Frontrunning was the greatest challenge with this. Our use case is a textbook example for frontrunning. Alice (stasher) wants to send money to anyone who knows a specific phrase (fetcher). How can you do that on the blockchain, where you can't store private data? First, we thought about ZK proofs because they're super cool. But gas cost of proving a zk-SNARK is still around 500k gwei, so we transitioned to a much cheapert two-phase time-locked transaction mechanism, which is also super cool. This has a smaller attack surface and is a more mature method of doing things. The way it works is this: when someone wants to claim funds, they make a deposit to lock the smart contract for 15 minutes to only allow retrievals to their address. The deposit only gets refunded if the fetcher then submits the correct passphrase. The required deposit amount can be set by the stasher. The two-phase mechanic prevents denial of service attacks and the deposits make DoS attacks very costly.

The second challenge was making this an unstoppable app. This means setting up the smart contract completely non-custodially, and hosting the frontend as a single static file. This is pretty unusual, and we had to adapt the tooling to us.

The third challenge was coming up with a cool story and design for this unstoppable app. We went with a space squirrel theme because we like to watch how squirrels stash stuff. And squirrels should not be sanctioned, because stashing and fetching is a right ʕ⁎̯͡⁎ʔ༄