code: public CertStore getCertificate(final X509Certificate identity,
              final PrivateKey key, final BigInteger serial)
              throws ClientException, OperationFailureException {
          return getCertificate(identity, key, serial, null);
      }
docs: Retrieves the certificate corresponding to the provided serial number.
  <p>
  This request relates only to the current CA certificate. If the CA
  certificate has changed since the requested certificate was issued, this
  operation will fail.
  
  @param identity
  the identity of the client.
  @param key
  the private key to sign the SCEP request.
  @param serial
  the serial number of the requested certificate.
  @return the certificate store containing the requested certificate.
  @throws ClientException
  if any client error occurs.
  @throws OperationFailureException
  if the SCEP server refuses to service the request.