Java Programming / Computer Security /

Digital signature:

I need help finishing the code below. The program is supposed to generate public and private key pairs and to sign a plaintext message using private key and verify the signature using public key.

In particular, I need to implement the follwoing constructor and methods in the Sig class:

1.) Sig constructor, where you initialize the fields keyGen, keyFactory, and signature.

2.) initKeyPair method, where you initialize the keyPair field.

3.) getPublicKey method, where you return the public key of the key pair as a string.

4.) getSignature method, where you return the string encoding of the digital signature of a message.

5.) verify method, where you verify the signature of a message given a public key.

Code:

package hwk8;

import java.nio.file.Files;

import java.nio.file.Paths;

import java.security.KeyFactory;

import java.security.KeyPair;

import java.security.KeyPairGenerator;

import java.security.PrivateKey;

import java.security.PublicKey;

import java.security.Signature;

import java.security.spec.X509EncodedKeySpec;

import java.util.Base64;

public class Sig {

KeyPairGenerator keyGen;

Signature signature;

KeyFactory keyFactory;

KeyPair keyPair;

/*

* initialize a RSA key generator object with given key size

* initialize a signature object with SHA1withRSA algorithm

* initialize a RSA key factory object

*/

Sig(int keySize) throws Exception {

// TODO

}

// geenerate a public/private key pair

void initKeyPair() {

// TODO

}

// return the string encoding of the public key

String getPublicKey() {

// TODO

}

// return the string encoding of the digital signature of the 'message'

String getSignature(String message) throws Exception {

// TODO

}

// return true if and only if the signature of the 'message' using the 'publicKey' matches the 'signatureString'

boolean verify(String message, String signatureString, String publicKey) throws Exception {

// TODO

}

public static void main(String[] args) {

try {

Sig sig = new Sig(1024);

sig.initKeyPair();

String publicKey = sig.getPublicKey();

String message = "This is a test message that needs to be signed";

String signatureString = sig.getSignature(message);

boolean verified = sig.verify(message, signatureString, publicKey);

System.out.printf("public key: %s signature: %s signature is verified: %s ",

publicKey, signatureString, verified);

String message1 = "This is a test message that needs to be verified";

publicKey = "MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQChBA91S3XSUYILZyhu5ilsiDjmzOZn830xxUEPLpeLQwr2BeLAtlCBqzihUcr5lGr9YvLN/jOta0TnEpQoa6kJYbUPnAZinh8E2Q4COfSx4Js3+TC64AD2yvlLaGOsBrDaBN15OGq9k2FgfxzxepVEPBW6DKHCeDQqPQAO8/UDXwIDAQAB";

signatureString = "VvLONhPbhkzhmR3Z5v4WQfleSm8CR1CLwDWXjDGu4NhzN6L7n7zJBxj+z6WT8W3hb2Le69gKO2WIy/7kNvfqOTE6hOGz68YB0idZoATB2C7m9dSjRcdL0nYxThoPrfZXVDuMVuSpDo2We/M/8a4oCAPzjCopaCsDtTac+ZcL7/w=";

verified = sig.verify(message1, signatureString, publicKey);

System.out.printf("public key: %s signature: %s signature is verified: %s ",

publicKey, signatureString, verified);

} catch(Exception e) {

System.out.println(e);

}

}

}