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Java Signature Certificate




Signature
A valid digital signature gives a recipient reason to believe that the message was created by a known sender (authentication), that the sender cannot deny having sent the message (non-repudiation), and that the message was not altered in transit (integrity)




 Original-data.getByte() + private key à signature


All are in byte array
As signature has to be passed through network, we convert into string but we cant use just string of original byte array because of security. Hence we do manipulation of byte array and convert into a different string.
We shoud not send new String(byte [] signature), we should send  new String(Base64.encode(byte[] signature))


Verification:
Client send (new data + signature)

object{
User name: Ram
Id : 678
Signature : ZTngSmnSpmWIj40r5TQ1hmec0UbfJLSCRSxbVBxCwchcFu6A8RS+O9BUFgG7U+UozVlrO5xGl9tARHxcIK4y2x/UHvhfYu74SOq22XgdGNuPMGQ560pUpiSkXspfGuFh9xHqovNGs7MQvWyESgurqehdsFD18sXV0z7gnqqFm78=
}

Server will receive new data and then again calculate a signature internally  and then it matches with th received signature.

Generate signature:
Signature signature = Signature.getInstance("SHA1WithRSA");
            signature.initSign(certDetails.getPrivateKey());

            // Supply the data to be signed to the Signature object
            // using the update() method and generate the digital
            // signature.
            byte[] databytes = Files.readAllBytes(Paths.get("D:\\user.txt"));
            signature.update(databytes);
            byte[] digitalSignature = signature.sign();

            // Save digital signature and the public key to a file.
            //Files.write(Paths.get("D:\\signature.txt"), digitalSignature);
            System.out.println(new String(Base64.encodeBase64(digitalSignature)));

Verify :
Signature signature = Signature.getInstance("SHA1WithRSA");
            signature.initVerify(publicKey);

            byte[] bytes = Files.readAllBytes(Paths.get("D:\\test1.txt"));
            signature.update(bytes);

            boolean verified = signature.verify(digitalSignature);





Java program :

package com.crypto.Digital;

import java.nio.file.Files;
import java.nio.file.Paths;
import java.security.*;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.X509EncodedKeySpec;

import javax.swing.JOptionPane;

import org.apache.commons.codec.binary.Base64;

public class GenerateDigitalSignature {
    public static void main(String[] args) {
        try {
           
            CertificateDetails certDetails = CertificateUtil.getCertificateDetails("d:\\test.keystore", "changeit");
            System.out.println(certDetails.getPrivateKey());
            System.out.println(certDetails.getX509Certificate());
           


            Signature signature = Signature.getInstance("SHA1WithRSA");
            signature.initSign(certDetails.getPrivateKey());

.
            byte[] bytes = Files.readAllBytes(Paths.get("D:\\test.txt"));
            signature.update(bytes);
            byte[] digitalSignature = signature.sign();
            System.out.println(new String(Base64.encodeBase64(digitalSignature)));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

package com.crypto.Digital;

import java.nio.file.Files;
import java.nio.file.Paths;
import java.security.KeyFactory;
import java.security.PublicKey;
import java.security.Signature;
import java.security.spec.X509EncodedKeySpec;

import javax.swing.JOptionPane;

import org.apache.commons.codec.binary.Base64;

public class VerifyDigitalSignature {
    public static void main(String[] args) {
        try {
            String digitalSignature1 = JOptionPane.showInputDialog("Type your signature here");
            byte[] digitalSignature =  Base64.decodeBase64(digitalSignature1.getBytes());
           
            CertificateDetails certDetails = CertificateUtil.getCertificateDetails("d:\\test.keystore", "changeit");
            System.out.println(certDetails.getPrivateKey());
            System.out.println(certDetails.getX509Certificate());

            PublicKey publicKey = certDetails.getX509Certificate().getPublicKey();
            Signature signature = Signature.getInstance("SHA1WithRSA");
            signature.initVerify(publicKey);

            byte[] bytes = Files.readAllBytes(Paths.get("D:\\test1.txt"));
            signature.update(bytes);

            boolean verified = signature.verify(digitalSignature);
            if (verified) {
                System.out.println("Data verified.");
            } else {
                System.out.println("Cannot verify data.");
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}






Geneate certificate and private key and put into JKS from program

note : With out program, we use keytool.


package com.crypto.Digital;

import java.io.FileOutputStream;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.security.KeyStore;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.cert.X509Certificate;
import java.util.Date;
import sun.security.x509.CertAndKeyGen;
import sun.security.x509.X500Name;

public class GenBothCertificateAndPrivateKey {
      
        private static final int keysize = 1024;
        private static final String commonName = "www.test.de";
        private static final String organizationalUnit = "IT";
        private static final String organization = "test";
        private static final String city = "test";
        private static final String state = "test";
        private static final String country = "DE";
        private static final long validity = 1096; // 3 years
        private static final String alias = "tomcat";
        private static final char[] keyPass = "changeit".toCharArray();

        // copied most ideas from sun.security.tools.KeyTool.java

        @SuppressWarnings("restriction")
        public static void main(String[] args) throws Exception {

            KeyStore keyStore = KeyStore.getInstance("JKS");
            keyStore.load(null, null);

            CertAndKeyGen keypair = new CertAndKeyGen("RSA", "SHA1WithRSA", null);

            X500Name x500Name = new X500Name(commonName, organizationalUnit, organization, city, state, country);

            keypair.generate(keysize);
            PrivateKey privKey = keypair.getPrivateKey();

            X509Certificate[] chain = new X509Certificate[1];

            chain[0] = keypair.getSelfCertificate(x500Name, new Date(), (long) validity * 24 * 60 * 60);

            keyStore.setKeyEntry(alias, privKey, keyPass, chain);

            keyStore.store(new FileOutputStream("d:\\test.keystore"), keyPass);
           
            //Files.write(Paths.get("D:\\PrivateKeyTest.txt"), privKey.getEncoded());


        }

}
--------------------------------------------------------

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