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Group Signatures and More from Isogenies and Lattices: Generic, Simple, and Efficient

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Advances in Cryptology – EUROCRYPT 2022 (EUROCRYPT 2022)

Abstract

We construct an efficient dynamic group signature (or more generally an accountable ring signature) from isogeny and lattice assumptions. Our group signature is based on a simple generic construction that can be instantiated by cryptographically hard group actions such as the CSIDH group action or an MLWE-based group action. The signature is of size \(O(\log N)\), where N is the number of users in the group. Our idea builds on the recent efficient OR-proof by Beullens, Katsumata, and Pintore (Asiacrypt’20), where we efficiently add a proof of valid ciphertext to their OR-proof and further show that the resulting non-interactive zero-knowledge proof system is online extractable.

Our group signatures satisfy more ideal security properties compared to previously known constructions, while simultaneously having an attractive signature size. The signature size of our isogeny-based construction is an order of magnitude smaller than all previously known post-quantum group signatures (e.g., 6.6 KB for 64 members). In comparison, our lattice-based construction has a larger signature size (e.g., either 126 KB or 89 KB for 64 members depending on the satisfied security property). However, since the \(O(\cdot )\)-notation hides a very small constant factor, it remains small even for very large group sizes, say \(2^{20}\).

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Notes

  1. 1.

    We note that their signature size grows by \(\log ^t N\) for a small constant \(t > 1\) rather than simply by \(\log N\). .

  2. 2.

    To be precise, they consider a weaker variant of standard accountable ring signature where no \(\mathsf {Judge}\) algorithm is considered.

  3. 3.

    Note that extractability via rewinding is insufficient for full anonymity as it will cause an exponential reduction loss when trying to extract the witness from adaptively chosen signatures [6].

  4. 4.

    The choice of what to encrypt is rather arbitrary. The same idea works if for instance we hash \(\mathsf {vk}\) into \(\mathcal {C\ell (O)}\) and view the digest as the message.

  5. 5.

    We note that we also have some independent looseness in the anonymity proof since we rely on the “multi-challenge” \(\mathsf {IND\text {-}CPA}\) security from our \(\mathsf {PKE}\). This is handled in a standard way, and this is also why we only achieve a truly tight group signature from lattices and not from isogenies.

  6. 6.

    We note that it seems difficult to use the parallel OR-proof for our sigma protocol since the challenge space is structured.

  7. 7.

    This is w.l.o.g., and guarantees that the list \(L_\mathcal {O}\) is updated with the input/output required to verify the proof \(\mathcal {A}\) receives or sends.

  8. 8.

    An astute reader may notice that the prover is only expected polynomial time. We can always assign an upper bound on the runtime of the prover, but did not do so for better readability. In practice, for concrete choices of the parameter, the number of repetition never exceeds, say 10.

  9. 9.

    Throughout the proof, we use overlines for \((\overline{\mathsf {com}}, \overline{\mathsf {chall}}, \overline{\mathsf {resp}})\) to indicate that it is a transcript of of \(\varPi _{\varSigma }^\mathsf {tOR}\). We use \(\mathsf {resp}_i\) without overlines to indicate elements of \(\overline{\mathsf {resp}}\).

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Acknowledgements

Yi-Fu Lai was supported by the Ministry for Business, Innovation and Employment in New Zealand. Shuichi Katsumata was supported by JST CREST Grant Number JPMJCR19F6, Japan. This work was supported by CyberSecurity Research Flanders with reference number VR20192203, and in part by the Research Council KU Leuven grant C14/18/067 on Cryptanalysis of post-quantum cryptography. Ward Beullens is funded by FWO Junior Postdoc- toral Fellowship 1S95620N.

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Beullens, W., Dobson, S., Katsumata, S., Lai, YF., Pintore, F. (2022). Group Signatures and More from Isogenies and Lattices: Generic, Simple, and Efficient. In: Dunkelman, O., Dziembowski, S. (eds) Advances in Cryptology – EUROCRYPT 2022. EUROCRYPT 2022. Lecture Notes in Computer Science, vol 13276. Springer, Cham. https://doi.org/10.1007/978-3-031-07085-3_4

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