Getting Started

				
					import org.bouncycastle.jce.provider.BouncyCastleProvider;

import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.security.KeyFactory;
import java.security.PrivateKey;
import java.security.Security;
import java.security.Signature;
import java.security.spec.PKCS8EncodedKeySpec;
import java.util.Base64;

public class Sign {
    public static void main(String[] args) throws Exception {
        Security.addProvider(new BouncyCastleProvider());

        String privateKeyString = fileToString(System.getProperty("private.key"));
        privateKeyString = removeRedundantPrivateKeyParts(privateKeyString);
        String certificateString = fileToString(System.getProperty("certificate.pem"));
        certificateString = removeRedundantCertParts(certificateString);
        String finalFormText = fileToString(System.getProperty("text.to.sign"))
                .replaceAll("\\n[ ]*", "")
                .replaceAll("\\r[ ]*", "")
                .replaceAll("\\t[ ]*", "")
                .replaceAll(":[ ]*", ":");
        byte[] data = finalFormText
                .getBytes("UTF-8");
        String signatureAlgorithm = "SHA256withRSA";

        KeyFactory kf = KeyFactory.getInstance("RSA");
        PKCS8EncodedKeySpec keySpecPKCS8 = new PKCS8EncodedKeySpec(Base64.getDecoder().decode(privateKeyString));
        PrivateKey privateKey = kf.generatePrivate(keySpecPKCS8);
        Signature sig = Signature.getInstance(signatureAlgorithm);
        sig.initSign(privateKey);
        sig.update(data);
        byte[] signatureBytes = sig.sign();
        System.out.println("Private: " + privateKeyString + "\n");
        System.out.println("Public: " + certificateString + "\n");
        System.out.println("Algorithm: " + signatureAlgorithm + "\n");
        System.out.println("Challenge: " + fileToString(System.getProperty("text.to.sign")) + "\n");
        System.out.println("Challenge after modifications : " + finalFormText + "\n");
        System.out.println("Signature: " + new String(Base64.getEncoder().encode(signatureBytes)));
    }

    private static String fileToString(String filePath) throws Exception {
        String content = "";
        try {
            content = new String(Files.readAllBytes(Paths.get(filePath)));
        } catch (IOException e) {
            e.printStackTrace();
        }
        return content;
    }

    private static String removeRedundantCertParts(String originalCert) {
        return originalCert
                .replace("\n", "")
                .replace("\r", "")
                .replace("\r\n", "")
                .replace("-----BEGIN CERTIFICATE-----", "")
                .replace("-----END CERTIFICATE-----", "")
                .replace(" ", "");
    }

    private static String removeRedundantPrivateKeyParts(String originalPrivateKey) {
        return originalPrivateKey
                .replace("\n", "")
                .replace("\r", "")
                .replace("\r\n", "")
                .replace("-----BEGIN PRIVATE KEY-----", "")
                .replace("-----END PRIVATE KEY-----", "")
                .replace(" ", "");
    }
}