Blockchain "on-chain" oracles are critical to the functioning of many Decentralized Finance (DeFi) protocols. We analyze these oracles for manipulation resistance. Specifically, we analyze the cost of manipulating on-chain time-weighted average price (TWAP) oracles that use the arithmetic mean. It has been assumed that manipulating a TWAP oracle with the well-known multi-block attack is expensive and scales linearly with the length of the TWAP. We question this assumption with two novel results. First, we describe a single-block attack that works under the same setting as the multi-block attack but costs less to execute. Second, we describe a multi-block MEV (MMEV) style attack where the attacker colludes with a miner/proposer who can mine/propose two blocks in a row. This MMEV style attack makes oracle manipulation orders of magnitude cheaper than previously known attacks. In the proof-of-work setting, MMEV can be done by selfish mining even with very low shares of hashpower.

Atomic Swaps enable exchanging crypto-assets with-out trusting a third party. To enable these swaps, both parties lock funds and let their counterparty withdraw them in exchange for a secret. This leads to the so-called griefing attack, or the emergence of an American Call option, where one party stops participating in the swap, thereby making their counterparty wait for a timelock to expire before they can withdraw their funds. The standard way to mitigate this attack is to make the attacker pay a premium for the emerging American Call option. In these premium-paying approaches, the premium itself ends up being locked for possibly an even longer duration than the swap amount itself. We propose a new Atomic Swap construction, where neither party exposes itself to a griefing attack by their counterparty. Notably, unlike previous constructions, ours can be implemented in Bitcoin as is. Our construction also takes fewer on-chain transactions and has a lower worst-case timelock.

In the context of second layer payments in Bitcoin, and specifically the Lightning Network, we propose a design for a lightweight watchtower that does not need to store signed justice transactions. We alter the structure of the opening and commitment transactions in Lightning channels to encode justice transactions as part of the commitment transactions. With that, a watchtower just needs to watch for specific cheating commitment transaction IDs on the blockchain and can extract signed justice transactions directly from these commitment transactions that appear on the blockchain. Our construction saves an order of magnitude in storage over existing watchtower designs. In addition, we let the watchtower prove to each channel that it has access to all the data required to do its job, and can therefore be paid-per-update.