Example on how to sign file content using a randomly generated private Key. The Code takes a file as input and generates a signature and a public key, that can be used to verify, that the signature was create by the generated private key.

MessageSignGenerator.java

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.PrivateKey;
import java.security.SecureRandom;
import java.security.Signature;

public class MessageSignGenerator {

    public static void main(String[] args) {
        
        if (args.length != 1) {
            System.out.println("Usage: MessageSignGenerator nameOfFileToSign");
        }  else try {
            createSignature(args[0]);
            System.out.println("Signed file " + args[0]);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private static void createSignature(String file) throws Exception, IOException {
        KeyPair keyPair = createKeyPair();
        byte[] sign = sign(Paths.get(file), keyPair.getPrivate());
        
        write(Paths.get(file + ".sig"), sign);
        write(Paths.get(file + ".suepk"), keyPair.getPublic().getEncoded());
    }

    private static KeyPair createKeyPair() throws Exception {
        KeyPairGenerator keyGen = KeyPairGenerator.getInstance("DSA");
        SecureRandom random = SecureRandom.getInstance("SHA1PRNG");
        keyGen.initialize(1024, random);
        return keyGen.generateKeyPair();
    }

    private static byte[] sign(Path path, PrivateKey privateKey) throws Exception {
        Signature dsa = Signature.getInstance("SHA1withDSA");
        dsa.initSign(privateKey);
        
        dsa.update(Files.readAllBytes(path));
        
        return dsa.sign();
    }
    
    private static void write(Path path, byte[] content) throws IOException {
        Files.write(path, content);
    }

}