A payment channel protocol does not only have to provide the payment functionality, it also has to fulfill security guarantees such as ensuring that an honest party receives their correct balance. For complexity reasons, it is typically difficult to assess the security of such a protocol or to find counterexamples in insecure protocols. In this poster, we present an approach to specify functional as well as security properties for a payment channel protocol in TLA+ and show that a Lightning Network-style protocol fulfills the required properties. In case a counterexample is found, we provide protocol developers with a graphical and intuitive output. We present the challenges we faced and our approach to meeting these challenges.