certGen.setSubjectDN(subjectName);
certGen.setPublicKey(keyPair.getPublic());
certGen.setSignatureAlgorithm("SHA256WithRSAEncryption");
certGen.addExtension(X509Extensions.AuthorityKeyIdentifier, false,
new AuthorityKeyIdentifierStructure(caCert));
certGen.addExtension(X509Extensions.SubjectKeyIdentifier, false,
new SubjectKeyIdentifierStructure(keyPair.getPublic()));
X509Certificate cert = certGen.generate(caPrivateKey, "BC"); // note: private key of CA
return cert;