X509 RSA bouncy castle sign and verify plain text in Java
<p>I am currently writing a Java program using BouncyCastle that generates an X509 SSL certificate with an RSA key pair.</p>

<p>I have been able to successfully create the SSL cert but I also want the ability to then sign arbitrary plain text and with that signature verify ownership of the keys by verifying the signature against the plain text.</p>

<p>The methods should look something like this:</p>

<pre><code>protected String SignData(String privateKey, String text)
{
    //return a signature
}
</code></pre>

<p>and</p>

<pre><code>protected boolean verifySignature(String text, String signature, String pubKey)
{
    //return either true or false depending on whether the signature is valid or not
}
</code></pre>

<p>I have successfully generated a self signed SSL certificate using the following:</p>

<pre><code>protected X509Certificate generateCert()
{
    try
    {
        Security.addProvider(new BouncyCastleProvider());
        // generate a key pair
        KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("RSA", "BC");
        keyPairGenerator.initialize(4096, new SecureRandom());
        KeyPair keyPair = keyPairGenerator.generateKeyPair();
        // build a certificate generator
        X509V3CertificateGenerator certGen = new X509V3CertificateGenerator();
        X500Principal dnName = new X500Principal("cn=example");
        // add some options
        certGen.setSerialNumber(BigInteger.valueOf(System.currentTimeMillis()));
        certGen.setSubjectDN(new X509Name("dc=name"));
        certGen.setIssuerDN(dnName); // use the same
        // yesterday
        certGen.setNotBefore(new Date(System.currentTimeMillis() - 24 * 60 * 60 * 1000));
        // in 2 years
        certGen.setNotAfter(new Date(System.currentTimeMillis() + 2 * 365 * 24 * 60 * 60 * 1000));
        certGen.setPublicKey(keyPair.getPublic());
        certGen.setSignatureAlgorithm("SHA256WithRSAEncryption");
        certGen.addExtension(X509Extensions.ExtendedKeyUsage, true, new ExtendedKeyUsage(KeyPurposeId.id_kp_timeStamping));

        // finally, sign the certificate with the private key of the same KeyPair
        X509Certificate cert = certGen.generate(keyPair.getPrivate(), "BC");

        return cert;
    }
    catch (Exception e)
    {
        e.printStackTrace();
        return null;
    }
}
</code></pre>