Creating an X509 Certificate in Java without BouncyCastle?

The ability to sign certificates is not part of a standard Java library or extension.

A lot of the code that is needed to do it yourself is part of the core. There are classes to encode and decode X.500 names, X.509 certificate extensions, public keys for various algorithms, and of course, for actually performing the digital signature.

Implementing this yourself is not trivial, but it is definitely doable—I probably spent 4 or 5 full days the first time I made a working prototype for certificate signing. It was a fantastic learning exercise for me, but it's hard to justify that expense when there are usable libraries available for free.

Yes, but not with publicly documented classes. I've documented the process in this article.

import sun.security.x509.*;
import java.security.cert.*;
import java.security.*;
import java.math.BigInteger;
import java.util.Date;
import java.io.IOException

/** 
 * Create a self-signed X.509 Certificate
 * @param dn the X.509 Distinguished Name, eg "CN=Test, L=London, C=GB"
 * @param pair the KeyPair
 * @param days how many days from now the Certificate is valid for
 * @param algorithm the signing algorithm, eg "SHA1withRSA"
 */ 
X509Certificate generateCertificate(String dn, KeyPair pair, int days, String algorithm)
  throws GeneralSecurityException, IOException
{
  PrivateKey privkey = pair.getPrivate();
  X509CertInfo info = new X509CertInfo();
  Date from = new Date();
  Date to = new Date(from.getTime() + days * 86400000l);
  CertificateValidity interval = new CertificateValidity(from, to);
  BigInteger sn = new BigInteger(64, new SecureRandom());
  X500Name owner = new X500Name(dn);
 
  info.set(X509CertInfo.VALIDITY, interval);
  info.set(X509CertInfo.SERIAL_NUMBER, new CertificateSerialNumber(sn));
  info.set(X509CertInfo.SUBJECT, new CertificateSubjectName(owner));
  info.set(X509CertInfo.ISSUER, new CertificateIssuerName(owner));
  info.set(X509CertInfo.KEY, new CertificateX509Key(pair.getPublic()));
  info.set(X509CertInfo.VERSION, new CertificateVersion(CertificateVersion.V3));
  AlgorithmId algo = new AlgorithmId(AlgorithmId.md5WithRSAEncryption_oid);
  info.set(X509CertInfo.ALGORITHM_ID, new CertificateAlgorithmId(algo));
 
  // Sign the cert to identify the algorithm that's used.
  X509CertImpl cert = new X509CertImpl(info);
  cert.sign(privkey, algorithm);
 
  // Update the algorith, and resign.
  algo = (AlgorithmId)cert.get(X509CertImpl.SIG_ALG);
  info.set(CertificateAlgorithmId.NAME + "." + CertificateAlgorithmId.ALGORITHM, algo);
  cert = new X509CertImpl(info);
  cert.sign(privkey, algorithm);
  return cert;
}   

Edit 2021 - unfortunately this approach won't work under Java 17, as the sun.* hierarchy can't be accessed. So it's back to BouncyCastle or a roll-your own ASN.1 serializer.

import sun.security.x509.*;

import java.security.cert.*;
import java.security.*;
import java.math.BigInteger;
import java.security.cert.Certificate;
import java.util.Date;
import java.io.IOException;

public class Example {
    /**
     * Create a self-signed X.509 Example
     *
     * @param dn        the X.509 Distinguished Name, eg "CN=Test, L=London, C=GB"
     * @param pair      the KeyPair
     * @param days      how many days from now the Example is valid for
     * @param algorithm the signing algorithm, eg "SHA1withRSA"
     */
    public X509Certificate generateCertificate(String dn, KeyPair pair, int days, String algorithm)
            throws GeneralSecurityException, IOException {
        PrivateKey privkey = pair.getPrivate();
        X509CertInfo info = new X509CertInfo();
        Date from = new Date();
        Date to = new Date(from.getTime() + days * 86400000l);
        CertificateValidity interval = new CertificateValidity(from, to);
        BigInteger sn = new BigInteger(64, new SecureRandom());
        X500Name owner = new X500Name(dn);

        info.set(X509CertInfo.VALIDITY, interval);
        info.set(X509CertInfo.SERIAL_NUMBER, new CertificateSerialNumber(sn));
        info.set(X509CertInfo.SUBJECT, owner);
        info.set(X509CertInfo.ISSUER, owner);
        info.set(X509CertInfo.KEY, new CertificateX509Key(pair.getPublic()));
        info.set(X509CertInfo.VERSION, new CertificateVersion(CertificateVersion.V3));
        AlgorithmId algo = new AlgorithmId(AlgorithmId.md5WithRSAEncryption_oid);
        info.set(X509CertInfo.ALGORITHM_ID, new CertificateAlgorithmId(algo));

        // Sign the cert to identify the algorithm that's used.
        X509CertImpl cert = new X509CertImpl(info);
        cert.sign(privkey, algorithm);

        // Update the algorith, and resign.
        algo = (AlgorithmId) cert.get(X509CertImpl.SIG_ALG);
        info.set(CertificateAlgorithmId.NAME + "." + CertificateAlgorithmId.ALGORITHM, algo);
        cert = new X509CertImpl(info);
        cert.sign(privkey, algorithm);
        return cert;
    }

    public static void main (String[] argv) throws Exception {
        KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("RSA");
        KeyPair keyPair = keyPairGenerator.generateKeyPair();
        Example example = new Example();
        String distinguishedName = "CN=Test, L=London, C=GB";
        Certificate certificate = example.generateCertificateOriginal(distinguishedName, keyPair, 365, "SHA256withRSA");
        System.out.println("it worked!");
    }
}

I liked vbence's answer, but I kept getting the following exception:

java.security.cert.CertificateException: Subject class type invalid.

After lots of attempts to find out was a valid subject class I found out that X509CerInfo wanted an instance of X500Name.

1 info.set(X509CertInfo.SERIAL_NUMBER, new CertificateSerialNumber(sn));
2 info.set(X509CertInfo.SUBJECT, new CertificateSubjectName(owner));
3 info.set(X509CertInfo.ISSUER, new CertificateIssuerName(owner));
4 info.set(X509CertInfo.KEY, new CertificateX509Key(pair.getPublic()));

So lines 2 & 3 needed to change to

2 info.set(X509CertInfo.SUBJECT, owner);
3 info.set(X509CertInfo.ISSUER, owner);