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The two-volume set LNCS 12110 and 12111 constitutes the refereed proceedings of the 23rd IACR International Conference on the Practice and Theory of Public-Key Cryptography, PKC 2020, held in Edinburgh, UK, in May 2020.

The 44 full papers presented were carefully reviewed and selected from 180 submissions. They are organized in topical sections such as: functional encryption; identity-based encryption; obfuscation and applications; encryption schemes; secure channels; basic primitives with special properties; proofs and arguments; lattice-based cryptography; isogeny-based cryptography; multiparty protocols; secure computation and related primitives; post-quantum primitives; and privacy-preserving schemes.



Functional Encryption


Fast, Compact, and Expressive Attribute-Based Encryption

Attribute-based encryption (ABE) is an advanced cryptographic tool and useful to build various types of access control systems. Toward the goal of making ABE more practical, we propose key-policy (KP) and ciphertext-policy (CP) ABE schemes, which first support unbounded sizes of attribute sets and policies with negation and multi-use of attributes, allow fast decryption, and are adaptively secure under a standard assumption, simultaneously. Our schemes are more expressive than previous schemes and efficient enough. To achieve the adaptive security along with the other properties, we refine the technique introduced by Kowalczyk and Wee (Eurocrypt’19) so that we can apply the technique more expressive ABE schemes. Furthermore, we also present a new proof technique that allows us to remove redundant elements used in their ABE schemes. We implement our schemes in 128-bit security level and present their benchmarks for an ordinary personal computer and smartphones. They show that all algorithms run in one second with the personal computer when they handle any policy or attribute set with one hundred attributes.
Junichi Tomida, Yuto Kawahara, Ryo Nishimaki

Adaptive Simulation Security for Inner Product Functional Encryption

Inner product functional encryption (\({\mathsf {IPFE}}\)) [1] is a popular primitive which enables inner product computations on encrypted data. In \({\mathsf {IPFE}}\), the ciphertext is associated with a vector \(\varvec{x}\), the secret key is associated with a vector \(\varvec{y}\) and decryption reveals the inner product \(\langle \varvec{x},\varvec{y}\rangle \). Previously, it was known how to achieve adaptive indistinguishability (\(\mathsf {IND}\)) based security for \({\mathsf {IPFE}}\) from the \(\mathsf {DDH}\), \(\mathsf {DCR}\) and \(\mathsf {LWE}\) assumptions [8]. However, in the stronger simulation (\(\mathsf {SIM}\)) based security game, it was only known how to support a restricted adversary that makes all its key requests either before or after seeing the challenge ciphertext, but not both. In more detail, Wee [46] showed that the \(\mathsf {DDH}\)-based scheme of Agrawal et al. (Crypto 2016) achieves semi-adaptive simulation-based security, where the adversary must make all its key requests after seeing the challenge ciphertext. On the other hand, O’Neill showed that all \(\mathsf {IND}\)-secure \({\mathsf {IPFE}}\) schemes (which may be based on \(\mathsf {DDH}\), \(\mathsf {DCR}\) and \(\mathsf {LWE}\)) satisfy \(\mathsf {SIM}\) based security in the restricted model where the adversary makes all its key requests before seeing the challenge ciphertext.
In this work, we resolve the question of \(\mathsf {SIM}\)-based security for \({\mathsf {IPFE}}\) by showing that variants of the \({\mathsf {IPFE}}\) constructions by Agrawal et al., based on \(\mathsf {DDH}\), Paillier and \(\mathsf {LWE}\), satisfy the strongest possible adaptive \(\mathsf {SIM}\)-based security where the adversary can make an unbounded number of key requests both before and after seeing the (single) challenge ciphertext. This establishes optimal security of the \({\mathsf {IPFE}}\) schemes, under all hardness assumptions on which it can (presently) be based.
Shweta Agrawal, Benoît Libert, Monosij Maitra, Radu Titiu

Verifiable Inner Product Encryption Scheme

In the standard setting of functional encryption (FE), we assume both the Central Authority (CA) and the encryptors to run their respective algorithms faithfully. Badrinarayanan et al. [ASIACRYPT 2016] proposed the concept of verifiable FE, which essentially guarantees that dishonest encryptors and authorities, even when colluding together, are not able to generate ciphertexts and tokens that give “inconsistent” results. They also provide a compiler turning any perfectly correct FE into a verifiable FE, but do not give efficient constructions.
In this paper we improve on this situation by considering Inner-Product Encryption (IPE), which is a special case of functional encryption and a primitive that has attracted wide interest from both practitioners and researchers in the last decade. Specifically, we construct the first efficient verifiable IPE (VIPE) scheme according to the inner-product functionality of Katz, Sahai and Waters [EUROCRYPT 2008]. To instantiate the general construction of Badrinarayanan et al. we need to solve several additional challenges. In particular, we construct the first efficient perfectly correct IPE scheme. Our VIPE satisfies unconditional verifiability, whereas its privacy relies on the DLin assumption.
Najmeh Soroush, Vincenzo Iovino, Alfredo Rial, Peter B. Roenne, Peter Y. A. Ryan

A New Paradigm for Public-Key Functional Encryption for Degree-2 Polynomials

We give the first public-key functional encryption that supports the generation of functional decryption keys for degree-2 polynomials, with succinct ciphertexts, whose semi-adaptive simulation-based security is proven under standard assumptions. At the heart of our new paradigm lies a so-called partially function-hiding functional encryption scheme for inner products, which admits public-key instances, and that is sufficient to build functional encryption for degree-2 polynomials. Doing so, we improve upon prior works, such as the constructions from Lin (CRYPTO 17) or Ananth Sahai (EUROCRYPT 17), both of which rely on function-hiding inner product FE, that can only exist in the private-key setting. The simplicity of our construction yields the most efficient FE for quadratic functions from standard assumptions (even those satisfying a weaker security notion). The interest of our methodology is that the FE for quadratic functions that builds upon any partially function-hiding FE for inner products inherits the security properties of the latter. In particular, we build a partially function-hiding FE for inner products that enjoys simulation security, in the semi-adaptive setting, where the challenge sent from the adversary can be chosen adaptively after seeing the public key (but before corrupting functional decryption keys). This is in contrast from prior public-key FE for quadratic functions from Baltico et al. (CRYPTO 17), which only achieved an indistinguishability-based, selective security. As a bonus, we show that we can obtain security against Chosen-Ciphertext Attacks straightforwardly. Even though this is the de facto security notion for encryption, this was not achieved by prior functional encryption schemes for quadratic functions, where the generic Fujisaki Okamoto transformation (CRYPTO 99) does not apply.
Romain Gay

Identity-Based Encryption


Master-Key KDM-Secure IBE from Pairings

Identity-based encryption (IBE) is a generalization of public-key encryption (PKE) by allowing encryptions to be made to user identities. In this work, we seek to obtain IBE schemes that achieve key-dependent-message (KDM) security with respect to messages that depend on the master secret key. Previous KDM-secure schemes only achieved KDM security in simpler settings, in which messages may only depend on user secret keys.
An important motivation behind studying master-KDM security is the application of this notion in obtaining generic constructions of KDM-CCA secure PKE, a primitive notoriously difficult to realize.
We give the first IBE that achieves master-KDM security from standard assumptions in pairing groups. Our construction is modular and combines techniques from KDM-secure PKE based from hash-proof systems, together with IBE that admits a tight security proof in the multi-challenge setting, which happens to be unexpectedly relevant in the context of KDM security. In fact, to the best of our knowledge, this is the first setting where techniques developed in the context of realizing tightly secure cryptosystems have led to a new feasibility result.
As a byproduct, our KDM-secure IBE, and thus the resulting KDM-CCA-secure PKE both enjoy a tight security reduction, independent of the number of challenge ciphertexts, which was not achieved before.
Sanjam Garg, Romain Gay, Mohammad Hajiabadi

Hierarchical Identity-Based Encryption with Tight Multi-challenge Security

We construct the first hierarchical identity-based encryption (HIBE) scheme with tight adaptive security in the multi-challenge setting, where adversaries are allowed to ask for ciphertexts for multiple adaptively chosen identities. Technically, we develop a novel technique that can tightly introduce randomness into user secret keys for hierarchical identities in the multi-challenge setting, which cannot be easily achieved by the existing techniques for tightly multi-challenge secure IBE.
In contrast to the previous constructions, the security of our scheme is independent of the number of user secret key queries and that of challenge ciphertext queries. We prove the tight security of our scheme based on the Matrix Decisional Diffie-Hellman Assumption, which is an abstraction of standard and simple decisional Diffie-Hellman assumptions, such as the k-Linear and SXDH assumptions.
Finally, we also extend our ideas to achieve tight chosen-ciphertext security and anonymity, respectively. These security notions for HIBE have not been tightly achieved in the multi-challenge setting before.
Roman Langrehr, Jiaxin Pan

Obfuscation and Applications


The Usefulness of Sparsifiable Inputs: How to Avoid Subexponential iO

We consider the problem of removing subexponential reductions to indistinguishability obfuscation (iO) in the context of obfuscating probabilistic programs. Specifically, we show how to apply complexity absorption (Zhandry Crypto 2016) to the recent notion of probabilistic indistinguishability obfuscation (piO, Canetti et al. TCC 2015). As a result, we obtain a variant of piO which allows to obfuscate a large class of probabilistic programs, from polynomially secure indistinguishability obfuscation and extremely lossy functions. Particularly, our piO variant is able to obfuscate circuits with specific input domains regardless of the performed computation. We then revisit several (direct or indirect) applications of piO, and obtain
– a fully homomorphic encryption scheme (without circular security assumptions),
– a multi-key fully homomorphic encryption scheme with threshold decryption,
– an encryption scheme secure under arbitrary key-dependent messages,
– a spooky encryption scheme for all circuits,
– a function secret sharing scheme with additive reconstruction for all circuits,
all from polynomially secure iO, extremely lossy functions, and, depending on the scheme, also other (but polynomial and comparatively mild) assumptions. All of these assumptions are implied by polynomially secure iO and the (non-polynomial, but very well-investigated) exponential DDH assumption. Previously, all the above applications required to assume the subexponential security of iO (and more standard assumptions).
Thomas Agrikola, Geoffroy Couteau, Dennis Hofheinz

Witness Maps and Applications

We introduce the notion of Witness Maps as a cryptographic notion of a proof system. A Unique Witness Map (UWM) deterministically maps all witnesses for an \(\mathbf {NP}\) statement to a single representative witness, resulting in a computationally sound, deterministic-prover, non-interactive witness independent proof system. A relaxation of UWM, called Compact Witness Map (CWM), maps all the witnesses to a small number of witnesses, resulting in a “lossy” deterministic-prover, non-interactive proof-system. We also define a Dual Mode Witness Map (DMWM) which adds an “extractable” mode to a CWM.
Our main construction is a DMWM for all \(\mathbf {NP}\) relations, assuming sub-exponentially secure indistinguishability obfuscation (\({i\mathcal {O}}\)), along with standard cryptographic assumptions. The DMWM construction relies on a CWM and a new primitive called Cumulative All-Lossy-But-One Trapdoor Functions (C-ALBO-TDF), both of which are in turn instantiated based on \({i\mathcal {O}}\) and other primitives. Our instantiation of a CWM is in fact a UWM; in turn, we show that a UWM implies Witness Encryption. Along the way to constructing UWM and C-ALBO-TDF, we also construct, from standard assumptions, Puncturable Digital Signatures and a new primitive called Cumulative Lossy Trapdoor Functions (C-LTDF). The former improves up on a construction of Bellare et al. (Eurocrypt 2016), who relied on sub-exponentially secure \({i\mathcal {O}}\) and sub-exponentially secure OWF.
As an application of our constructions, we show how to use a DMWM to construct the first leakage and tamper-resilient signatures with a deterministic signer, thereby solving a decade old open problem posed by Katz and Vaikunthanathan (Asiacrypt 2009), by Boyle, Segev and Wichs (Eurocrypt 2011), as well as by Faonio and Venturi (Asiacrypt 2016). Our construction achieves the optimal leakage rate of \(1 - o(1)\).
Suvradip Chakraborty, Manoj Prabhakaran, Daniel Wichs

Encryption Schemes


Memory-Tight Reductions for Practical Key Encapsulation Mechanisms

The efficiency of a black-box reduction is an important goal of modern cryptography. Traditionally, the time complexity and the success probability were considered as the main aspects of efficiency measurements. In CRYPTO 2017, Auerbach et al. introduced the notion of memory-tightness in cryptographic reductions and showed a memory-tight reduction of the existential unforgeability of the RSA-FDH signature scheme. Unfortunately, their techniques do not extend directly to the reductions involving intricate RO-programming. The problem seems to be inherent as all the other existing results on memory-tightness are lower bounds and impossibility results. In fact, Auerbach et al. conjectured that a memory-tight reduction for security of Hashed-ElGamal KEM is impossible.
  • We refute the above conjecture. Using a simple RO simulation technique, we provide memory-tight reductions of security of the Cramer-Shoup and the ECIES version of Hashed-ElGamal KEM.
  • We prove memory-tight reductions for different variants of Fujisaki-Okamoto Transformation. We analyze the modular transformations introduced by Hofheinz, Hövermanns and Kiltz (TCC 2017). In addition to the constructions involving implicit rejection, we present a memory-tight reduction for the security of the transformation \(\mathsf{\text {QFO}_m^\perp }\). Our techniques can withstand correctness-errors, and applicable to several lattice-based KEM candidates.
Rishiraj Bhattacharyya

Toward RSA-OAEP Without Random Oracles

We show new partial and full instantiation results under chosen-ciphertext security for the widely implemented and standardized RSA-OAEP encryption scheme of Bellare and Rogaway (EUROCRYPT 1994) and two variants. Prior work on such instantiations either showed negative results or settled for “passive” security notions like IND-CPA. More precisely, recall that RSA-OAEP adds redundancy and randomness to a message before composing two rounds of an underlying Feistel transform, whose round functions are modeled as random oracles (ROs), with RSA. Our main results are:
  • Either of the two oracles (while still modeling the other as a RO) can be instantiated in RSA-OAEP under IND-CCA2 using mild standard-model assumptions on the round functions and generalizations of algebraic properties of RSA shown by Barthe, Pointcheval, and Báguelin (CCS 2012). The algebraic properties are only shown to hold at practical parameters for small encryption exponent (\(e=3\)), but we argue they have value for larger e as well.
  • Both oracles can be instantiated simultaneously for two variants of RSA-OAEP, called “t-clear” and “s-clear” RSA-OAEP. For this we use extractability-style assumptions in the sense of Canetti and Dakdouk (TCC 2010) on the round functions, as well as novel yet plausible “XOR-type” assumptions on RSA. While admittedly strong, such assumptions may nevertheless be necessary at this point to make positive progress.
In particular, our full instantiations evade impossibility results of Shoup (J. Cryptology 2002), Kiltz and Pietrzak (EUROCRYPT 2009), and Bitansky et al. (STOC 2014). Moreover, our results for s-clear RSA-OAEP yield the most efficient RSA-based encryption scheme proven IND-CCA2 in the standard model (using bold assumptions on cryptographic hashing) to date.
Nairen Cao, Adam O’Neill, Mohammad Zaheri

Public-Key Puncturable Encryption: Modular and Compact Constructions

We revisit the method of designing public-key puncturable encryption schemes and present a generic conversion by leveraging the techniques of distributed key-distribution and revocable encryption. In particular, we first introduce a refined version of identity-based revocable encryption, named key-homomorphic identity-based revocable key encapsulation mechanism with extended correctness. Then, we propose a generic construction of puncturable key encapsulation mechanism from the former by merging the idea of distributed key-distribution. Compared to the state-of-the-art, our generic construction supports unbounded number of punctures and multiple tags per message, thus achieving more fine-grained revocation of decryption capability. Further, it does not rely on random oracles, not suffer from non-negligible correctness error, and results in a variety of efficient schemes with distinct features. More precisely, we obtain the first scheme with very compact ciphertexts in the standard model, and the first scheme with support for both unbounded size of tags per ciphertext and unbounded punctures as well as constant-time puncture operation. Moreover, we get a comparable scheme proven secure under the standard DBDH assumption, which enjoys both faster encryption and decryption than previous works based on the same assumption, especially when the number of tags associated with the ciphertext is large.
Shi-Feng Sun, Amin Sakzad, Ron Steinfeld, Joseph K. Liu, Dawu Gu

Secure Channels


Flexible Authenticated and Confidential Channel Establishment (fACCE): Analyzing the Noise Protocol Framework

The Noise protocol framework is a suite of channel establishment protocols, of which each individual protocol ensures various security properties of the transmitted messages, but keeps specification, implementation, and configuration relatively simple. Implementations of the Noise protocols are themselves, due to the employed primitives, very performant. Thus, despite its relative youth, Noise is already used by large-scale deployed applications such as WhatsApp and Slack. Though the Noise specification describes and claims the security properties of the protocol patterns very precisely, there has been no computational proof yet. We close this gap.
Noise uses only a limited number of cryptographic primitives which makes it an ideal candidate for reduction-based security proofs. Due to its patterns’ characteristics as channel establishment protocols, and the usage of established keys within the handshake, the authenticated and confidential channel establishment (ACCE) model (Jager et al. CRYPTO 2012) seems to perfectly fit for an analysis of Noise. However, the ACCE model strictly divides protocols into two non-overlapping phases: the pre-accept phase (i.e., the channel establishment) and post-accept phase (i.e., the channel). In contrast, Noise allows the transmission of encrypted messages as soon as any key is established (for instance, before authentication between parties has taken place), and then incrementally increases the channel’s security guarantees. By proposing a generalization of the original ACCE model, we capture security properties of such staged channel establishment protocols flexibly – comparably to the multi-stage key exchange model (Fischlin and Günther CCS 2014).
We give security proofs for eight of the 15 basic Noise patterns in the full version (EPRINT 2019/436) and exemplify them by the proof of the XK pattern in this article.
Benjamin Dowling, Paul Rösler, Jörg Schwenk

Limits on the Efficiency of (Ring) LWE Based Non-interactive Key Exchange

\(\mathsf {LWE}\) based key-exchange protocols lie at the heart of post-quantum public-key cryptography. However, all existing protocols either lack the non-interactive nature of Diffie-Hellman key-exchange or polynomial \(\mathsf {LWE}\)-modulus, resulting in unwanted efficiency overhead.
We study the possibility of designing non-interactive \(\mathsf {LWE}\)-based protocols with polynomial \(\mathsf {LWE}\)-modulus. To this end,
  • We identify and formalize simple non-interactive and polynomial \(\mathsf {LWE}\)-modulus variants of existing protocols, where Alice and Bob simultaneously exchange one or more (ring) \(\mathsf {LWE}\) samples with polynomial \(\mathsf {LWE}\)-modulus and then run individual key reconciliation functions to obtain the shared key.
  • We point out central barriers and show that such non-interactive key-exchange protocols are impossible if:
    the reconciliation functions first compute the inner product of the received \(\mathsf {LWE}\) sample with their private \(\mathsf {LWE}\) secret. This impossibility is information theoretic.
    One of the reconciliation functions does not depend on the error of the transmitted \(\mathsf {LWE}\) sample. This impossibility assumes hardness of \(\mathsf {LWE}\).
  • We give further evidence that progress in either direction, of giving an \(\mathsf {LWE}\)-based \(\mathrm {NIKE}\) protocol or proving impossibility of one will lead to progress on some other well-studied questions in cryptography.
Overall, our results show possibilities and challenges in designing simple (ring) \(\mathsf {LWE}\)-based non-interactive key exchange protocols.
Siyao Guo, Pritish Kamath, Alon Rosen, Katerina Sotiraki

PAKEs: New Framework, New Techniques and More Efficient Lattice-Based Constructions in the Standard Model

Password-based authenticated key exchange (PAKE) allows two parties with a shared password to agree on a session key. In the last decade, the design of PAKE protocols from lattice assumptions has attracted lots of attention. However, existing solutions in the standard model do not have appealing efficiency. In this work, we first introduce a new PAKE framework. We then provide two realizations in the standard model, under the Learning With Errors (LWE) and Ring-LWE assumptions, respectively. Our protocols are much more efficient than previous proposals, thanks to three novel technical ingredients that may be of independent interests. The first ingredient consists of two approximate smooth projective hash (ASPH) functions from LWE, as well as two ASPHs from Ring-LWE. The latter are the first ring-based constructions in the literature, one of which only has a quasi-linear runtime while its function value contains \(\varTheta (n)\) field elements (where n is the degree of the polynomial defining the ring). The second ingredient is a new key conciliation scheme that is approximately rate-optimal and that leads to a very efficient key derivation for PAKE protocols. The third one is a new authentication code that allows to verify a MAC with a noisy key.
Shaoquan Jiang, Guang Gong, Jingnan He, Khoa Nguyen, Huaxiong Wang

Basic Primitives with Special Properties


Constraining and Watermarking PRFs from Milder Assumptions

Constrained pseudorandom functions (C-PRFs) let the possessor of a secret key delegate the ability to evaluate the function on certain authorized inputs, while keeping the remaining function values pseudorandom. A constraint-hiding constrained PRF (CHC-PRF) additionally conceals the predicate that determines which inputs are authorized. These primitives have a wealth of applications, including watermarking schemes, symmetric deniable encryption, and updatable garbled circuits.
Recent works have constructed (CH)C-PRFs from rather aggressive parameterizations of Learning With Errors (LWE) with subexponential modulus-noise ratios, even for relatively simple “puncturing” or \(\text {NC}^{1}\) circuit constraints. This corresponds to strong lattice assumptions and inefficient constructions, and stands in contrast to LWE-based unconstrained PRFs and fully homomorphic encryption schemes, which can be based on quasi-polynomial or even (nearly) polynomial modulus-noise ratios.
In this work we considerably improve the LWE assumptions needed for building (constraint-hiding) constrained PRFs and watermarking schemes. In particular, for CHC-PRFs and related watermarking schemes we improve the modulus-noise ratio to \(\lambda ^{O((d+\log \lambda ) \log \lambda )}\) for depth-d circuit constraints, which is merely quasi-polynomial for \(\text {NC}^{1}\) circuits and closely related watermarking schemes. For (constraint-revealing) C-PRFs for \(\text {NC}^{1}\) we do even better, obtaining a nearly polynomial \(\lambda ^{\omega (1)}\) ratio. These improvements are partly enabled by slightly modifying the definition of C-PRFs, in a way that is still compatible with many of their applications. Finally, as a contribution of independent interest we build CHC-PRFs for special constraint classes from generic, weaker assumptions: we obtain bit-fixing constraints based on the minimal assumption of one-way functions, and hyperplane-membership constraints based on key-homomorphic PRFs.
Chris Peikert, Sina Shiehian

Bringing Order to Chaos: The Case of Collision-Resistant Chameleon-Hashes

Chameleon-hash functions, introduced by Krawczyk and Rabin at NDSS 2000, are trapdoor collision-resistant hash-functions parametrized by a public key. If the corresponding secret key is known, arbitrary collisions for the hash function can be efficiently found. Chameleon-hash functions have prominent applications in the design of cryptographic primitives, such as lifting non-adaptively secure signatures to adaptively secure ones. Recently, this primitive also received a lot of attention as a building block in more complex cryptographic applications ranging from editable blockchains to advanced signature and encryption schemes.
We observe that in latter applications various different notions of collision-resistance are used, and it is not always clear if the respective notion does really cover what seems intuitively required by the application. Therefore, we revisit existing collision-resistance notions in the literature, study their relations, and—using the example of the recent redactable blockchain proposals—discuss which practical impact different notions of collision-resistance might have. Moreover, we provide a stronger, and arguably more desirable, notion of collision-resistance than what is known from the literature. Finally, we present a surprisingly simple and efficient black-box construction of chameleon-hash functions achieving this strong notion.
David Derler, Kai Samelin, Daniel Slamanig

Proofs and Arguments I


Concretely-Efficient Zero-Knowledge Arguments for Arithmetic Circuits and Their Application to Lattice-Based Cryptography

In this work we present a new interactive Zero-Knowledge Argument of knowledge for general arithmetic circuits. Our protocol is based on the “MPC-in-the-head”-paradigm of Ishai et al. (STOC 2009) and follows the recent “MPC-in-the-head with Preprocessing” as proposed by Katz, Kolesnikov and Wang (ACM CCS 2018). However, in contrast to Katz et al. who used the “cut-and-choose” approach for pre-processing, we show how to incorporate the well-known “sacrificing” paradigm into “MPC-in-the-head”, which reduces the proof size when working over arithmetic circuits. Our argument system uses only lightweight symmetric-key primitives and utilizes a simplified version of the so-called SPDZ-protocol.
Based on specific properties of our protocol we then show how it can be used to construct an efficient Zero-Knowledge Argument of Knowledge for instances of the Short Integer Solution (SIS) problem. We present different protocols that are tailored to specific uses of SIS, while utilizing the advantages of our scheme. In particular, we present a variant of our argument system that allows the parties to sample the circuit “on the fly”, which may be of independent interest.
We furthermore implemented our Zero-Knowledge argument for SIS and show that using our protocols it is possible to run a complete interactive proof, even for general SIS instances which result in a circuit with \({>}10^6\) gates, in less than 0.5 s.
Carsten Baum, Ariel Nof

Updateable Inner Product Argument with Logarithmic Verifier and Applications

We propose an improvement for the inner product argument of Bootle et al. (EUROCRYPT’16). The new argument replaces the unstructured common reference string (the commitment key) by a structured one. We give two instantiations of this argument, for two different distributions of the CRS. In the designated verifier setting, this structure can be used to reduce verification from linear to logarithmic in the circuit size. The argument can be compiled to the publicly verifiable setting in asymmetric bilinear groups. The new common reference string can easily be updateable. The argument can be directly used to improve verification of Bulletproofs range proofs (IEEE SP’18). On the other hand, to use the improved argument to prove circuit satisfiability with logarithmic verification, we adapt recent techniques from Sonic (ACM CCS’19) to work with the new common reference string. The resulting argument is secure under standard assumptions (in the Random Oracle Model), in contrast with Sonic and recent works that improve its efficiency (Plonk, Marlin, AuroraLight), which, apart from the Random Oracle Model, need either the Algebraic Group Model or Knowledge Type assumptions.
Vanesa Daza, Carla Ràfols, Alexandros Zacharakis

On Black-Box Extensions of Non-interactive Zero-Knowledge Arguments, and Signatures Directly from Simulation Soundness

Highly efficient non-interactive zero-knowledge arguments (NIZK) are often constructed for limited languages and it is not known how to extend them to cover wider classes of languages in general. In this work we initiate a study on black-box language extensions for conjunctive and disjunctive relations, that is, building a NIZK system for \({\mathcal L}\diamond \hat{{\mathcal L}}\) (with \(\diamond \in \{\wedge , \vee \}\)) based on NIZK systems for languages \({\mathcal L}\) and \(\hat{{\mathcal L}}\). While the conjunctive extension of NIZKs is straightforward by simply executing the given NIZKs in parallel, it is not known how disjunctive extensions could be achieved in a black-box manner. Besides, observe that the simple conjunctive extension does not work in the case of simulation-sound NIZKs (SS-NIZKs), as pointed out by Sahai (Sahai, FOCS 1999). Our main contribution is an impossibility result that negates the existence of the above extensions and implies other non-trivial separations among NIZKs, SS-NIZKs, and labelled SS-NIZKs.
Motivated by the difficulty of such transformations, we additionally present an efficient construction of signature schemes based on unbounded simulation-sound NIZKs (USS-NIZKs) for any language without language extensions.
Masayuki Abe, Miguel Ambrona, Miyako Ohkubo

On QA-NIZK in the BPK Model

Recently, Bellare et al. defined subversion-resistance (security in the case the CRS creator may be malicious) for NIZK. In particular, a Sub-ZK NIZK is zero-knowledge, even in the case of subverted CRS. We study Sub-ZK QA-NIZKs, where the CRS can depend on the language parameter. First, we observe that subversion zero-knowledge (Sub-ZK) in the CRS model corresponds to no-auxiliary-string non-black-box NIZK in the Bare Public Key model, and hence, the use of non-black-box techniques is needed to obtain Sub-ZK. Second, we give a precise definition of Sub-ZK QA-NIZKs that are (knowledge-)sound if the language parameter but not the CRS is subverted and zero-knowledge even if both are subverted. Third, we prove that the most efficient known QA-NIZK for linear subspaces by Kiltz and Wee is Sub-ZK under a new knowledge assumption that by itself is secure in (a weaker version of) the algebraic group model. Depending on the parameter setting, it is (knowledge-)sound under different non-falsifiable assumptions, some of which do not belong to the family of knowledge assumptions.
Behzad Abdolmaleki, Helger Lipmaa, Janno Siim, Michał Zając

Lattice-Based Cryptography


Improved Discrete Gaussian and Subgaussian Analysis for Lattice Cryptography

Discrete Gaussian distributions over lattices are central to lattice-based cryptography, and to the computational and mathematical aspects of lattices more broadly. The literature contains a wealth of useful theorems about the behavior of discrete Gaussians under convolutions and related operations. Yet despite their structural similarities, most of these theorems are formally incomparable, and their proofs tend to be monolithic and written nearly “from scratch,” making them unnecessarily hard to verify, understand, and extend.
In this work we present a modular framework for analyzing linear operations on discrete Gaussian distributions. The framework abstracts away the particulars of Gaussians, and usually reduces proofs to the choice of appropriate linear transformations and elementary linear algebra. To showcase the approach, we establish several general properties of discrete Gaussians, and show how to obtain all prior convolution theorems (along with some new ones) as straightforward corollaries. As another application, we describe a self-reduction for Learning With Errors (LWE) that uses a fixed number of samples to generate an unlimited number of additional ones (having somewhat larger error). The distinguishing features of our reduction are its simple analysis in our framework, and its exclusive use of discrete Gaussians without any loss in parameters relative to a prior mixed discrete-and-continuous approach.
As a contribution of independent interest, for subgaussian random matrices we prove a singular value concentration bound with explicitly stated constants, and we give tighter heuristics for specific distributions that are commonly used for generating lattice trapdoors. These bounds yield improvements in the concrete bit-security estimates for trapdoor lattice cryptosystems.
Nicholas Genise, Daniele Micciancio, Chris Peikert, Michael Walter

Almost Tight Security in Lattices with Polynomial Moduli – PRF, IBE, All-but-many LTF, and More

Achieving tight security is a fundamental task in cryptography. While one of the most important purposes of this task is to improve the overall efficiency of a construction (by allowing smaller security parameters), many current lattice-based instantiations do not completely achieve the goal. Particularly, a super-polynomial modulus seems to be necessary in all prior work for (almost) tight schemes that allow the adversary to conduct queries, such as PRF, IBE, and Signatures. As the super-polynomial modulus would affect the noise-to-modulus ratio and thus increase the parameters, this might cancel out the advantages (in efficiency) brought from the tighter analysis. To determine the full power of tight security/analysis in lattices, it is necessary to determine whether the super-polynomial modulus restriction is inherent.
In this work, we remove the super-polynomial modulus restriction for many important primitives – PRF, IBE, All-but-many Lossy Trapdoor Functions, and Signatures. The crux relies on an improvement over the framework of Boyen and Li (Asiacrypt 16), and an almost tight reduction from LWE to LWR, which improves prior work by Alwen et al. (Crypto 13), Bogdanov et al. (TCC 16), and Bai et al. (Asiacrypt 15). By combining these two advances, we are able to derive these almost tight schemes under LWE with a polynomial modulus.
Qiqi Lai, Feng-Hao Liu, Zhedong Wang


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