## CryptoDB

### Ryo Nishimaki

#### Publications

**Year**

**Venue**

**Title**

2022

PKC

The Direction of Updatable Encryption Does Matter
📺
Abstract

We introduce a new definition for key updates, called backward-leak uni-directional key updates, in updatable encryption (UE). This notion is a variant of uni-directional key updates for UE. We show that existing secure UE schemes in the bi-directional key updates setting are not secure in the backward-leak uni-directional key updates setting. Thus, security in the backward-leak uni-directional key updates setting is strictly stronger than security in the bi-directional key updates setting. This result is in sharp contrast to the equivalence theorem by Jiang (Asiacrypt 2020), which says security in the bi-directional key updates setting is equivalent to security in the existing uni-directional key updates setting. We call the existing uni-directional key updates ``forward-leak uni-directional'' key updates to distinguish two types of uni-directional key updates in this paper.
We also present two UE schemes with the following features.
- The first scheme is post-quantum secure in the backward-leak uni-directional key updates setting under the learning with errors assumption.
- The second scheme is secure in the no-directional key updates setting and based on indistinguishability obfuscation and one-way functions. This result solves the open problem left by Jiang (Asiacrypt 2020).

2022

PKC

KDM Security for the Fujisaki-Okamoto Transformations in the QROM
📺
Abstract

Key dependent message (KDM) security is a security notion that guarantees confidentiality of communication even if secret keys are encrypted.
KDM security has found a number of applications in practical situations such as hard-disk encryption systems, anonymous credentials, and bootstrapping of fully homomorphic encryptions. Recently, it also found an application in quantum delegation protocols as shown by Zhang (TCC 2019).
In this work, we investigate the KDM security of existing practical public-key encryption (PKE) schemes proposed in the quantum random oracle model (QROM).
Concretely, we study a PKE scheme whose KEM is constructed by using Fujisaki-Okamoto (FO) transformations in the QROM.
FO transformations are applied to an IND-CPA secure PKE schemes and yield IND-CCA secure key encapsulation mechanisms (KEM).
Then, we show the following results.
- We can reduce the KDM-CPA security in the QROM of a PKE scheme whose KEM is derived from any of the FO transformations proposed by Hofheinz et al. (TCC 2017) to the IND-CPA security of the underlying PKE scheme, without square root security loss.
For this result we use one-time-pad (OTP) as DEM to convert KEM into PKE.
- We can reduce the KDM-CCA security in the QROM of a PKE scheme whose KEM is derived from a single variant of the FO transformation proposed by Hofheinz et al. (TCC 2017) to the IND-CPA security of the underlying PKE scheme, without square root security loss. For this result, we use OTP-then-MAC construction as DEM to convert KEM into PKE. Also, we require a mild injectivity assumption for the underlying IND-CPA secure PKE scheme.
In order to avoid square root security loss, we use a double-sided one-way to hiding (O2H) lemma proposed by Kuchta et al. (EUROCRYPT 2020).
In the context of KDM security, there is a technical hurdle for using double-sided O2H lemma due to the circularity issue.
Our main technical contribution is to overcome the hurdle.

2022

EUROCRYPT

Watermarking PRFs against Quantum Adversaries
Abstract

We initiate the study of software watermarking against quantum adversaries.
A quantum adversary generates a quantum state as a pirate software that potentially removes an embedded message from a classical marked software.
Extracting an embedded message from quantum pirate software is difficult since measurement could irreversibly alter the quantum state.
In software watermarking against classical adversaries, a message extraction algorithm crucially uses the (input-output) behavior of a classical pirate software to extract an embedded message. Even if we instantiate existing watermarking PRFs with quantum-safe building blocks, it is not clear whether they are secure against quantum adversaries due to the quantum-specific property above.
Thus, we need entirely new techniques to achieve software watermarking against quantum adversaries.
In this work, we define secure watermarking PRFs for quantum adversaries (unremovability against quantum adversaries). We also present two watermarking PRFs as follows.
- We construct a privately extractable watermarking PRF against quantum adversaries from the quantum hardness of the learning with errors (LWE) problem. The marking and extraction algorithms use a public parameter and a private extraction key, respectively. The watermarking PRF is unremovable even if adversaries have (the public parameter and) access to the extraction oracle, which returns a result of extraction for a queried quantum circuit.
- We construct a publicly extractable watermarking PRF against quantum adversaries from indistinguishability obfuscation (IO) and the quantum hardness of the LWE problem. The marking and extraction algorithms use a public parameter and a public extraction key, respectively. The watermarking PRF is unremovable even if adversaries have the extraction key (and the public parameter).
We develop a quantum extraction technique to extract information (a classical string) from a quantum state without destroying the state too much.
We also introduce the notion of extraction-less watermarking PRFs as a crucial building block to achieve the results above by combining the tool with our quantum extraction technique.

2021

EUROCRYPT

Round-Optimal Blind Signatures in the Plain Model from Classical and Quantum Standard Assumptions
📺
Abstract

Blind signatures, introduced by Chaum (Crypto'82), allows a user to obtain a signature on a message without revealing the message itself to the signer. Thus far, all existing constructions of round-optimal blind signatures are known to require one of the following: a trusted setup, an interactive assumption, or complexity leveraging. This state-of-the-affair is somewhat justified by the few known impossibility results on constructions of round-optimal blind signatures in the plain model (i.e., without trusted setup) from standard assumptions. However, since all of these impossibility results only hold \emph{under some conditions}, fully (dis)proving the existence of such round-optimal blind signatures has remained open.
In this work, we provide an affirmative answer to this problem and construct the first round-optimal blind signature scheme in the plain model from standard polynomial-time assumptions. Our construction is based on various standard cryptographic primitives and also on new primitives that we introduce in this work, all of which are instantiable from __classical and post-quantum__ standard polynomial-time assumptions. The main building block of our scheme is a new primitive called a blind-signature-conforming zero-knowledge (ZK) argument system. The distinguishing feature is that the ZK property holds by using a quantum polynomial-time simulator against non-uniform classical polynomial-time adversaries.
Syntactically one can view this as a delayed-input three-move ZK argument with a reusable first message, and we believe it would be of independent interest.

2021

PKC

Universal Proxy Re-Encryption
📺
Abstract

We put forward the notion of universal proxy re-encryption (UPRE). A UPRE scheme enables a proxy to convert a ciphertext under a (delegator) public key of any existing public-key encryption (PKE) scheme into another ciphertext under a (delegatee) public key of any existing PKE scheme (possibly different from the delegator one). The proxy has a re-encryption key generated from the delegator's secret key and the delegatee public key. Thus UPRE generalizes proxy re-encryption by supporting arbitrary PKE schemes and allowing to convert ciphertexts into ones of possibly different PKE schemes. In this work, we
- provide syntax and definitions for both UPRE and a variant we call relaxed UPRE. The relaxed variant means that decryption algorithms for re-encrypted ciphertexts are slightly modified but still only use the original delegatee secret keys for decryption.
- construct a UPRE based on probabilistic indistinguishability obfuscation (PIO). It allows us to re-encrypt ciphertexts polynomially many times.
- construct relaxed UPRE from garbled circuits (GCs). We provide two variants of this construction, one which allows us to re-encrypt ciphertexts polynomially many times, and a second one which satisfies a stronger security requirement but only allows us to re-encrypt ciphertexts a constant number of times.

2021

ASIACRYPT

Quantum Encryption with Certified Deletion, Revisited: Public Key, Attribute-Based, and Classical Communication
📺
Abstract

Broadbent and Islam (TCC '20) proposed a quantum cryptographic primitive called quantum encryption with certified deletion.
In this primitive, a receiver in possession of a quantum ciphertext can generate a classical certificate that the encrypted message is deleted.
Although their construction is information-theoretically secure, it is limited to the setting of one-time symmetric key encryption (SKE), where a sender and receiver have to share a common key in advance and the key can be used only once. Moreover, the sender has to generate a quantum state and send it to the receiver over a quantum channel in their construction.
Deletion certificates are privately verifiable, which means a verification key for a certificate must be kept secret, in the definition by Broadbent and Islam. However, we can also consider public verifiability.
In this work, we present various constructions of encryption with certified deletion.
- Quantum communication case: We achieve (reusable-key) public key encryption (PKE) and attribute-based encryption (ABE) with certified deletion.
Our PKE scheme with certified deletion is constructed assuming the existence of IND-CPA secure PKE, and our ABE scheme with certified deletion is constructed assuming the existence of indistinguishability obfuscation and one-way function. These two schemes are privately verifiable.
- Classical communication case: We also achieve interactive encryption with certified deletion that uses only classical communication.
We give two schemes, a privately verifiable one and a publicly verifiable one. The former is constructed assuming the LWE assumption in the quantum random oracle model. The latter is constructed assuming the existence of one-shot signatures and extractable witness encryption.

2021

TCC

Secure Software Leasing from Standard Assumptions
📺
Abstract

Secure software leasing (SSL) is a quantum cryptographic primitive that enables an authority to lease software to a user by encoding it into a quantum state. SSL prevents users from generating authenticated pirated copies of leased software, where authenticated copies indicate those run on legitimate platforms. Although SSL is a relaxed variant of quantum copy protection that prevents users from generating any copy of leased softwares, it is still meaningful and attractive. Recently, Ananth and La Placa proposed the first SSL scheme. It satisfies a strong security notion called infinite-term security. On the other hand, it has a drawback that it is based on public key quantum money, which is not instantiated with standard cryptographic assumptions so far. Moreover, their scheme only supports a subclass of evasive functions.
In this work, we present SSL schemes that satisfy a security notion called finite-term security based on the learning with errors assumption (LWE). Finite-term security is weaker than infinite-term security, but it still provides a reasonable security guarantee. Specifically, our contributions consist of the following.
- We construct a finite-term secure SSL scheme for pseudorandom functions from the LWE assumption against quantum adversaries.
- We construct a finite-term secure SSL scheme for a subclass of evasive functions from the LWE assumption against sub-exponential quantum adversaries.
- We construct finite-term secure SSL schemes for the functionalities above with classical communication from the LWE assumption against (sub-exponential) quantum adversaries.
SSL with classical communication means that entities exchange only classical information though they run quantum computation locally.
Our crucial tool is two-tier quantum lightning, which is introduced in this work and a relaxed version of quantum lighting. In two-tier quantum lightning schemes, we have a public verification algorithm called semi-verification and a private verification algorithm called full-verification. An adversary cannot generate possibly entangled two quantum states whose serial numbers are the same such that one passes the semi-verification, and the other also passes the full-verification. We show that we can construct a two-tier quantum lightning scheme from the LWE assumption.

2021

JOFC

Compact Designated Verifier NIZKs from the CDH Assumption Without Pairings
Abstract

In a non-interactive zero-knowledge (NIZK) proof, a prover can non-interactively convince a verifier of a statement without revealing any additional information. A useful relaxation of NIZK is a designated verifier NIZK (DV-NIZK) proof, where proofs are verifiable only by a designated party in possession of a verification key. A crucial security requirement of DV-NIZKs is unbounded-soundness, which guarantees soundness even if the verification key is reused for multiple statements. Most known DV-NIZKs (except standard NIZKs) for $$\mathbf{NP} $$ NP do not have unbounded-soundness. Existing DV-NIZKs for $$\mathbf{NP} $$ NP satisfying unbounded-soundness are based on assumptions which are already known to imply standard NIZKs. In particular, it is an open problem to construct (DV-)NIZKs from weak paring-free group assumptions such as decisional Diffie–Hellman (DH). As a further matter, all constructions of (DV-)NIZKs from DH type assumptions (regardless of whether it is over a paring-free or paring group) require the proof size to have a multiplicative-overhead $$|C| \cdot \mathsf {poly}(\kappa )$$ | C | · poly ( κ ) , where | C | is the size of the circuit that computes the $$\mathbf{NP} $$ NP relation. In this work, we make progress of constructing DV-NIZKs from DH-type assumptions that are not known to imply standard NIZKs. Our results are summarized as follows: DV-NIZKs for $$\mathbf{NP} $$ NP from the computational DH assumption over pairing-free groups. This is the first construction of such NIZKs on pairing-free groups and resolves the open problem posed by Kim and Wu (CRYPTO’18). DV-NIZKs for $$\mathbf{NP} $$ NP with proof size $$|C|+\mathsf {poly}(\kappa )$$ | C | + poly ( κ ) from the computational DH assumption over specific pairing-free groups. This is the first DV-NIZK that achieves a compact proof from a standard DH type assumption. Moreover, if we further assume the $$\mathbf{NP} $$ NP relation to be computable in $$\mathbf{NC} ^1$$ NC 1 and assume hardness of a (non-static) falsifiable DH type assumption over specific pairing-free groups, the proof size can be made as small as $$|w| + \mathsf {poly}(\kappa )$$ | w | + poly ( κ ) .

2021

JOFC

Simple and Generic Constructions of Succinct Functional Encryption
Abstract

We propose simple generic constructions of succinct functional encryption. Our key tool is strong exponentially efficient indistinguishability obfuscator (SXIO), which is the same as indistinguishability obfuscator (IO) except that the size of an obfuscated circuit and the running time of an obfuscator are slightly smaller than that of a brute-force canonicalizer that outputs the entire truth table of a circuit to be obfuscated. A “compression factor” of SXIO indicates how much SXIO compresses the brute-force canonicalizer. In this study, we propose a significantly simple framework to construct succinct functional encryption via SXIO and show that SXIO is powerful enough to achieve cutting-edge cryptography. In particular, we propose the following constructions: Single-key weakly succinct secret-key functional encryption (SKFE) is constructed from SXIO (even with a bad compression factor) and one-way functions. Single-key weakly succinct public-key functional encryption (PKFE) is constructed from SXIO with a good compression factor and public-key encryption. Single-key weakly succinct PKFE is constructed from SXIO (even with a bad compression factor) and identity-based encryption. Our new framework has side benefits. Our constructions do not rely on any number theoretic or lattice assumptions such as decisional Diffie–Hellman and learning with errors assumptions. Moreover, all security reductions incur only polynomial security loss. Known constructions of weakly succinct SKFE or PKFE from SXIO with polynomial security loss rely on number theoretic or lattice assumptions. As corollaries of our results, relationships among SXIO, a few variants of SKFE, and a variant of randomized encoding are discovered.

2020

EUROCRYPT

Compact NIZKs from Standard Assumptions on Bilinear Maps
📺
Abstract

A non-interactive zero-knowledge (NIZK) protocol enables a prover to convince a verifier of the truth of a statement without leaking any other information by sending a single message. The main focus of this work is on exploring short pairing-based NIZKs for all NP languages based on standard assumptions. In this regime, the seminal work of Groth, Ostrovsky, and Sahai (J.ACM'12) (GOS-NIZK) is still considered to be the state-of-the-art. Although fairly efficient, one drawback of GOS-NIZK is that the proof size is multiplicative in the circuit size computing the NP relation. That is, the proof size grows by $O(|C|k)$, where $C$ is the circuit for the NP relation and $k$ is the security parameter.
By now, there have been numerous follow-up works focusing on shortening the proof size of pairing-based NIZKs, however, thus far, all works come at the cost of relying either on a non-standard knowledge-type assumption or a non-static $q$-type assumption. Specifically, improving the proof size of the original GOS-NIZK under the same standard assumption has remained as an open problem.
Our main result is a construction of a pairing-based NIZK for all of NP whose proof size is additive in $|C|$, that is, the proof size only grows by $|C| +poly(k)$, based on the decisional linear (DLIN) assumption. Since the DLIN assumption is the same assumption underlying GOS-NIZK, our NIZK is a strict improvement on their proof size.
As by-products of our main result, we also obtain the following two results: (1) We construct a perfectly zero-knowledge NIZK (NIPZK) for NP relations computable in NC1 with proof size $|w|poly(k)$ where $|w|$ is the witness length based on the DLIN assumption. This is the first pairing-based NIPZK for a non-trivial class of NP languages whose proof size is independent of $|C|$ based on a standard assumption. (2) We construct a universally composable (UC) NIZK for NP relations computable in NC1 in the erasure-free adaptive setting whose proof size is $|w|poly(k)$ from the DLIN assumption. This is an improvement over the recent result of Katsumata, Nishimaki, Yamada, and Yamakawa (CRYPTO'19), which gave a similar scheme based on a non-static $q$-type assumption.
The main building block for all of our NIZKs is a constrained signature scheme with decomposable online-offline efficiency. This is a property which we newly introduce in this paper and construct from the DLIN assumption. We believe this construction is of an independent interest.

2020

PKC

Fast, Compact, and Expressive Attribute-Based Encryption
📺
Abstract

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.

2020

CRYPTO

Adaptively Secure Constrained Pseudorandom Functions in the Standard Model
📺
Abstract

Constrained pseudorandom functions (CPRFs) allow learning "constrained" PRF keys that can evaluate the PRF on a subset of the input space, or based on some predicate.
First introduced by Boneh and Waters [AC’13], Kiayias et al. [CCS’13] and Boyle et al. [PKC’14], they have shown to be a useful cryptographic primitive with many applications.
These applications often require CPRFs to be adaptively secure, which allows the adversary to learn PRF values and constrained keys in an arbitrary order.
However, there is no known construction of adaptively secure CPRFs based on a standard assumption in the standard model for any non-trivial class of predicates.
Moreover, even if we rely on strong tools such as indistinguishability obfuscation (IO), the state-of-the-art construction of adaptively secure CPRFs in the standard model only supports the limited class of NC1 predicates.
In this work, we develop new adaptively secure CPRFs for various predicates from different types of assumptions in the standard model. Our results are summarized below.
- We construct adaptively secure and O(1)-collusion-resistant CPRFs for t-conjunctive normal form (t-CNF) predicates from one-way functions (OWFs) where t is a constant. Here, O(1)-collusion-resistance means that we can allow the adversary to obtain a constant number of constrained keys. Note that t-CNF includes bit-fixing predicates as a special case.
- We construct adaptively secure and single-key CPRFs for inner-product predicates from the learning with errors (LWE) assumption. Here, single-key means that we only allow the adversary to learn one constrained key. Note that inner-product predicates include t-CNF predicates for a constant t as a special case. Thus, this construction supports a more expressive class of predicates than that supported by the first construction though it loses the collusion-resistance and relies on a stronger assumption.
- We construct adaptively secure and O(1)-collusion-resistant CPRFs for all circuits from the LWE assumption and indistinguishability obfuscation (IO).
The first and second constructions are the first CPRFs for any non-trivial predicates to achieve adaptive security outside of the random oracle model or relying on strong cryptographic assumptions. Moreover, the first construction is also the first to achieve any notion of collusion-resistance in this setting. Besides, we prove that the first and second constructions satisfy weak 1-key privacy, which roughly means that a constrained key does not reveal the corresponding constraint. The third construction is an improvement over previous adaptively secure CPRFs for less expressive predicates based on IO in the standard model.

2020

TCC

Equipping Public-Key Cryptographic Primitives with Watermarking (or: A Hole Is to Watermark)
📺
Abstract

Program watermarking enables users to embed an arbitrary string called a mark into a program while preserving the functionality of the program. Adversaries cannot remove the mark without destroying the functionality. Although there exist generic constructions of watermarking schemes for public-key cryptographic (PKC) primitives, those schemes are constructed from scratch and not efficient.
In this work, we present a general framework to equip a broad class of PKC primitives with an efficient watermarking scheme. The class consists of PKC primitives that have a canonical all-but-one (ABO) reduction. Canonical ABO reductions are standard techniques to prove selective security of PKC primitives, where adversaries must commit a target attribute at the beginning of the security game. Thus, we can obtain watermarking schemes for many existing efficient PKC schemes from standard cryptographic assumptions via our framework. Most well-known selectively secure PKC schemes have canonical ABO reductions. Notably, we can achieve watermarking for public-key encryption whose ciphertexts and secret-keys are constant-size, and that is chosen-ciphertext secure.
Our approach accommodates the canonical ABO reduction technique to the puncturable pseudorandom function (PRF) technique, which is used to achieve watermarkable PRFs. We find that canonical ABO reductions are compatible with such puncturable PRF-based watermarking schemes.

2020

ASIACRYPT

Adaptively Secure Inner Product Encryption from LWE
📺
Abstract

Attribute-based encryption (ABE) is an advanced form of encryption scheme allowing for access policies to be embedded within the secret keys and ciphertexts. By now, we have ABEs supporting numerous types of policies based on hardness assumptions over bilinear maps and lattices. However, one of the distinguishing differences between ABEs based on these two breeds of assumptions is that the former can achieve adaptive security for quite expressible policies (e.g., inner-products, boolean formula) while the latter can not. Recently, two adaptively secure lattice-based ABEs have appeared and changed the state of affairs: a non-zero inner-product (NIPE) encryption by Katsumata and Yamada (PKC'19) and an ABE for t-CNF policies by Tsabary (CRYPTO'19). However, the policies supported by these ABEs are still quite limited and do not embrace the more interesting policies that have been studied in the literature. Notably, constructing an adaptively secure inner-product encryption (IPE) based on lattices still remains open.
In this work, we propose the first adaptively secure IPE based on the learning with errors (LWE) assumption with sub-exponential modulus size (without resorting to complexity leveraging). Concretely, our IPE supports inner-products over the integers Z with polynomial sized entries and
satisfies adaptively weakly-attribute-hiding security.
We also show how to convert such an IPE to an IPE supporting inner-products over Z_p for a polynomial-sized p and a fuzzy identity-based encryption (FIBE) for small and large universes. Our result builds on the ideas presented in Tsabary (CRYPTO'19), which uses constrained pseudorandom functions (CPRF) in a semi-generic way to achieve adaptively secure ABEs, and the recent lattice-based adaptively secure CPRF for inner-products by Davidson et al. (CRYPTO'20). Our main observation is realizing how to weaken the conforming CPRF property introduced in Tsabary (CRYPTO'19) by taking advantage of the specific linearity property enjoyed by the lattice evaluation algorithms by Boneh et al. (EUROCRYPT'14).

2019

PKC

Leakage-Resilient Identity-Based Encryption in Bounded Retrieval Model with Nearly Optimal Leakage-Ratio
Abstract

We propose new constructions of leakage-resilient public-key encryption (PKE) and identity-based encryption (IBE) schemes in the bounded retrieval model (BRM). In the BRM, adversaries are allowed to obtain at most
$$\ell $$
-bit leakage from a secret key and we can increase
$$\ell $$
only by increasing the size of secret keys without losing efficiency in any other performance measure. We call
$$\ell /|\mathsf {sk}|$$
leakage-ratio where
$$|\mathsf {sk}|$$
denotes a bit-length of a secret key. Several PKE/IBE schemes in the BRM are known. However, none of these constructions achieve a constant leakage-ratio under a standard assumption in the standard model. Our PKE/IBE schemes are the first schemes in the BRM that achieve leakage-ratio
$$1-\epsilon $$
for any constant
$$\epsilon >0$$
under standard assumptions in the standard model.As previous works, we use identity-based hash proof systems (IB-HPS) to construct IBE schemes in the BRM. It is known that a parameter for IB-HPS called the universality-ratio is translated into the leakage-ratio of the resulting IBE scheme in the BRM. We construct an IB-HPS with universality-ratio
$$1-\epsilon $$
for any constant
$$\epsilon >0$$
based on any inner-product predicate encryption (IPE) scheme with compact secret keys. Such IPE schemes exist under the d-linear, subgroup decision, learning with errors, or computational bilinear Diffie-Hellman assumptions. As a result, we obtain IBE schemes in the BRM with leakage-ratio
$$1-\epsilon $$
under any of these assumptions. Our PKE schemes are immediately obtained from our IBE schemes.

2019

PKC

Adaptively Single-Key Secure Constrained PRFs for $\mathrm {NC}^1$
Abstract

We present a construction of an adaptively single-key secure constrained PRF (CPRF) for $$\mathbf {NC}^1$$ assuming the existence of indistinguishability obfuscation (IO) and the subgroup hiding assumption over a (pairing-free) composite order group. This is the first construction of such a CPRF in the standard model without relying on a complexity leveraging argument.To achieve this, we first introduce the notion of partitionable CPRF, which is a CPRF accommodated with partitioning techniques and combine it with shadow copy techniques often used in the dual system encryption methodology. We present a construction of partitionable CPRF for $$\mathbf {NC}^1$$ based on IO and the subgroup hiding assumption over a (pairing-free) group. We finally prove that an adaptively single-key secure CPRF for $$\mathbf {NC}^1$$ can be obtained from a partitionable CPRF for $$\mathbf {NC}^1$$ and IO.

2019

EUROCRYPT

Designated Verifier/Prover and Preprocessing NIZKs from Diffie-Hellman Assumptions
📺
Abstract

In a non-interactive zero-knowledge (NIZK) proof, a prover can non-interactively convince a verifier of a statement without revealing any additional information. Thus far, numerous constructions of NIZKs have been provided in the common reference string (CRS) model (CRS-NIZK) from various assumptions, however, it still remains a long standing open problem to construct them from tools such as pairing-free groups or lattices. Recently, Kim and Wu (CRYPTO’18) made great progress regarding this problem and constructed the first lattice-based NIZK in a relaxed model called NIZKs in the preprocessing model (PP-NIZKs). In this model, there is a trusted statement-independent preprocessing phase where secret information are generated for the prover and verifier. Depending on whether those secret information can be made public, PP-NIZK captures CRS-NIZK, designated-verifier NIZK (DV-NIZK), and designated-prover NIZK (DP-NIZK) as special cases. It was left as an open problem by Kim and Wu whether we can construct such NIZKs from weak paring-free group assumptions such as DDH. As a further matter, all constructions of NIZKs from Diffie-Hellman (DH) type assumptions (regardless of whether it is over a paring-free or paring group) require the proof size to have a multiplicative-overhead $$|C| \cdot \mathsf {poly}(\kappa )$$|C|·poly(κ), where |C| is the size of the circuit that computes the $$\mathbf {NP}$$NP relation.In this work, we make progress of constructing (DV, DP, PP)-NIZKs with varying flavors from DH-type assumptions. Our results are summarized as follows:DV-NIZKs for $$\mathbf {NP}$$NP from the CDH assumption over pairing-free groups. This is the first construction of such NIZKs on pairing-free groups and resolves the open problem posed by Kim and Wu (CRYPTO’18).DP-NIZKs for $$\mathbf {NP}$$NP with short proof size from a DH-type assumption over pairing groups. Here, the proof size has an additive-overhead $$|C|+\mathsf {poly}(\kappa )$$|C|+poly(κ) rather then an multiplicative-overhead $$|C| \cdot \mathsf {poly}(\kappa )$$|C|·poly(κ). This is the first construction of such NIZKs (including CRS-NIZKs) that does not rely on the LWE assumption, fully-homomorphic encryption, indistinguishability obfuscation, or non-falsifiable assumptions.PP-NIZK for $$\mathbf {NP}$$NP with short proof size from the DDH assumption over pairing-free groups. This is the first PP-NIZK that achieves a short proof size from a weak and static DH-type assumption such as DDH. Similarly to the above DP-NIZK, the proof size is $$|C|+\mathsf {poly}(\kappa )$$|C|+poly(κ). This too serves as a solution to the open problem posed by Kim and Wu (CRYPTO’18).
Along the way, we construct two new homomorphic authentication (HomAuth) schemes which may be of independent interest.

2019

CRYPTO

Adaptively Secure and Succinct Functional Encryption: Improving Security and Efficiency, Simultaneously
Abstract

Functional encryption (FE) is advanced encryption that enables us to issue functional decryption keys where functions are hardwired. When we decrypt a ciphertext of a message m by a functional decryption key where a function f is hardwired, we can obtain f(m) and nothing else. We say FE is selectively or adaptively secure when target messages are chosen at the beginning or after function queries are sent, respectively. In the weakly-selective setting, function queries are also chosen at the beginning. We say FE is single-key/collusion-resistant when it is secure against adversaries that are given only-one/polynomially-many functional decryption keys, respectively. We say FE is sublinearly-succinct/succinct when the running time of an encryption algorithm is sublinear/poly-logarithmic in the function description size, respectively.In this study, we propose a generic transformation from weakly-selectively secure, single-key, and sublinearly-succinct (we call “building block”) PKFE for circuits into adaptively secure, collusion-resistant, and succinct (we call “fully-equipped”) one for circuits. Our transformation relies on neither concrete assumptions such as learning with errors nor indistinguishability obfuscation (IO). This is the first generic construction of fully-equipped PKFE that does not rely on IO.As side-benefits of our results, we obtain the following primitives from the building block PKFE for circuits: (1) laconic oblivious transfer (2) succinct garbling scheme for Turing machines (3) selectively secure, collusion-resistant, and succinct PKFE for Turing machines (4) low-overhead adaptively secure traitor tracing (5) key-dependent message secure and leakage-resilient public-key encryption. We also obtain a generic transformation from simulation-based adaptively secure garbling schemes that satisfy a natural decomposability property into adaptively indistinguishable garbling schemes whose online complexity does not depend on the output length.

2019

CRYPTO

Exploring Constructions of Compact NIZKs from Various Assumptions
📺
Abstract

A non-interactive zero-knowledge (NIZK) protocol allows a prover to non-interactively convince a verifier of the truth of the statement without leaking any other information. In this study, we explore shorter NIZK proofs for all $$\mathbf{NP }$$ languages. Our primary interest is NIZK proofs from falsifiable pairing/pairing-free group-based assumptions. Thus far, NIZKs in the common reference string model (CRS-NIZKs) for $$\mathbf{NP }$$ based on falsifiable pairing-based assumptions all require a proof size at least as large as $$O(|C| \kappa )$$, where C is a circuit computing the $$\mathbf{NP }$$ relation and $$\kappa $$ is the security parameter. This holds true even for the weaker designated-verifier NIZKs (DV-NIZKs). Notably, constructing a (CRS, DV)-NIZK with proof size achieving an additive-overhead $$O(|C|) + \mathsf {poly}(\kappa )$$, rather than a multiplicative-overhead $$|C| \cdot \mathsf {poly}(\kappa )$$, based on any falsifiable pairing-based assumptions is an open problem.In this work, we present various techniques for constructing NIZKs with compact proofs, i.e., proofs smaller than $$O(|C|) + \mathsf {poly}(\kappa )$$, and make progress regarding the above situation. Our result is summarized below.
We construct CRS-NIZK for all $$\mathbf{NP }$$ with proof size $$|C| +\mathsf {poly}(\kappa )$$ from a (non-static) falsifiable Diffie-Hellman (DH) type assumption over pairing groups. This is the first CRS-NIZK to achieve a compact proof without relying on either lattice-based assumptions or non-falsifiable assumptions. Moreover, a variant of our CRS-NIZK satisfies universal composability (UC) in the erasure-free adaptive setting. Although it is limited to $$\mathbf{NP }$$ relations in $$\mathbf{NC }^1$$, the proof size is $$|w| \cdot \mathsf {poly}(\kappa )$$ where w is the witness, and in particular, it matches the state-of-the-art UC-NIZK proposed by Cohen, shelat, and Wichs (CRYPTO’19) based on lattices.We construct (multi-theorem) DV-NIZKs for $$\mathbf{NP }$$ with proof size $$|C|+\mathsf {poly}(\kappa )$$ from the computational DH assumption over pairing-free groups. This is the first DV-NIZK that achieves a compact proof from a standard DH type assumption. Moreover, if we further assume the $$\mathbf{NP }$$ relation to be computable in $$\mathbf{NC }^1$$ and assume hardness of a (non-static) falsifiable DH type assumption over pairing-free groups, the proof size can be made as small as $$|w| + \mathsf {poly}(\kappa )$$.
Another related but independent issue is that all (CRS, DV)-NIZKs require the running time of the prover to be at least $$|C|\cdot \mathsf {poly}(\kappa )$$. Considering that there exists NIZKs with efficient verifiers whose running time is strictly smaller than |C|, it is an interesting problem whether we can construct prover-efficient NIZKs. To this end, we construct prover-efficient CRS-NIZKs for $$\mathbf{NP }$$ with compact proof through a generic construction using laconic functional evaluation schemes (Quach, Wee, and Wichs (FOCS’18)). This is the first NIZK in any model where the running time of the prover is strictly smaller than the time it takes to compute the circuit C computing the $$\mathbf{NP }$$ relation.Finally, perhaps of an independent interest, we formalize the notion of homomorphic equivocal commitments, which we use as building blocks to obtain the first result, and show how to construct them from pairing-based assumptions.

2019

JOFC

From Cryptomania to Obfustopia Through Secret-Key Functional Encryption
Abstract

Functional encryption lies at the frontiers of the current research in cryptography; some variants have been shown sufficiently powerful to yield indistinguishability obfuscation (IO), while other variants have been constructed from standard assumptions such as LWE. Indeed, most variants have been classified as belonging to either the former or the latter category. However, one mystery that has remained is the case of secret-key functional encryption with an unbounded number of keys and ciphertexts. On the one hand, this primitive is not known to imply anything outside of minicrypt, the land of secret-key cryptography, but, on the other hand, we do no know how to construct it without the heavy hammers in obfustopia. In this work, we show that (subexponentially secure) secret-key functional encryption is powerful enough to construct indistinguishability obfuscation if we additionally assume the existence of (subexponentially secure) plain public-key encryption. In other words, secret-key functional encryption provides a bridge from cryptomania to obfustopia. On the technical side, our result relies on two main components. As our first contribution, we show how to use secret-key functional encryption to get “exponentially efficient indistinguishability obfuscation” (XIO), a notion recently introduced by Lin et al. (PKC’16) as a relaxation of IO. Lin et al. show how to use XIO and the LWE assumption to build IO. As our second contribution, we improve on this result by replacing its reliance on the LWE assumption with any plain public-key encryption scheme. Lastly, we ask whether secret-key functional encryption can be used to construct public-key encryption itself and therefore take us all the way from minicrypt to obfustopia. A result of Asharov and Segev (FOCS’15) shows that this is not the case under black-box constructions, even for exponentially secure functional encryption. We show, through a non-black-box construction, that subexponentially secure-key functional encryption indeed leads to public-key encryption. The resulting public-key encryption scheme, however, is at most quasi-polynomially secure, which is insufficient to take us to obfustopia.

2018

CRYPTO

Constrained PRFs for $\mathrm{NC}^1$ in Traditional Groups
📺
Abstract

We propose new constrained pseudorandom functions (CPRFs) in traditional groups. Traditional groups mean cyclic and multiplicative groups of prime order that were widely used in the 1980s and 1990s (sometimes called “pairing free” groups). Our main constructions are as follows.
We propose a selectively single-key secure CPRF for circuits with depth$$O(\log n)$$(that is,NC$$^1$$circuits) in traditional groups where n is the input size. It is secure under the L-decisional Diffie-Hellman inversion (L-DDHI) assumption in the group of quadratic residues $$\mathbb {QR}_q$$ and the decisional Diffie-Hellman (DDH) assumption in a traditional group of order qin the standard model.We propose a selectively single-key private bit-fixing CPRF in traditional groups. It is secure under the DDH assumption in any prime-order cyclic group in the standard model.We propose adaptively single-key secure CPRF for NC$$^1$$ and private bit-fixing CPRF in the random oracle model.
To achieve the security in the standard model, we develop a new technique using correlated-input secure hash functions.

2018

PKC

Simple and Generic Constructions of Succinct Functional Encryption
Abstract

We propose simple and generic constructions of succinct functional encryption. Our key tool is exponentially-efficient indistinguishability obfuscator (XIO), which is the same as indistinguishability obfuscator (IO) except that the size of an obfuscated circuit (or the running-time of an obfuscator) is slightly smaller than that of a brute-force canonicalizer that outputs the entire truth table of a circuit to be obfuscated. A “compression factor” of XIO indicates how much XIO compresses the brute-force canonicalizer. In this study, we propose a significantly simple framework to construct succinct functional encryption via XIO and show that XIO is a powerful enough to achieve cutting-edge cryptography. In particular, we prove the followings:Single-key weakly succinct secret-key functional encryption (SKFE) is constructed from XIO (even with a bad compression factor) and one-way function.Single-key weakly succinct public-key functional encryption (PKFE) is constructed from XIO with a good compression factor and public-key encryption.Single-key weakly succinct PKFE is constructed from XIO (even with a bad compression factor) and identity-based encryption.
Our new framework has side benefits. Our constructions do not rely on any number theoretic or lattice assumptions such as decisional Diffie-Hellman and learning with errors assumptions. Moreover, all security reductions incur only polynomial security loss. Known constructions of weakly succinct SKFE or PKFE from XIO with polynomial security loss rely on number theoretic or lattice assumptions.

2016

JOFC

2012

ASIACRYPT

#### Program Committees

- Crypto 2022
- Crypto 2021
- TCC 2021
- Eurocrypt 2020
- PKC 2020
- Crypto 2019
- TCC 2019
- PKC 2018

#### Coauthors

- Masayuki Abe (4)
- Nuttapong Attrapadung (2)
- Nir Bitansky (2)
- Nishanth Chandran (1)
- Melissa Chase (3)
- Bernardo David (3)
- Alex Davidson (1)
- Nico Döttling (1)
- Taiga Hiroka (1)
- Dennis Hofheinz (1)
- Shuichi Katsumata (7)
- Yuto Kawahara (1)
- Fuyuki Kitagawa (7)
- Markulf Kohlweiss (3)
- Feng-Hao Liu (1)
- Toshihide Matsuda (1)
- Takahiro Matsuda (2)
- Tomoyuki Morimae (1)
- Miyako Ohkubo (4)
- Jiaxin Pan (1)
- Alain Passelègue (2)
- Keisuke Tanaka (5)
- Junichi Tomida (1)
- Daniel Wichs (3)
- Keita Xagawa (2)
- Shota Yamada (9)
- Takashi Yamakawa (13)
- Mark Zhandry (1)