## CryptoDB

### Shota Yamada

#### Publications

**Year**

**Venue**

**Title**

2024

CRYPTO

Attribute Based Encryption for Turing Machines from Lattices
Abstract

We provide the first attribute based encryption (ABE) scheme for Turing machines supporting unbounded collusions from lattice assumptions. In more detail, the encryptor encodes an attribute x together with a bound t on the machine running time and a message m into the ciphertext, the key generator embeds a Turing machine M into the secret key and decryption returns m if and only if M (x) = 1. Crucially, the input x and machine M can be of unbounded size, the time bound t can be chosen dynamically for each input and decryption runs in input specific time.
Previously the best known ABE for uniform computation supported only non-deterministic log space Turing machines (NL from pairings (Lin and Luo, Eurocrypt 2020). In the post-quantum regime, the state of the art supports non-deterministic finite automata from LWE in the symmetric key setting (Agrawal, Maitra and Yamada, Crypto 2019).
In more detail, our results are:
1. We construct the first ABE for NL from the LWE and evasive LWE assumptions. This yields the first (conjectured) post-quantum ABE for NL.
2. Relying on new and arguably natural assumptions which we call path LWE, evasive path LWE and circular tensor LWE, in addition to standard LWE, we construct ABE for all Turing machines.
Towards our ABE for Turing machines, we obtain the first CP-ABE for circuits of unbounded depth and size from the same assumptions – this may be of independent interest. At a high level, our path LWE assumption states that the joint distribution of specially constructed FHE and ABE encodings are pseudorandom. The evasive path LWE assumption incorporates path LWE into the celebrated evasive LWE assumption (Wee, Eurocrypt 2022 and Tsabary, Crypto 2022), while the circular tensor LWE assumption incorporates circularity into the tensor LWE (Wee, Eurocrypt 2022) assumption. We believe these assumptions provide an important new tool for encrypted computation and are likely to find other applications.

2023

EUROCRYPT

Public Key Encryption with Secure Key Leasing
Abstract

We introduce the notion of public key encryption with secure key leasing (PKE-SKL). Our notion supports the leasing of decryption keys so that a leased key achieves the decryption functionality but comes with the guarantee that if the quantum decryption key returned by a user passes a validity test, then the user has lost the ability to decrypt. Our notion is similar in spirit to the notion of secure software leasing (SSL) introduced by Ananth and La Placa (Eurocrypt 2021) but captures significantly more general adversarial strategies. In more detail, our adversary is not restricted to use an honest evaluation algorithm to run pirated software. Our results can be summarized as follows:
1. Definitions: We introduce the definition of PKE with secure key leasing and formalize a security notion that we call indistinguishability against key leasing attacks (IND-KLA security). We also define a one-wayness notion for PKE-SKL that we call OW-KLA security and show that an OW-KLA secure PKE-SKL scheme can be lifted to an IND-KLA secure one by using the (quantum) Goldreich-Levin lemma.
2. Constructing IND-KLA PKE with Secure Key Leasing: We provide a construction of OW-KLA secure PKE-SKL (which implies IND-KLA secure PKE-SKL as discussed above) by leveraging a PKE scheme that satisfies a new security notion that we call consistent or inconsistent security against key leasing attacks (CoIC-KLA security). We then construct a CoIC-KLA secure PKE scheme using 1-key Ciphertext-Policy Functional Encryption (CPFE) that in turn can be based on any IND-CPA secure PKE scheme.
3. Identity Based Encryption, Attribute Based Encryption and Functional Encryption with Secure Key Leasing: We provide definitions of secure key leasing in the context of advanced encryption schemes such as identity based encryption (IBE), attribute-based encryption (ABE) and functional encryption (FE). Then we provide constructions by combining the above PKE-SKL with standard IBE, ABE and FE schemes.
Notably, our definitions allow the adversary to request distinguishing keys in the security game, namely, keys that distinguish the challenge bit by simply decrypting the challenge ciphertext, so long as it returns them (and they pass the validity test) before it sees the challenge ciphertext. All our constructions satisfy this stronger definition, albeit with the restriction that only a bounded number of such keys be allowed to the adversary in the IBE and ABE (but not FE) security games.
Prior to our work, the notion of single decryptor encryption (SDE) has been studied in the context of PKE (Georgiou and Zhandry, Eprint 2020) and FE (Kitigawa and Nishimaki, Asiacrypt 2022) but all their constructions rely on strong assumptions including indistinguishability obfuscation. In contrast, our constructions do not require any additional assumptions, showing that PKE/IBE/ABE/FE can be upgraded to support secure key leasing for free.

2023

EUROCRYPT

Broadcast, Trace and Revoke with Optimal Parameters from Polynomial Hardness
Abstract

A {\it broadcast, trace and revoke} system generalizes broadcast encryption as well as traitor tracing. In such a scheme, an encryptor can specify a list $L \subseteq N$ of revoked users so that (i) users in $L$ can no longer decrypt ciphertexts, (ii) ciphertext size is independent of $L$, (iii) a pirate decryption box supports tracing of compromised users. The ``holy grail'' of this line of work is a construction which resists unbounded collusions, achieves all parameters (including public and secret key) sizes independent of $|L|$ and $|N|$, and is based on polynomial hardness assumptions. In this work we make the following contributions:
1. {\it Public Trace Setting:} We provide a construction which (i) achieves optimal parameters, (ii) supports embedding identities (from an exponential space) in user secret keys, (iii) relies on polynomial hardness assumptions, namely compact functional encryption (${\sf FE}$) and a key-policy attribute based encryption (${\sf ABE}$) with special efficiency properties, and (iv) enjoys adaptive security with respect to the revocation list. The previous best known construction by Nishimaki, Wichs and Zhandry (Eurocrypt 2016) which achieved optimal parameters and embedded identities, relied on indistinguishability obfuscation, which is considered an inherently subexponential assumption and achieved only selective security with respect to the revocation list.
2. {\it Secret Trace Setting:} We provide the first construction with optimal ciphertext, public and secret key sizes and embedded identities from any assumption outside Obfustopia. In detail, our construction relies on Lockable Obfuscation which can be constructed using ${\sf LWE}$ (Goyal, Koppula, Waters and Wichs, Zirdelis, Focs 2017) and two ${\sf ABE}$ schemes: (i) the key-policy scheme with special efficiency properties by Boneh et al. (Eurocrypt 2014) and (ii) a ciphertext-policy ${\sf ABE}$ for ${\sf P}$ which was recently constructed by Wee (Eurocrypt 2022) using a new assumption called {\it evasive and tensor} ${\sf LWE}$. This assumption, introduced to build an ${\sf ABE}$, is believed to be much weaker than lattice based assumptions underlying ${\sf FE}$ or ${\sf iO}$ -- in particular it is required even for lattice based broadcast, without trace.
Moreover, by relying on subexponential security of ${\sf LWE}$, both our constructions can also support a {\it super-polynomial} sized revocation list, so long as it allows efficient representation and membership testing. Ours is the first work to achieve this, to the best of our knowledge.

2023

CRYPTO

Constant Input Attribute Based (and Predicate) Encryption from Evasive and Tensor LWE
Abstract

Constructing advanced cryptographic primitives such as obfuscation or broadcast encryption from standard hardness assumptions in the post quantum regime is an important area of research, which has met with limited success despite significant effort. It is therefore extremely important to find new, simple to state assumptions in this regime which can be used to fill this gap. An important step was taken recently by Wee (Eurocrypt '22) who identified two new assumptions from lattices, namely evasive LWE and tensor LWE, and used these to construct broadcast encryption and ciphertext policy attribute based encryption for P with optimal parameters. Independently, Tsabary formulated a similar assumption and used it to construct witness encryption (Crypto '22). Following Wee's work, Vaikuntanathan, Wee and Wichs independently provided a construction of witness encryption (Asiacrypt '22).
In this work, we advance this line of research by providing the first construction of multi-input attribute based encryption (miABE) for the function class NC_1 for any constant arity from evasive LWE. Our construction can be extended to support the function class P} by using evasive and a suitable strengthening of tensor LWE. In more detail, our construction supports k encryptors, for any constant k, where each encryptor uses the master secret key msk to encode its input (x_i, m_i), the key generator computes a key sk_f for a function f \in NC_1 and the decryptor can recover (m_1,...,m_k) if and only if f(x_1,...,x_k)=1. The only known construction for miABE for NC_1 by Agrawal, Yadav and Yamada (Crypto '22) supports arity 2 and relies on pairings in the generic group model (or with a non-standard knowledge assumption) in addition to LWE. Furthermore, it is completely unclear how to go beyond arity 2 using this approach due to the reliance on pairings.
Using a compiler from Agrawal, Yadav and Yamada (Crypto '22), our miABE can be upgraded to multi-input predicate encryption for the same arity and function class. Thus, we obtain the first constructions for constant-arity predicate and attribute based encryption for a generalized class such as NC_1 or P from simple assumptions that may be conjectured post-quantum secure. Along the way, we show that the tensor LWE assumption can be reduced to standard LWE in an important special case which was not known before. This adds confidence to the plausibility of the assumption and may be of wider interest.

2022

TCC

Bounded Functional Encryption for Turing Machines: Adaptive Security from General Assumptions
Abstract

The recent work of Agrawal et al., [Crypto '21] and Goyal et al. [Eurocrypt '22] concurrently introduced the notion of dynamic bounded collusion security for functional encryption (FE) and showed a construction satisfying the notion from identity based encryption (IBE). Agrawal et al., [Crypto '21] further extended it to FE for Turing machines in non-adaptive simulation setting from the sub-exponential learining with errors assumption (LWE). Concurrently, the work of Goyal et al. [Asiacrypt '21] constructed attribute based encryption (ABE) for Turing machines achieving adaptive indistinguishability based security against bounded (static) collusions from IBE, in the random oracle model. In this work, we significantly improve the state of art for dynamic bounded collusion FE and ABE for Turing machines by achieving \emph{adaptive} simulation style security from a broad class of assumptions, in the standard model. In more detail, we obtain the following results:
\begin{enumerate}
\item We construct an adaptively secure (AD-SIM) FE for Turing machines, supporting dynamic bounded collusion, from sub-exponential LWE. This improves the result of Agrawal et al. which achieved only non-adaptive (NA-SIM) security in the dynamic bounded collusion model.
\item Towards achieving the above goal, we construct a \emph{ciphertext policy} FE scheme (CPFE) for circuits of \emph{unbounded} size and depth, which achieves AD-SIM security in the dynamic bounded collusion model from IBE and \emph{laconic oblivious transfer} (LOT). Both IBE and LOT can be instantiated from a large number of mild assumptions such as the computational Diffie-Hellman assumption, the factoring assumption, and polynomial LWE. This improves the construction of Agrawal et al. which could only achieve NA-SIM security for CPFE supporting circuits of unbounded depth from IBE.
\item We construct an AD-SIM secure FE for Turing machines, supporting dynamic bounded collusions, from LOT, ABE for NC1 (or NC) and private information retrieval (PIR) schemes which satisfy certain properties. This significantly expands the class of assumptions on which AD-SIM secure FE for Turing machines can be based. In particular, it leads to new constructions of FE for Turing machines including one based on polynomial LWE and one based on the combination of the bilinear decisional Diffie-Hellman assumption and the decisional Diffie-Hellman assumption on some specific groups. In contrast the only prior construction by Agrawal et al. achieved only NA-SIM security and relied on \emph{sub-exponential} LWE.
To achieve the above result, we define the notion of CPFE for read only RAM programs and succinct FE for LOT, which may be of independent interest.
\item We also construct an \emph{ABE} scheme for Turing machines which achieves AD-IND security in the \emph{standard model} supporting dynamic bounded collusions. Our scheme is based on IBE and LOT. Previously, the only known candidate that achieved AD-IND security from IBE by Goyal et al. relied on the random oracle model. \end{enumerate}

2022

CRYPTO

Multi-Input Attribute Based Encryption and Predicate Encryption
📺
Abstract

Motivated by several new and natural applications, we initiate the study of multi-input predicate encryption (${\sf miPE}$) and further develop multi-input attribute based encryption (${\sf miABE}$). Our contributions are:
1. {\it Formalizing Security:} We provide definitions for ${\sf miABE}$ and ${\sf miPE}$ in the {symmetric} key setting and formalize security in the standard {\it indistinguishability} (IND) paradigm, against {\it unbounded} collusions.
2. {\it Two-input ${\sf ABE}$ for ${\sf NC}_1$ from ${\sf LWE}$ and Pairings:} We provide the first constructions for two-input {\it key-policy} ${\sf ABE}$ for ${\sf NC}_1$ from ${\sf LWE}$ and pairings. Our construction leverages a surprising connection between techniques recently developed by Agrawal and Yamada (Eurocrypt, 2020) in the context of succinct {\it single}-input {\it ciphertext-policy} ${\sf ABE}$, to the seemingly unrelated problem of {\it two}-input {\it key-policy} ${\sf ABE}$. Similarly to Agrawal-Yamada, our construction is proven secure in the bilinear generic group model. By leveraging inner product functional encryption and using (a variant of) the KOALA knowledge assumption, we obtain a construction in the standard model analogously to Agrawal, Wichs and Yamada (TCC, 2020).
3. {\it Heuristic two-input ${\sf ABE}$ for ${\sf P}$ from Lattices:} We show that techniques developed for succinct single-input ciphertext-policy ${\sf ABE}$ by Brakerski and Vaikuntanathan (ITCS 2022) can also be seen from the lens of ${\sf miABE}$ and obtain the first two-input key-policy ${\sf ABE}$ from lattices for ${\sf P}$.
4. {\it Heuristic three-input ${\sf ABE}$ and ${\sf PE}$ for ${\sf NC}_1$ from Pairings and Lattices:} We obtain the first {\it three}-input ${\sf ABE}$ for ${\sf NC}_1$ by harnessing the powers of both the Agrawal-Yamada and the Brakerski-Vaikuntanathan constructions.
5. {\it Multi-input ${\sf ABE}$ to multi-input ${\sf PE}$ via Lockable Obfuscation:} We provide a generic compiler that lifts multi-input ${\sf ABE}$ to multi-input $\PE$ by relying on the hiding properties of Lockable Obfuscation (${\sf LO}$) by Wichs-Zirdelis and Goyal-Koppula-Waters (FOCS 2018), which can be based on ${\sf LWE}$. Our compiler generalises such a compiler for single-input setting to the much more challenging setting of multiple inputs. By instantiating our compiler with our new two and three-input ${\sf ABE}$ schemes, we obtain the first constructions of two and three-input ${\sf PE}$ schemes.
Our constructions of multi-input ${\sf ABE}$ provide the first improvement to the compression factor of {\it non-trivially exponentially efficient Witness Encryption} defined by Brakerski et al. (SCN 2018) without relying on compact functional encryption or indistinguishability obfuscation. We believe that the unexpected connection between succinct single-input ciphertext-policy ${\sf ABE}$ and multi-input key-policy ${\sf ABE}$ may lead to a new pathway for witness encryption. We remark that our constructions provide the first candidates for a nontrivial class of ${\sf miFE}$ without needing ${\sf LPN}$ or low depth ${\sf PRG}$.
\keywords{Multi-input \and Attribute Based Encryption \and Predicate Encryption.}

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

CRYPTO

Non-Interactive Secure Multiparty Computation for Symmetric Functions, Revisited: More Efficient Constructions and Extensions
📺
Abstract

Non-interactive secure multiparty computation (NIMPC) is a variant of secure computation which allows each of $n$ players to send only a single message depending on his input and correlated randomness.
Abelian programs, which can realize any symmetric function, are defined as functions on the sum of the players' inputs over an abelian group and provide useful functionalities for real-world applications.
We improve and extend the previous results in the following ways:
\begin{itemize}
\item We present NIMPC protocols for abelian programs that improve the best known communication complexity.
If inputs take any value of an abelian group $\mathbb{G}$, our protocol achieves the communication complexity $O(|\mathbb{G}|(\log|\mathbb{G}|)^2)$ improving $O(|\mathbb{G}|^2n^2)$ of Beimel et al. (Crypto 2014).
If players are limited to inputs from subsets of size at most $d$, our protocol achieves $|\mathbb{G}|(\log|\mathbb{G}|)^2(\max\{n,d\})^{(1+o(1))t}$ where $t$ is a corruption threshold.
This result improves $|\mathbb{G}|^3(nd)^{(1+o(1))t}$ of Beimel et al. (Crypto 2014), and even $|\mathbb{G}|^{\log n+O(1)}n$ of Benhamouda et al. (Crypto 2017) if $t=o(\log n)$ and $|\mathbb{G}|=n^{\Theta(1)}$.
\item We propose for the first time NIMPC protocols for linear classifiers that are more efficient than those obtained from the generic construction.
\item We revisit a known transformation of Benhamouda et al. (Crypto 2017) from Private Simultaneous Messages (PSM) to NIMPC, which we repeatedly use in the above results.
We reveal that a sub-protocol used in the transformation does not satisfy the specified security.
We also fix their protocol with only constant overhead in the communication complexity.
As a byproduct, we obtain an NIMPC protocol for indicator functions with asymptotically optimal communication complexity with respect to the input length.
\end{itemize}

2021

CRYPTO

Functional Encryption for Turing Machines with Dynamic Bounded Collusion from LWE
📺
Abstract

The classic work of Gorbunov, Vaikuntanathan and Wee (CRYPTO 2012) and follow-ups provided constructions of bounded collusion Functional Encryption (FE) for circuits from mild assumptions. In this work, we improve the state of affairs for bounded collusion FE in several ways:
1. {\it New Security Notion.} We introduce the notion of {\it dynamic} bounded collusion FE, where the declaration of collusion bound is delayed to the time of encryption. This enables the encryptor to dynamically choose the collusion bound for different ciphertexts depending on their individual level of sensitivity. Hence, the ciphertext size grows linearly with its own collusion bound and the public key size is independent of collusion bound. In contrast, all prior constructions have public key and ciphertext size that grow at least linearly with a fixed bound $Q$.
2. {\it CPFE for circuits with Dynamic Bounded Collusion.} We provide the first CPFE schemes for circuits enjoying dynamic bounded collusion security. By assuming identity based encryption (IBE), we construct CPFE for circuits of {\it unbounded} size satisfying {\it non-adaptive} simulation based security. By strengthening the underlying assumption to IBE with receiver selective opening security, we obtain CPFE for circuits of {\it bounded} size enjoying {\it adaptive} simulation based security. Moreover, we show that IBE is a necessary assumption for these primitives. Furthermore, by relying on the Learning With Errors (LWE) assumption, we obtain the first {\it succinct} CPFE for circuits, i.e. supporting circuits with unbounded size, but fixed output length and depth. This scheme achieves {\it adaptive} simulation based security.
3. {\it KPFE for circuits with dynamic bounded collusion.} We provide the first KPFE for circuits of unbounded size, but bounded depth and output length satisfying dynamic bounded collusion security from LWE. Our construction achieves {\it adaptive} simulation security improving security of \cite{GKPVZ13a}.
4. {\it KP and CP FE for TM/NL with dynamic bounded collusion.} We provide the first KPFE and CPFE constructions of bounded collusion functional encryption for Turing machines in the public key setting from LWE. Our constructions achieve non-adaptive simulation based security. Both the input and the machine in our construction can be of {\it unbounded} polynomial length.
We provide a variant of the above scheme that satisfies {\it adaptive} security, but at the cost of supporting a smaller class of computation, namely Nondeterministic Logarithmic-space (NL). Since NL contains Nondeterministic Finite Automata (NFA), this result subsumes {\it all} prior work of bounded collusion FE for uniform models from standard assumptions \cite{AMY19,AS17}.

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

Tighter Security Proofs for GPV-IBE in the Quantum Random Oracle Model
Abstract

In (STOC, 2008), Gentry, Peikert, and Vaikuntanathan proposed the first identity-based encryption (GPV-IBE) scheme based on a post-quantum assumption, namely the learning with errors assumption. Since their proof was only made in the random oracle model (ROM) instead of the quantum random oracle model (QROM), it remained unclear whether the scheme was truly post-quantum or not. In (CRYPTO, 2012), Zhandry developed new techniques to be used in the QROM and proved security of GPV-IBE in the QROM, hence answering in the affirmative that GPV-IBE is indeed post-quantum. However, since the general technique developed by Zhandry incurred a large reduction loss, there was a wide gap between the concrete efficiency and security level provided by GPV-IBE in the ROM and QROM. Furthermore, regardless of being in the ROM or QROM, GPV-IBE is not known to have a tight reduction in the multi-challenge setting. Considering that in the real-world an adversary can obtain many ciphertexts, it is desirable to have a security proof that does not degrade with the number of challenge ciphertext. In this paper, we provide a much tighter proof for the GPV-IBE in the QROM in the single-challenge setting. In addition, we show that a slight variant of the GPV-IBE has an almost tight reduction in the multi-challenge setting both in the ROM and QROM, where the reduction loss is independent of the number of challenge ciphertext. Our proof departs from the traditional partitioning technique and resembles the approach used in the public key encryption scheme of Cramer and Shoup (CRYPTO, 1998). Our proof strategy allows the reduction algorithm to program the random oracle the same way for all identities and naturally fits the QROM setting where an adversary may query a superposition of all identities in one random oracle query. Notably, our proofs are much simpler than the one by Zhandry and conceptually much easier to follow for cryptographers not familiar with quantum computation. Although at a high level, the techniques used for the single- and multi-challenge setting are similar, the technical details are quite different. For the multi-challenge setting, we rely on the Katz–Wang technique (CCS, 2003) to overcome some obstacles regarding the leftover hash lemma.

2020

EUROCRYPT

Optimal Broadcast Encryption from Pairings and LWE
★
Abstract

Boneh, Waters and Zhandry (CRYPTO 2014) used multilinear maps to provide a solution to the long-standing problem of public-key broadcast encryption (BE) where all parameters in the system are small. In this work, we improve their result by providing a solution that uses only {\it bilinear} maps and Learning With Errors (LWE). Our scheme is fully collusion-resistant against any number of colluders, and can be generalized to an identity-based broadcast system with short parameters. Thus, we reclaim the problem of optimal broadcast encryption from the land of ``Obfustopia''.
Our main technical contribution is a ciphertext policy attribute based encryption (CP-ABE) scheme which achieves special efficiency properties -- its ciphertext size, secret key size, and public key size are all independent of the size of the circuits supported by the scheme. We show that this special CP-ABE scheme implies BE with optimal parameters; but it may also be of independent interest. Our constructions rely on a novel interplay of bilinear maps and LWE, and are proven secure in the generic group model.

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

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

CP-ABE for Circuits (and more) in the Symmetric Key Setting
📺
Abstract

The celebrated work of Gorbunov, Vaikuntanathan and Wee [GVW13] provided the first key policy attribute based encryption scheme (ABE) for circuits from the Learning With Errors (LWE) assumption. However, the arguably more natural ciphertext policy variant has remained elusive, and is a central primitive not yet known from LWE.
In this work, we construct the first symmetric key ciphertext policy attribute based encryption scheme (CP-ABE) for all polynomial sized circuits from the learning with errors (LWE) assumption. In more detail, the ciphertext for a message m is labelled with an access control policy f, secret keys are labelled with public attributes x from the domain of f and decryption succeeds to yield the hidden message m if and only if f(x) = 1. The size of our public and secret key do not depend on the size of the circuits supported by the scheme – this enables our construction to support circuits of unbounded size (but bounded depth). Our construction is secure against collusions of unbounded size. We note that current best CP-ABE schemes [BSW07, Wat11, LOS+10, OT10, LW12, RW13, Att14, Wee14, AHY15, CGW15, AC17, KW19] rely on pairings and only support circuits in the class NC1 (albeit in the public key setting).
We adapt our construction to the public key setting for the case of bounded size circuits. The size of the ciphertext and secret key as well as running time of encryption, key generation and decryption satisfy the efficiency properties desired from CP-ABE, assuming that all algorithms have RAM access to the public key. However, the running time of the setup algorithm and size of the public key depends on the circuit size bound, restricting the construction to support circuits of a-priori bounded size. We remark that the inefficiency of setup is somewhat mitigated by the fact that setup must only be run once.
We generalize our construction to consider attribute and function hiding. The compiler of lockable obfuscation upgrades any attribute based encryption scheme to predicate encryption, i.e. with attribute hiding [GKW17, WZ17]. Since lockable obfuscation can be constructed from LWE, we achieve ciphertext policy predicate encryption immediately. For function privacy, we show that the most natural notion of function hiding ABE for circuits, even in the symmetric key setting, is sufficient to imply indistinguishability obfuscation. We define a suitable weakening of function hiding to sidestep the implication and provide a construction to achieve this notion for both the key policy and ciphertext policy case. Previously, the largest function class for which function private predicate encryption (supporting unbounded keys) could be achieved was inner product zero testing, by Shen, Shi and Waters [SSW09].

2020

TCC

Optimal Broadcast Encryption from LWE and Pairings in the Standard Model
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Abstract

Broadcast Encryption with optimal parameters was a long-standing problem, whose first solution was provided in an elegant work by Boneh, Waters and Zhandry \cite{BWZ14}. However, this work relied on multilinear maps of logarithmic degree, which is not considered a standard assumption. Recently, Agrawal and Yamada \cite{AY20} improved this state of affairs by providing the first construction of optimal broadcast encryption from Bilinear Maps and Learning With Errors (LWE). However, their proof of security was in the generic bilinear group model. In this work, we improve upon their result by providing a new construction and proof in the standard model. In more detail, we rely on the Learning With Errors (LWE) assumption and the Knowledge of OrthogonALity Assumption (KOALA) \cite{BW19} on bilinear groups.
Our construction combines three building blocks: a (computational) nearly linear secret sharing scheme with compact shares which we construct from LWE, an inner-product functional encryption scheme with special properties which is constructed from the bilinear Matrix Decision Diffie Hellman (MDDH) assumption, and a certain form of hyperplane obfuscation, which is constructed using the KOALA assumption. While similar to that of Agrawal and Yamada, our construction provides a new understanding of how to decompose the construction into simpler, modular building blocks with concrete and easy-to-understand security requirements for each one. We believe this sheds new light on the requirements for optimal broadcast encryption, which may lead to new constructions in the future.

2020

ASIACRYPT

Adaptively Secure Inner Product Encryption from LWE
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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

Non-zero Inner Product Encryption Schemes from Various Assumptions: LWE, DDH and DCR
Abstract

In non-zero inner product encryption (NIPE) schemes, ciphertexts and secret keys are associated with vectors and decryption is possible whenever the inner product of these vectors does not equal zero. So far, much effort on constructing bilinear map-based NIPE schemes have been made and this has lead to many efficient schemes. However, the constructions of NIPE schemes without bilinear maps are much less investigated. The only known other NIPE constructions are based on lattices, however, they are all highly inefficient due to the need of converting inner product operations into circuits or branching programs.To remedy our rather poor understanding regarding NIPE schemes without bilinear maps, we provide two methods for constructing NIPE schemes: a direct construction from lattices and a generic construction from schemes for inner products (LinFE). For our first direct construction, it highly departs from the traditional lattice-based constructions and we rely heavily on new tools concerning Gaussian measures over multi-dimensional lattices to prove security. For our second generic construction, using the recent constructions of LinFE schemes as building blocks, we obtain the first NIPE constructions based on the DDH and DCR assumptions. In particular, we obtain the first NIPE schemes without bilinear maps or lattices.

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
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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

EUROCRYPT

Group Signatures Without NIZK: From Lattices in the Standard Model
Abstract

In a group signature scheme, users can anonymously sign messages on behalf of the group they belong to, yet it is possible to trace the signer when needed. Since the first proposal of lattice-based group signatures in the random oracle model by Gordon, Katz, and Vaikuntanathan (ASIACRYPT 2010), the realization of them in the standard model from lattices has attracted much research interest, however, it has remained unsolved. In this paper, we make progress on this problem by giving the first such construction. Our schemes satisfy CCA-selfless anonymity and full traceability, which are the standard security requirements for group signatures proposed by Bellare, Micciancio, and Warinschi (EUROCRYPT 2003) with a slight relaxation in the anonymity requirement suggested by Camenisch and Groth (SCN 2004). We emphasize that even with this relaxed anonymity requirement, all previous group signature constructions rely on random oracles or NIZKs, where currently NIZKs are not known to be implied from lattice-based assumptions. We propose two constructions that provide tradeoffs regarding the security assumption and efficiency:Our first construction is proven secure assuming the standard LWE and the SIS assumption. The sizes of the public parameters and the signatures grow linearly in the number of users in the system.Our second construction is proven secure assuming the standard LWE and the subexponential hardness of the SIS problem. The sizes of the public parameters and the signatures are independent of the number of users in the system.
Technically, we obtain the above schemes by combining a secret key encryption scheme with additional properties and a special type of attribute-based signature (ABS) scheme, thus bypassing the utilization of NIZKs. More specifically, we introduce the notion of indexed ABS, which is a relaxation of standard ABS. The above two schemes are obtained by instantiating the indexed ABS with different constructions. One is a direct construction we propose and the other is based on previous work.

2019

CRYPTO

Attribute Based Encryption (and more) for Nondeterministic Finite Automata from LWE
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Abstract

Constructing Attribute Based Encryption (ABE) [56] for uniform models of computation from standard assumptions, is an important problem, about which very little is known. The only known ABE schemes in this setting that (i) avoid reliance on multilinear maps or indistinguishability obfuscation, (ii) support unbounded length inputs and (iii) permit unbounded key requests to the adversary in the security game, are by Waters from Crypto, 2012 [57] and its variants. Waters provided the first ABE for Deterministic Finite Automata (DFA) satisfying the above properties, from a parametrized or “q-type” assumption over bilinear maps. Generalizing this construction to Nondeterministic Finite Automata (NFA) was left as an explicit open problem in the same work, and has seen no progress to date. Constructions from other assumptions such as more standard pairing based assumptions, or lattice based assumptions has also proved elusive.In this work, we construct the first symmetric key attribute based encryption scheme for nondeterministic finite automata (NFA) from the learning with errors (LWE) assumption. Our scheme supports unbounded length inputs as well as unbounded length machines. In more detail, secret keys in our construction are associated with an NFA M of unbounded length, ciphertexts are associated with a tuple $$(\mathbf {x}, m)$$ where $$\mathbf {x}$$ is a public attribute of unbounded length and m is a secret message bit, and decryption recovers m if and only if $$M(\mathbf {x})=1$$.Further, we leverage our ABE to achieve (restricted notions of) attribute hiding analogous to the circuit setting, obtaining the first predicate encryption and bounded key functional encryption schemes for NFA from LWE. We achieve machine hiding in the single/bounded key setting to obtain the first reusable garbled NFA from standard assumptions. In terms of lower bounds, we show that secret key functional encryption even for DFAs, with security against unbounded key requests implies indistinguishability obfuscation ($$\mathsf {iO}$$) for circuits; this suggests a barrier in achieving full fledged functional encryption for NFA.

2019

TCC

Attribute Based Encryption for Deterministic Finite Automata from $\mathsf{DLIN}$
Abstract

Waters [Crypto, 2012] provided the first attribute based encryption scheme ABE for Deterministic Finite Automata (DFA) from a parametrized or “q-type” assumption over bilinear maps. Obtaining a construction from static assumptions has been elusive, despite much progress in the area of ABE.In this work, we construct the first attribute based encryption scheme for DFA from static assumptions on pairings, namely, the $$\mathsf{DLIN}$$ assumption. Our scheme supports unbounded length inputs, unbounded length machines and unbounded key requests. In more detail, secret keys in our construction are associated with a DFA M of unbounded length, ciphertexts are associated with a tuple $$(\mathbf {x}, \mathsf {\mu })$$ where $$\mathbf {x}$$ is a public attribute of unbounded length and $$\mathsf {\mu }$$ is a secret message bit, and decryption recovers $$\mathsf {\mu }$$ if and only if $$M(\mathbf {x})=1$$.Our techniques are at least as interesting as our final result. We present a simple compiler that combines constructions of unbounded ABE schemes for monotone span programs (MSP) in a black box way to construct ABE for DFA. In more detail, we find a way to embed DFA computation into monotone span programs, which lets us compose existing constructions (modified suitably) of unbounded key-policy ABE ($${\mathsf {kpABE}}$$) and unbounded ciphertext-policy ABE ($${\mathsf {cpABE}}$$) for MSP in a simple and modular way to obtain key-policy ABE for DFA. Our construction uses its building blocks in a symmetric way – by swapping the use of the underlying $${\mathsf {kpABE}}$$ and $${\mathsf {cpABE}}$$, we also obtain a construction of ciphertext-policy ABE for DFA.Our work extends techniques developed recently by Agrawal, Maitra and Yamada [Crypto 2019], which show how to construct ABE that support unbounded machines and unbounded inputs by combining ABE schemes that are bounded in one co-ordinate. At the heart of our work is the observation that unbounded, multi-use ABE for MSP already achieve most of what we need to build ABE for DFA.

2019

CRYPTO

Exploring Constructions of Compact NIZKs from Various Assumptions
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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.

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

ASIACRYPT

Tighter Security Proofs for GPV-IBE in the Quantum Random Oracle Model
Abstract

In (STOC, 2008), Gentry, Peikert, and Vaikuntanathan proposed the first identity-based encryption (GPV-IBE) scheme based on a post-quantum assumption, namely, the learning with errors (LWE) assumption. Since their proof was only made in the random oracle model (ROM) instead of the quantum random oracle model (QROM), it remained unclear whether the scheme was truly post-quantum or not. In (CRYPTO, 2012), Zhandry developed new techniques to be used in the QROM and proved security of GPV-IBE in the QROM, hence answering in the affirmative that GPV-IBE is indeed post-quantum. However, since the general technique developed by Zhandry incurred a large reduction loss, there was a wide gap between the concrete efficiency and security level provided by GPV-IBE in the ROM and QROM. Furthermore, regardless of being in the ROM or QROM, GPV-IBE is not known to have a tight reduction in the multi-challenge setting. Considering that in the real-world an adversary can obtain many ciphertexts, it is desirable to have a security proof that does not degrade with the number of challenge ciphertext.In this paper, we provide a much tighter proof for the GPV-IBE in the QROM in the single-challenge setting. In addition, we also show that a slight variant of the GPV-IBE has an almost tight reduction in the multi-challenge setting both in the ROM and QROM, where the reduction loss is independent of the number of challenge ciphertext. Our proof departs from the traditional partitioning technique and resembles the approach used in the public key encryption scheme of Cramer and Shoup (CRYPTO, 1998). Our proof strategy allows the reduction algorithm to program the random oracle the same way for all identities and naturally fits the QROM setting where an adversary may query a superposition of all identities in one random oracle query. Notably, our proofs are much simpler than the one by Zhandry and conceptually much easier to follow for cryptographers not familiar with quantum computation. Although at a high level, the techniques used for the single and multi-challenge setting are similar, the technical details are quite different. For the multi-challenge setting, we rely on the Katz-Wang technique (CCS, 2003) to overcome some obstacles regarding the leftover hash lemma.

2017

CRYPTO

2016

EUROCRYPT

2016

CRYPTO

2016

ASIACRYPT

2015

ASIACRYPT

2014

CRYPTO

#### Program Committees

- TCC 2023
- Crypto 2022
- Asiacrypt 2022

#### Coauthors

- Shweta Agrawal (12)
- Nuttapong Attrapadung (7)
- Alex Davidson (1)
- Keita Emura (1)
- Reo Eriguchi (1)
- Goichiro Hanaoka (9)
- Shuichi Katsumata (12)
- Fuyuki Kitagawa (2)
- Simran Kumari (2)
- Noboru Kunihiro (6)
- Narasimha Sai Vempati (1)
- Monosij Maitra (3)
- Takahiro Matsuda (3)
- Anuja Modi (1)
- Ryo Nishimaki (11)
- Koji Nuida (1)
- Kazuma Ohara (1)
- Go Ohtake (1)
- Mélissa Rossi (1)
- Bagus Santoso (1)
- Jacob C. N. Schuldt (1)
- Daniel Wichs (1)
- Anshu Yadav (3)
- Takashi Yamakawa (15)