Paper 2026/395

How To Make Delegated Payments on Bitcoin: A Question for the AI Agentic Future

Paul Gerhart, TU Wien

Sri AravindaKrishnan Thyagarajan, University of Sydney

Abstract

AI agents and custodial services are increasingly being entrusted as intermediaries to conduct transactions on behalf of institutions. The stakes are high: The digital asset market is projected to exceed \$16 trillion by 2030, where exchanges often involve proprietary, time-sensitive goods. Although industry efforts like Google’s Agent-to-Payments (AP2) protocol standardize how agents authorize payments, they leave open the core challenge of fair exchange: ensuring that a buyer obtains the asset if and only if the seller is compensated without exposing sensitive information. We introduce proxy adaptor signatures (PAS), a new cryptographic primitive that enables fair exchange through delegation without sacrificing atomicity or privacy. A stateless buyer issues a single request and does not need to manage long-term cryptographic secrets while proxies complete the exchange with a seller. The seller is guaranteed payment if the buyer can later reconstruct the purchased witness; meanwhile, the proxies remain oblivious to the witness throughout the protocol. We formalize PAS under a threshold model that tolerates the collusion of up to $t-1$ proxies. We also present an efficient construction from standard primitives that is compatible with Bitcoin, Cardano, and Ethereum. Finally, we evaluate a Rust implementation that supports up to 30 proxies. Our prototype is concretely efficient: buyer and seller computations take place in microseconds, proxy operations in milliseconds, and on-chain costs are equivalent to those of a standard transaction without fair exchange.

@misc{cryptoeprint:2026/395,
      author = {Jay Taylor and Paul Gerhart and Sri AravindaKrishnan Thyagarajan},
      title = {How To Make Delegated Payments on Bitcoin: A Question for the {AI} Agentic Future},
      howpublished = {Cryptology {ePrint} Archive, Paper 2026/395},
      year = {2026},
      url = {https://eprint.iacr.org/2026/395}
}