* This method checks if a Certificate with a given serial number is already issued.
* @param sNo The serial number of the the certificate to be looked for
* @return true if a certificate with the specified serial number has already been issued
*/
public boolean isCertificateIssued(BigInteger sNo) throws CertificationAuthorityException {
if(isLocked()) throw new CertificationAuthorityException("CA is locked.");
return certStore.containsCertificate(sNo);
}