */
       public static String toHex(byte[] bytes)
       {
          StringBuilder sb = new StringBuilder();
          for (byte b : bytes) {
             sb.append(String.format("%02X", b));
          }
          return sb.toString();
       }

       /**
        * Converts a hex formatted string back to a byte array.
        *
        * @param   hexText
        *          Hex formatted string.
        *
        * @return  Byte array.
        */
       public static byte[] fromHex(String hexText)
       {
          byte[] bytes = new BigInteger(hexText, 16).toByteArray();
          if (bytes[0] == 0) {
             // cleans up the leading zero
             byte[] newBytes = new byte[bytes.length - 1];
             System.arraycopy(bytes, 1, newBytes, 0, newBytes.length);
             bytes = newBytes;
          }
          return bytes;
       }

       /**
        * Signs the message with the private key and returns the signed value.
        *
        * @param    message
        *           The message to sign.
        *
        * @return   The signed message.
        *
        * @throws   SignatureException
        * @throws   InvalidKeyException
        * @throws   NoSuchAlgorithmException
        */
       public byte[] sign(String message)
       throws SignatureException,
              InvalidKeyException,
              NoSuchAlgorithmException
       {
          Signature signature = Signature.getInstance(ALGORITHM);
          signature.initSign(privateKey);
          signature.update(message.getBytes());

          return signature.sign();
       }

       /**
        * Signs the message with the private key and returns the signed value.
        *
        * @param    originalMessage
        *           The original message.
        * @param    signedMessage
        *           The signed message.
        *
        * @return   <code>true</code> if the signedMessage is generated by the key, <code>false</code> otherwise.
        *
        * @throws   SignatureException
        * @throws   InvalidKeyException
        * @throws   NoSuchAlgorithmException
        */
       public boolean verify(String originalMessage, byte[] signedMessage)
       throws SignatureException,
              InvalidKeyException,
              NoSuchAlgorithmException
       {
          Signature signature = Signature.getInstance(ALGORITHM);
          signature.initVerify(publicKey);
          signature.update(originalMessage.getBytes());

          return signature.verify(signedMessage);
       }

       /**
        * Test for signing messages.
        *
        * @param    args
        *           Unused.
        */
       public static void main(String[] args)
       throws Exception
       {
          DigitalSignature deviceIdSignature = fromServerKeyStore();

          String originalMessage = "test";
          byte[] signedMessage = deviceIdSignature.sign(originalMessage);

          String hex = toHex(signedMessage);
          byte[] signedMessage1 = fromHex(hex);

          boolean isValid = deviceIdSignature.verify(originalMessage, signedMessage1);
          CentralLogger.get(DigitalSignature.class).warning("isValid: " + isValid);
       }
    }