Paper 2021/1182

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

Nicholas Stifter, Aljosha Judmayer, Philipp Schindler, and Edgar Weippl


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.

Available format(s)
Publication info
Preprint. Minor revision.
algorithmic double-spendingcryptocurrencyblockchain
Contact author(s)
nstifter @ sba-research org
2021-09-14: received
Short URL
Creative Commons Attribution


      author = {Nicholas Stifter and Aljosha Judmayer and Philipp Schindler and Edgar Weippl},
      title = {Opportunistic Algorithmic Double-Spending: How I learned to stop worrying and hedge the Fork},
      howpublished = {Cryptology ePrint Archive, Paper 2021/1182},
      year = {2021},
      note = {\url{}},
      url = {}
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