Paper 2023/833

Anonymous, Timed and Revocable Proxy Signatures

Abstract

A proxy signature enables a party to delegate her signing power to another. This is useful in practice to achieve goals related to robustness, crowd-sourcing, and workload sharing. Such applications usually require delegation to satisfy several properties, including time bounds, anonymity, revocability, and policy enforcement. Despite the large amount of work on proxy signatures in the literature, none of the existing schemes satisfy all these properties; even there is no unified formal notion that captures them. In this work, we close this gap and propose an anonymous, timed, and revocable proxy signature scheme. We achieve this in two steps: First, we introduce a tokenizable digital signature based on Schnorr signature allowing for secure distribution of signing tokens (which could be of independent interest). Second, we utilize a public bulletin board and timelock encryption to support: (1) one-time usage of the signing tokens by tracking tokens used so far based on unique values associated to them, (2) timed delegation so that a proxy signer cannot sign outside a given period, and (3) delegation revocation allowing the original signer to end a delegation earlier than provisioned. All of these are done in a decentralized and anonymous way; no trusted party is involved, and no one can tell that someone else signed on behalf of the original signer or even that a delegation took place. We define a formal notion for proxy signatures capturing all these properties, and prove that our construction realizes this notion. We also introduce several design considerations addressing issues related to deployment in practice.