Connecting foreign blockchain assets to Ethereum requires a light-client following the consensus algorithm of the other chain. In the case of BTC it entails the relay and verification of all Bitcoin block headers in an Ethereum smart contract. Once the canonical chain is determined transaction inclusion proofs can be used to prove deposits or burns without trust in any intermediaries.
The tBTC project by Keep Network and Strudel.Finance alike use a Relay contract deployed at 0x1531b6e3d51bf80f634957df81a990b92da4b15. A relayer service was initially operated by tBTC to feed the contract, providing vital functionality to both bridges. At Ethereum block 11030571 (at 10.50 pm UTC on Saturday) it stopped operation due to a low ETH balance and accordingly no possibility to pay for transactions.
The Strudel community detected the anomaly of the Relayer within two hours following the event. The relay rewards built into the Strudel protocol kicked in and animated community members to relay blocks manually, before a relayer script was automated only a few hours later.
In the meantime, the tBTC and vBTC bridges could continue operations seamlessly. An example for a successful transaction from the tBTC project calling this Relay on Monday can be found here.
The original relayer service went back online after being funded on Monday, the 12th of October, at around 3 pm UTC.
At Strudel, we are thankful to the Keep Network team for creating the relay contract and to our active community for jumping in at the right time to keep it going. We will continue the collaboration between the teams emerging from this incident and operate the relay collectively as a public good in the future.