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This paper extends the concept of withdrawable signatures by integrating lattice-based constructions into the Fiat-Shamir with aborts paradigm, enhancing signer anonymity through a claimable ring signature approach. By employing a one-out-of-N signature as a black box, the method ensures that no signer-derived information is leaked, even under full key exposure. The authors provide comprehensive proofs demonstrating the system's correctness, extended withdrawability, and unforgeability, relying on established lattice problems and security models, thus advancing the robustness of cryptographic signatures in quantum contexts.
Signer anonymity is preserved even under full key exposure, revolutionizing the security of withdrawable signatures in lattice-based cryptography.
We extend the extended withdrawable signatures of Liu, Susilo and Baek to lattice-based constructions built on the Fiat-Shamir with aborts paradigm. Departing from an earlier draft that transported a per-signer shift in the clear, which leaks the signer, we realise extended withdrawable signatures as a claimable ring signature: signer ambiguity is provided by a one-out-of-N signature used as a black box (anonymity under full key exposure), and confirmation is the signer's claim, a binding signature together with the opening of a hiding index commitment bound into the transcript. No signer-derived value is published in the clear. We give complete proofs of correctness, extended withdrawability (as anonymity-until-claim), unforgeability under insider corruption, and claimability soundness, reducing to decisional MLWE (commitment hiding), MSIS (commitment binding), the anonymity of the one-out-of-$N$ scheme, and the EUF-CMA security of the base signature, in the (quantum) random-oracle model. We instantiate the base signature with a no-hint, full-$t$ Dilithium-style scheme and the one-out-of-$N$ layer with an established lattice one-out-of-many proof.