The problem SSPC: Stupid Simple Payment Channel solves

Rollups are great for increasing the scalability of blockchains, however transactions costs still hover in the 5-20 cent range. Even with EIP-4844 sub-cent microtransactions will be infeasible on Rollups.

Payment Channels Networks (where transactions are routed via multiple participants) have the disadvantage of requiring intermediate peers to always be online and requiring a lot of capital to be locked. They are very complex to build, as channels need to have sufficient balance to route payments.

An advantage of Simple Bi-directional Payment Channels over Payment Channel Networks (like Raiden or the Lightning Network) is that its way easier to gauge the amount of capital a channel should have. As you need to estimate how many payments will go over your channels when opening them and its easier to estimate it for a single service compared to multiple services.

Simple Bi-directional Payment Channels are easy to build and solve most use cases. Additionally they can support truly offline payments without compromising on security. With openings and closings happening on Rollups, they can help scale Ethereum even further.

Challenges we ran into

We found a bug in the Fe compiler that prevented us from adding error messages to our assertions in our tests. We overcame this by not adding specific error messages and just relying on the testing framework to fail with a generic error.

Another challenge we ran into was that the NFC library we used for our wallet did not work correctly and was very hard to test. Thus we could not fully end-to-end test our workflow on mobile devices. The smart contracts are tested and working correctly, only the mobile wallet flow could not be fully verified.