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Documents authored by Rotem, Lior

Document
DOI: 10.4230/LIPIcs.AFT.2023.26
Post-Quantum Single Secret Leader Election (SSLE) from Publicly Re-Randomizable Commitments

Authors: Dan Boneh, Aditi Partap, and Lior Rotem

Published in: LIPIcs, Volume 282, 5th Conference on Advances in Financial Technologies (AFT 2023)

Abstract
A Single Secret Leader Election (SSLE) enables a group of parties to randomly choose exactly one leader from the group with the restriction that the identity of the leader will be known to the chosen leader and nobody else. At a later time, the elected leader should be able to publicly reveal her identity and prove that she is the elected leader. The election process itself should work properly even if many registered users are passive and do not send any messages. SSLE is used to strengthen the security of proof-of-stake consensus protocols by ensuring that the identity of the block proposer remains unknown until the proposer publishes a block. Boneh, Eskandarian, Hanzlik, and Greco (AFT'20) defined the concept of an SSLE and gave several constructions. Their most efficient construction is based on the difficulty of the Decision Diffie-Hellman problem in a cyclic group. In this work we construct the first efficient SSLE protocols based on the standard Learning With Errors (LWE) problem on integer lattices, as well as the Ring-LWE problem. Both are believed to be post-quantum secure. Our constructions generalize the paradigm of Boneh et al. by introducing the concept of a re-randomizable commitment (RRC). We then construct several post-quantum RRC schemes from lattice assumptions and prove the security of the derived SSLE protocols. Constructing a lattice-based RRC scheme is non-trivial, and may be of independent interest.
Cite as

Dan Boneh, Aditi Partap, and Lior Rotem. Post-Quantum Single Secret Leader Election (SSLE) from Publicly Re-Randomizable Commitments. In 5th Conference on Advances in Financial Technologies (AFT 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 282, pp. 26:1-26:23, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@InProceedings{boneh_et_al:LIPIcs.AFT.2023.26,
  author =	{Boneh, Dan and Partap, Aditi and Rotem, Lior},
  title =	{{Post-Quantum Single Secret Leader Election (SSLE) from Publicly Re-Randomizable Commitments}},
  booktitle =	{5th Conference on Advances in Financial Technologies (AFT 2023)},
  pages =	{26:1--26:23},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-303-4},
  ISSN =	{1868-8969},
  year =	{2023},
  volume =	{282},
  editor =	{Bonneau, Joseph and Weinberg, S. Matthew},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.AFT.2023.26},
  URN =		{urn:nbn:de:0030-drops-192158},
  doi =		{10.4230/LIPIcs.AFT.2023.26},
  annote =	{Keywords: Consensus, Leader Election, Post-Quantum, Lattice Cryptography, Blockchain}
}
Document
DOI: 10.4230/LIPIcs.ITC.2022.12
A Fully-Constructive Discrete-Logarithm Preprocessing Algorithm with an Optimal Time-Space Tradeoff

Authors: Lior Rotem and Gil Segev

Published in: LIPIcs, Volume 230, 3rd Conference on Information-Theoretic Cryptography (ITC 2022)

Abstract
Identifying the concrete hardness of the discrete logarithm problem is crucial for instantiating a vast range of cryptographic schemes. Towards this goal, Corrigan-Gibbs and Kogan (EUROCRYPT '18) extended the generic-group model for capturing "preprocessing" algorithms, offering a tradeoff between the space S required for storing their preprocessing information, the time T required for their online phase, and their success probability. Corrigan-Gibbs and Kogan proved an upper bound of Õ(S T²/N) on the success probability of any such algorithm, where N is the prime order of the group, matching the known preprocessing algorithms. However, the known algorithms assume the availability of truly random hash functions, without taking into account the space required for storing them as part of the preprocessing information, and the time required for evaluating them in essentially each and every step of the online phase. This led Corrigan-Gibbs and Kogan to pose the open problem of designing a discrete-logarithm preprocessing algorithm that is fully constructive in the sense that it relies on explicit hash functions whose description lengths and evaluation times are taken into account in the algorithm’s space-time tradeoff. We present a fully constructive discrete-logarithm preprocessing algorithm with an asymptotically optimal space-time tradeoff (i.e., with success probability Ω̃(S T²/N)). In addition, we obtain an algorithm that settles the corresponding tradeoff for the computational Diffie-Hellman problem. Our approach is based on derandomization techniques that provide rather weak independence guarantees. On the one hand, we show that such guarantees can be realized in our setting with only a minor efficiency overhead. On the other hand, exploiting such weak guarantees requires a more subtle and in-depth analysis of the underlying combinatorial structure compared to that of the known preprocessing algorithms and their analyses.
Cite as

Lior Rotem and Gil Segev. A Fully-Constructive Discrete-Logarithm Preprocessing Algorithm with an Optimal Time-Space Tradeoff. In 3rd Conference on Information-Theoretic Cryptography (ITC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 230, pp. 12:1-12:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{rotem_et_al:LIPIcs.ITC.2022.12,
  author =	{Rotem, Lior and Segev, Gil},
  title =	{{A Fully-Constructive Discrete-Logarithm Preprocessing Algorithm with an Optimal Time-Space Tradeoff}},
  booktitle =	{3rd Conference on Information-Theoretic Cryptography (ITC 2022)},
  pages =	{12:1--12:16},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-238-9},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{230},
  editor =	{Dachman-Soled, Dana},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITC.2022.12},
  URN =		{urn:nbn:de:0030-drops-164905},
  doi =		{10.4230/LIPIcs.ITC.2022.12},
  annote =	{Keywords: Discrete logarithm, Preprocessing}
}
Document
DOI: 10.4230/LIPIcs.ITC.2022.13
Revisiting the Uber Assumption in the Algebraic Group Model: Fine-Grained Bounds in Hidden-Order Groups and Improved Reductions in Bilinear Groups

Authors: Lior Rotem

Published in: LIPIcs, Volume 230, 3rd Conference on Information-Theoretic Cryptography (ITC 2022)

Abstract
We prove strong security guarantees for a wide array of computational and decisional problems, both in hidden-order groups and in bilinear groups, within the algebraic group model (AGM) of Fuchsbauer, Kiltz and Loss (CRYPTO '18). As our first contribution, we put forth a new fine-grained variant of the Uber family of assumptions in hidden-order groups. This family includes in particular the repeated squaring function of Rivest, Shamir and Wagner, which underlies their time-lock puzzle as well as the main known candidates for verifiable delay functions; and a computational variant of the generalized BBS problem, which underlies the timed commitments of Boneh and Naor (CRYPTO '00). We then provide two results within a variant of the AGM, which show that the hardness of solving problems in this family in a less-than-trivial number of steps is implied by well-studied assumptions. The first reduction may be applied in any group (and in particular, class groups), and is to the RSA assumption; and our second reduction is in RSA groups with a modulus which is the product of two safe primes, and is to the factoring assumption. Additionally, we prove that the hardness of any computational problem in the Uber family of problems in bilinear groups is implied by the hardness of the q-discrete logarithm problem. The parameter q in our reduction is the maximal degree in which a variable appears in the polynomials which define the specific problem within the Uber family. This improves upon a recent result of Bauer, Fuchsbauer and Loss (CRYPTO '20), who obtained a similar implication but for a parameter q which is lower bounded by the maximal total degree of one of the above polynomials. We discuss the implications of this improvement to prominent group key-exchange protocols.
Cite as

Lior Rotem. Revisiting the Uber Assumption in the Algebraic Group Model: Fine-Grained Bounds in Hidden-Order Groups and Improved Reductions in Bilinear Groups. In 3rd Conference on Information-Theoretic Cryptography (ITC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 230, pp. 13:1-13:13, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{rotem:LIPIcs.ITC.2022.13,
  author =	{Rotem, Lior},
  title =	{{Revisiting the Uber Assumption in the Algebraic Group Model: Fine-Grained Bounds in Hidden-Order Groups and Improved Reductions in Bilinear Groups}},
  booktitle =	{3rd Conference on Information-Theoretic Cryptography (ITC 2022)},
  pages =	{13:1--13:13},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-238-9},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{230},
  editor =	{Dachman-Soled, Dana},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITC.2022.13},
  URN =		{urn:nbn:de:0030-drops-164910},
  doi =		{10.4230/LIPIcs.ITC.2022.13},
  annote =	{Keywords: Algebraic group model, Uber assumption, Bilinear groups, Hidden-order groups}
}
Document
DOI: 10.4230/LIPIcs.ITC.2020.9
Out-Of-Band Authenticated Group Key Exchange: From Strong Authentication to Immediate Key Delivery

Authors: Moni Naor, Lior Rotem, and Gil Segev

Published in: LIPIcs, Volume 163, 1st Conference on Information-Theoretic Cryptography (ITC 2020)

Abstract
Given the inherent ad-hoc nature of popular communication platforms, out-of-band authenticated key-exchange protocols are becoming widely deployed: Key exchange protocols that enable users to detect man-in-the-middle attacks by manually authenticating one short value. In this work we put forward the notion of immediate key delivery for such protocols, requiring that even if some users participate in the protocol but do not complete it (e.g., due to losing data connectivity or to other common synchronicity issues), then the remaining users should still agree on a shared secret. A property of a similar flavor was introduced by Alwen, Coretti and Dodis (EUROCRYPT '19) asking for immediate decryption of messages in user-to-user messaging while assuming that a shared secret has already been established - but the underlying issue is crucial already during the initial key exchange and goes far beyond the context of messaging. Equipped with our immediate key delivery property, we formalize strong notions of security for out-of-band authenticated group key exchange, and demonstrate that the existing protocols either do not satisfy our notions of security or are impractical (these include, in particular, the protocols deployed by Telegram, Signal and WhatsApp). Then, based on the existence of any passively-secure key-exchange protocol (e.g., the Diffie-Hellman protocol), we construct an out-of-band authenticated group key-exchange protocol satisfying our notions of security. Our protocol is inspired by techniques that have been developed in the context of fair string sampling in order to minimize the effect of adversarial aborts, and offers the optimal tradeoff between the length of its out-of-band value and its security.
Cite as

Moni Naor, Lior Rotem, and Gil Segev. Out-Of-Band Authenticated Group Key Exchange: From Strong Authentication to Immediate Key Delivery. In 1st Conference on Information-Theoretic Cryptography (ITC 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 163, pp. 9:1-9:25, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)

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@InProceedings{naor_et_al:LIPIcs.ITC.2020.9,
  author =	{Naor, Moni and Rotem, Lior and Segev, Gil},
  title =	{{Out-Of-Band Authenticated Group Key Exchange: From Strong Authentication to Immediate Key Delivery}},
  booktitle =	{1st Conference on Information-Theoretic Cryptography (ITC 2020)},
  pages =	{9:1--9:25},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-151-1},
  ISSN =	{1868-8969},
  year =	{2020},
  volume =	{163},
  editor =	{Tauman Kalai, Yael and Smith, Adam D. and Wichs, Daniel},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITC.2020.9},
  URN =		{urn:nbn:de:0030-drops-121146},
  doi =		{10.4230/LIPIcs.ITC.2020.9},
  annote =	{Keywords: End-to-end encryption, out-of-band authentication, key exchange}
}
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