How to store and reuse keypair in Java?

Question

I am wanting generate a keypair once and reuse it.

public static KeyPair generateKeyPair() throws Exception {
        KeyPairGenerator generator = KeyPairGenerator.getInstance("RSA");
        generator.initialize(2048, new SecureRandom());
        KeyPair pair = generator.generateKeyPair();

        return pair;
}

How do I go about this?

Answer 1

There is a bit of a problem here: Java's focus is almost entirely on TLS and the cryptography required to implement TLS. For TLS a private key and a certificate is required. So you get into a situation where you:

1. have to generate a (bogus) self signed certificate to go with your public key or;
2. have to find another way of storing the private key without a certificate.

However, with (2.) you quickly get a method that isn't very compatible. If you want to go that way, you could create a PKCS#8 encrypted private key that is encrypted using PBE / CBC.

So here's some code to create a self signed certificate and use that to store the key. Note the expiration date, you could set it to 100 years to be on the safe side (or you could actually do some key management).

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.math.BigInteger;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.KeyStore;
import java.security.KeyStore.Entry;
import java.security.KeyStore.PrivateKeyEntry;
import java.security.KeyStoreException;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.Provider;
import java.security.PublicKey;
import java.security.SecureRandom;
import java.security.Security;
import java.security.UnrecoverableEntryException;
import java.security.cert.Certificate;
import java.security.cert.CertificateException;
import java.security.interfaces.RSAPrivateKey;
import java.security.interfaces.RSAPublicKey;
import java.util.Calendar;
import java.util.Date;

import org.bouncycastle.asn1.x500.X500Name;
import org.bouncycastle.asn1.x509.SubjectPublicKeyInfo;
import org.bouncycastle.cert.X509CertificateHolder;
import org.bouncycastle.cert.X509v3CertificateBuilder;
import org.bouncycastle.cert.jcajce.JcaX509CertificateConverter;
import org.bouncycastle.jce.provider.BouncyCastleProvider;
import org.bouncycastle.operator.ContentSigner;
import org.bouncycastle.operator.OperatorCreationException;
import org.bouncycastle.operator.jcajce.JcaContentSignerBuilder;

public class StoreKeyPair {
    public static KeyPair generateKeyPair() throws Exception {
        KeyPairGenerator generator = KeyPairGenerator.getInstance("RSA");
        generator.initialize(2048, new SecureRandom());
        KeyPair pair = generator.generateKeyPair();

        return pair;
    }

    public static Certificate selfSign(KeyPair keyPair, String subjectDN)
            throws OperatorCreationException, CertificateException, IOException
    {
        Provider bcProvider = new BouncyCastleProvider();
        Security.addProvider(bcProvider);

        long now = System.currentTimeMillis();
        Date startDate = new Date(now);

        X500Name dnName = new X500Name(subjectDN);

        // Using the current timestamp as the certificate serial number
        BigInteger certSerialNumber = new BigInteger(Long.toString(now));

        Calendar calendar = Calendar.getInstance();
        calendar.setTime(startDate);
        // 1 Yr validity
        calendar.add(Calendar.YEAR, 1);

        Date endDate = calendar.getTime();

        // Use appropriate signature algorithm based on your keyPair algorithm.
        String signatureAlgorithm = "SHA256WithRSA";

        SubjectPublicKeyInfo subjectPublicKeyInfo = SubjectPublicKeyInfo.getInstance(keyPair
                .getPublic().getEncoded());

        X509v3CertificateBuilder certificateBuilder = new X509v3CertificateBuilder(dnName,
                certSerialNumber, startDate, endDate, dnName, subjectPublicKeyInfo);

        ContentSigner contentSigner = new JcaContentSignerBuilder(signatureAlgorithm).setProvider(
                bcProvider).build(keyPair.getPrivate());

        X509CertificateHolder certificateHolder = certificateBuilder.build(contentSigner);

        Certificate selfSignedCert = new JcaX509CertificateConverter()
                .getCertificate(certificateHolder);

        return selfSignedCert;
    }

    public static void main(String[] args) throws Exception {
        KeyPair generatedKeyPair = generateKeyPair();

        String filename = "test_gen_self_signed.pkcs12";
        char[] password = "test".toCharArray();

        storeToPKCS12(filename, password, generatedKeyPair);

        KeyPair retrievedKeyPair = loadFromPKCS12(filename, password);

        // you can validate by generating a signature and verifying it or by
        // comparing the moduli by first casting to RSAPublicKey, e.g.:

        RSAPublicKey pubKey = (RSAPublicKey) generatedKeyPair.getPublic();
        RSAPrivateKey privKey = (RSAPrivateKey) retrievedKeyPair.getPrivate();
        System.out.println(pubKey.getModulus().equals(privKey.getModulus()));
    }

    private static KeyPair loadFromPKCS12(String filename, char[] password)
            throws KeyStoreException, NoSuchAlgorithmException, CertificateException,
            FileNotFoundException, IOException, UnrecoverableEntryException {
        KeyStore pkcs12KeyStore = KeyStore.getInstance("PKCS12");

        try (FileInputStream fis = new FileInputStream(filename);) {
            pkcs12KeyStore.load(fis, password);
        }

        KeyStore.ProtectionParameter param = new KeyStore.PasswordProtection(password);
        Entry entry = pkcs12KeyStore.getEntry("owlstead", param);
        if (!(entry instanceof PrivateKeyEntry)) {
            throw new KeyStoreException("That's not a private key!");
        }
        PrivateKeyEntry privKeyEntry = (PrivateKeyEntry) entry;
        PublicKey publicKey = privKeyEntry.getCertificate().getPublicKey();
        PrivateKey privateKey = privKeyEntry.getPrivateKey();
        return new KeyPair(publicKey, privateKey);
    }

    private static void storeToPKCS12(
            String filename, char[] password,
            KeyPair generatedKeyPair) throws KeyStoreException, IOException,
            NoSuchAlgorithmException, CertificateException, FileNotFoundException,
            OperatorCreationException {

        Certificate selfSignedCertificate = selfSign(generatedKeyPair, "CN=owlstead");

        KeyStore pkcs12KeyStore = KeyStore.getInstance("PKCS12");
        pkcs12KeyStore.load(null, null);

        KeyStore.Entry entry = new PrivateKeyEntry(generatedKeyPair.getPrivate(),
                new Certificate[] { selfSignedCertificate });
        KeyStore.ProtectionParameter param = new KeyStore.PasswordProtection(password);

        pkcs12KeyStore.setEntry("owlstead", entry, param);

        try (FileOutputStream fos = new FileOutputStream(filename)) {
            pkcs12KeyStore.store(fos, password);
        }
    }
}

Note that I was too lazy to properly handle the exceptions.

This code uses a slightly altered version of this answer, see my comments for why I changed the code.

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The public key can of course be stored separately as well. Just call getEncoded and store the resulting SubjectPublicKeyInfo structure.