Beholder signatures We introduce a new primitive, called beholder signatures (Full version [14] of the paper is available at https://eprint.iacr.org/2025/1900), which, in some sense, are the opposite of blind signatures. In a beholder signature, one signs a commitment to a (potentially very long) message, and the signature attests that the parties participating in the signing process, who know the secret key, jointly also know the entire committed message. This guarantee holds even against distributed adversaries that use secure multi-party computation (MPC) to produce the signature. We work in the distributed adversarial model (Dziembowski, Faust, and Lizurej, Crypto’23), where one assumes that it is infeasible to evaluate a large number of hash queries without any of the participating parties learning the input. We propose a construction of beholder signatures in the random oracle model. The starting point of our construction is proofs of complete knowledge, recently proposed by (Kelkar et al. CCS’24), which build on Fischlin’s transformation of a sigma protocol to a non-interactive, straight-line extractable zero-knowledge proof of knowledge. Our scheme is concretely efficient and comes with a proof-of-concept implementation using Schnorr as the underlying sigma protocol. The primary applications of beholder signatures can be found within the blockchain space. In particular, we describe how to use them to construct proofs of custody (Feist, 2021) that do not require ephemeral keys and are non-interactive. We also outline applications to data dissemination, data availability, and proofs of replication. 16801 134-165 134-165 Springer