DSA data conversion in java

Question

I would like like to check if the public key corresponds to the private key - is it correct.

There is also no provider called BC. I have this implementation but i don't know what Utils.createFixedRandom() suppose to do. I don't have such a library and method.

Besides if you have an raw implementation without java.security it would be awesome.

import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.SecureRandom;
import java.security.Signature;

public class BasicDSAExample {
    public static void main(String[] args) throws Exception {
        KeyPairGenerator keyGen = KeyPairGenerator.getInstance("DSA", "BC");
        keyGen.initialize(512, new SecureRandom());
        KeyPair keyPair = keyGen.generateKeyPair();
        Signature signature = Signature.getInstance("DSA", "BC");

        signature.initSign(keyPair.getPrivate(), Utils.createFixedRandom() );
        byte[] message = new byte[] { (byte) 'a', (byte) 'b', (byte) 'c' };
        signature.update(message);
        byte[] sigBytes = signature.sign();

        signature.initVerify(keyPair.getPublic());
        signature.update(message);
        if (signature.verify(sigBytes)) {
            System.out.println("pow");
        } else {
            System.out.println("nie");
        }
    }
}

David M · Accepted Answer · 2012-07-04T16:59:24.607

1

You can't encrypt and decrypt using DSA, it's a digital signature algorithm. Digital signatures are created by encrypting a hash of the message using the signer's private key, so that it can be verified using their public key. But because it is hashed, the message cannot be recovered.

Following your edit:

If you have a private and a public key, the public key can be derived from the private one anyway, no need to use the key and verify anything. The private key contains all the information to construct a KeyPair, and then the getPublic() method will retrieve the equivalent public key. If you want to see if a given public key is correct, just compare with this.

edited Jul 04 '12 at 16:59

answered Jul 04 '12 at 16:19

David M

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Person X - has a private key Perosn Y - has a public key M is a message What is purpose of that if we can't code it? – Yoda Jul 04 '12 at 16:37
It's to sign the message, which guarantees its integrity. It's not about confidentiality - if you have secret information in it, you need an **encryption algorithm**. If it's a public document but you want people to be confident that (a) it comes from you and (b) hasn't been tampered with, you attached a digital signature to it generated with your private key. Two fundemantally different types of algorithm for different purposes. – David M Jul 04 '12 at 16:39
ok so how to attach private key to the message and then check if person has the proper key? how to confirm it? – Yoda Jul 04 '12 at 16:45
Not sure I understand the question? What's your use case for cryptography here - what problem are you trying to solve? Is it to send a message that must not be read in transit? If so, how many recipients, what relationship does the sender have with the recipient? All of these factors affect the choice of algorithm. – David M Jul 04 '12 at 16:46
1

Cannot mod you up again David. You can of course generate a signature with the private key and verify it with the public key, but without any guarantees that the entire key pair hasn't been replaced, that does not accomplish a whole lot either (except that creation/encoding/decoding seemed to work ok). – Maarten Bodewes Jul 04 '12 at 22:29

DSA data conversion in java

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