def check_validity(issuer_cert = nil, time = Time.now)
      ret = if @not_before && @not_before > time
              [false, :expired, "not valid before '#@not_before'"]
            elsif @not_after && @not_after < time
              [false, :expired, "not valid after '#@not_after'"]
            elsif issuer_cert && !verify(issuer_cert.public_key)
              [false, :issuer, "#{issuer_cert.subject} is not issuer"]
            else
              [true, :ok, 'Valid certificate']
            end

      # return hash
      { :is_valid => ret[0], :error => ret[1], :desc => ret[2] }
    end
Check the validity of this certificate.