Opportunistic Algorithmic Double-Spending: How I learned to stop worrying and hedge the Fork

In this paper we outline a novel form of attack we refer to as Opportunistic Algorithmic Double-Spending (OpAl). OpAl attacks not only avoid equivocation, i.e., do not require conflicting transactions, the attack is also carried out programmatically. Algorithmic double-spending is facilitated through transaction semantics that dynamically depend on the context and ledger state at the time of execution. Hence, OpAl evades common double-spending detection mechanisms and can opportunistically leverage forks, even if the malicious sender itself is not aware of their existence. Furthermore, the cost of modifying a regular transaction to opportunistically perform an OpAl attack is low enough to consider it a viable default strategy for most use cases. Our analysis suggests that while Bitcoin’s stateless UTXO model is more robust against OpAl , designs with expressive transaction semantics, especially stateful smart contract platforms such as Ethereum, are particularly vulnerable.