code: public static byte[] getExtensionValue(X509Certificate cert, String oid)
      throws IOException {
      if (cert == null) {
          throw new IllegalArgumentException(i18n.getMessage("certNull"));
      }
      if (oid == null) {
          throw new IllegalArgumentException(i18n.getMessage("oidNull"));
      }
  
      byte [] value = cert.getExtensionValue(oid);
      if (value == null) {
          return null;
      }
  
      return getExtensionValue(value);
      }
docs: Returns the actual value of the extension.
  
  @param cert the certificate that contains the extensions to retrieve.
  @param oid the oid of the extension to retrieve.
  @return the actual value of the extension (not octet string encoded)
  @exception IOException if decoding the extension fails.