Paper 2023/054

On the Incoercibility of Digital Signatures

Abstract

We introduce incoercible digital signature schemes, a variant of a standard digital signature. Incoercible signatures enable signers, when coerced to produce a signature for a message chosen by an attacker, to generate fake signatures that are indistinguishable from real signatures, even if the signer is compelled to reveal their full history (including their secret signing keys and any randomness used to produce keys/signatures) to the attacker. Additionally, we introduce an authenticator that can detect fake signatures, which ensures that coercion is identified. We present a formal security model for incoercible signature schemes that comprises an established definition of unforgeability and captures new notions of weak receipt-freeness, strong receipt-freeness and coercion-resistance. We demonstrate that an incoercible signature scheme can be viewed as a transformation of any generic signature scheme. Indeed, we present two incoercible signature scheme constructions that are built from a standard signature scheme and a sender-deniable encryption scheme. We prove that our first construction satisfies coercion-resistance, and our second satisfies strong receipt-freeness. We conclude by presenting an extension to our security model: we show that our security model can be extended to the designated verifier signature scheme setting in an intuitive way as the designated verifier can assume the role of the authenticator and detect coercion during the verification process.