Abstract
This work proposes a new lattice two-stage sampling technique, generalizing the prior two-stage sampling method of Gentry, Peikert, and Vaikuntanathan (STOC ’08). By using our new technique as a key building block, we can significantly improve security and efficiency of the current state of the arts of simulation-based functional encryption. Particularly, our functional encryption achieves \((Q,\mathsf {poly})\) simulation-based semi-adaptive security that allows arbitrary pre- and post-challenge key queries, and has succinct ciphertexts with only an additive O(Q) overhead.
Additionally, our two-stage sampling technique can derive new feasibilities of indistinguishability-based adaptively-secure \(\mathsf {IB} \)-\(\mathsf {FE} \) for inner products and semi-adaptively-secure \(\mathsf {AB} \)-\(\mathsf {FE} \) for inner products, breaking several technical limitations of the recent work by Abdalla, Catalano, Gay, and Ursu (Asiacrypt ’20).
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- 1.
We note that both the names “index” and “attribute” have been used interchangeably in the literature.
- 2.
For example in \(\mathsf {IBE}\) and \(\mathsf {ABE}\), 0-keys are useful for decrypting other ciphertexts with satisfying indices. They just cannot decrypt the specific (challenge) index.
- 3.
We notice that \((\mathsf {poly},\mathsf {poly})\) \(\mathsf {SIM}\)-based security is not possible by the lower bound of [4]. Thus, \((Q,\mathsf {poly})\) \(\mathsf {SIM}\)-based security is the best we can hope for in this model.
- 4.
A very-selective scheme requires the adversary to commit to both the challenge index and function in the very beginning of the security experiment.
- 5.
- 6.
Notice that the reusable garbled circuits following from our \(\mathsf {SIM}\)-secure \(\mathsf {FE}\) can achieve \(\mathsf {SA}\)-\(\mathsf {SIM}\) security, and support general circuits and any arbitrary pre- and post-challenge key query, for one query.
- 7.
To sample \(\mathcal {D}_{\Lambda _q^{\boldsymbol{u}}(\mathbf {A}),s} \), the current sampling algorithm requires that \(s> \Vert \widetilde{\mathbf {T}}_{\mathbf {A}}\Vert \omega (\sqrt{\log m})\). According to the best known (to our knowledge) trapdoor generation, the smallest s we can sample would be \(\omega (\sqrt{n\log q} \cdot \sqrt{\log m})\), which is much larger than the required bound for Lemma 4.1.
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Acknowledgements
We would like to thank the anonymous reviewers of Eurocrypt 2021 for their insightful advices. Qiqi Lai is supported by the National Key R&D Program of China (2017YFB0802000), the National Natural Science Foundation of China (61802241, U2001205, 61772326, 61802242), the Natural Science Basic Research Plan in Shaanxi Province of China (2019JQ-360), the National Cryptography Development Foundation during the 13th Five-year Plan Period (MMJJ20180217), and the Fundamental Research Funds for the Central Universities (GK202103093). Feng-Hao Liu and Zhedong Wang are supported by an NSF Award CNS-1657040 and an NSF Career Award CNS-1942400. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the sponsors.
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Lai, Q., Liu, FH., Wang, Z. (2021). New Lattice Two-Stage Sampling Technique and Its Applications to Functional Encryption – Stronger Security and Smaller Ciphertexts. In: Canteaut, A., Standaert, FX. (eds) Advances in Cryptology – EUROCRYPT 2021. EUROCRYPT 2021. Lecture Notes in Computer Science(), vol 12696. Springer, Cham. https://doi.org/10.1007/978-3-030-77870-5_18
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