## CryptoDB

### Fuyuki Kitagawa

#### Publications

**Year**

**Venue**

**Title**

2024

EUROCRYPT

Certified Everlasting Secure Collusion-Resistant Functional Encryption, and More
Abstract

We study certified everlasting secure functional encryption (FE) and many other cryptographic primitives in this work.
Certified everlasting security roughly means the following.
A receiver possessing a quantum cryptographic object (such as ciphertext) can issue a certificate showing that the receiver has deleted the cryptographic object and information included in the object (such as plaintext) was lost.
If the certificate is valid, the security is guaranteed even if the receiver becomes computationally unbounded after the deletion.
Many cryptographic primitives are known to be impossible (or unlikely) to have information-theoretical security even in the quantum world.
Hence, certified everlasting security is a nice compromise (intrinsic to quantum).
In this work, we define certified everlasting secure versions of FE, compute-and-compare obfuscation, predicate encryption (PE), secret-key encryption (SKE), public-key encryption (PKE), receiver non-committing encryption (RNCE), and garbled circuits.
We also present the following constructions:
- Adaptively certified everlasting secure collusion-resistant public-key FE for all polynomial-size circuits from indistinguishability obfuscation and one-way functions.
- Adaptively certified everlasting secure bounded collusion-resistant public-key FE for $\mathsf{NC}^1$ circuits from standard PKE.
- Certified everlasting secure compute-and-compare obfuscation from standard fully homomorphic encryption and standard compute-and-compare obfuscation
- Adaptively (resp., selectively) certified everlasting secure PE from standard adaptively (resp., selectively) secure attribute-based encryption and certified everlasting secure compute-and-compare obfuscation.
- Certified everlasting secure SKE and PKE from standard SKE and PKE, respectively.
- Cetified everlasting secure RNCE from standard PKE.
- Cetified everlasting secure garbled circuits from standard SKE.

2024

CRYPTO

Quantum Public-Key Encryption with Tamper-Resilient Public Keys from One-Way Functions
Abstract

We construct quantum public-key encryption from one-way functions. In our construction, public keys are quantum, but ciphertexts are classical. Quantum public-key encryption from one-way functions (or weaker primitives such as pseudorandom function-like states) are also proposed in some recent works [Morimae-Yamakawa, eprint:2022/1336; Coladangelo, eprint:2023/282; Barooti-Grilo-Malavolta-Sattath-Vu-Walter, TCC 2023]. However, they have a huge drawback: they are secure only when quantum public keys can be transmitted to the sender (who runs the encryption algorithm) without being tampered with by the adversary, which seems to require unsatisfactory physical setup assumptions such as secure quantum channels. Our construction is free from such a drawback: it guarantees the secrecy of the encrypted messages even if we assume only unauthenticated quantum channels. Thus, the encryption is done with adversarially tampered quantum public keys. Our construction is the first quantum public-key encryption that achieves the goal of classical public-key encryption, namely, to establish secure communication over insecure channels, based only on one-way functions. Moreover, we show a generic compiler to upgrade security against chosen plaintext attacks (CPA security) into security against chosen ciphertext attacks (CCA security) only using one-way functions. As a result, we obtain CCA secure quantum public-key encryption based only on one-way functions.

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

JOFC

NIZK from SNARGs
Abstract

We give a construction of a non-interactive zero-knowledge (NIZK) argument for all $${\textsf{NP}}$$ NP languages based on a succinct non-interactive argument (SNARG) for all $${\textsf{NP}}$$ NP languages and a one-way function. The succinctness requirement for the SNARG is rather mild: We only require that the proof size be $$|\pi |={\textsf{poly}}(\lambda )(|x|+|w|)^\delta $$ | π | = poly ( λ ) ( | x | + | w | ) δ for some constant $$\delta <1$$ δ < 1 , where | x | is the statement length, | w | is the witness length, and $$\lambda $$ λ is the security parameter. Especially, we do not require the efficiency of the verification to be sublinear in | x | or | w |. As a corollary, we give a generic conversion from a SNARK to a zero-knowledge SNARG assuming the existence of one-way functions where SNARK is a SNARG with knowledge-extractability. For this conversion, we require the SNARK to be fully succinct, i.e., the proof size is $${\textsf{poly}}(\lambda )(|x|+|w|)^{o(1)}$$ poly ( λ ) ( | x | + | w | ) o ( 1 ) . Before this work, such a conversion was only known if we additionally assume the existence of a NIZK. Along the way of obtaining our result, we give a generic compiler to upgrade a NIZK for all $${\textsf{NP}}$$ NP languages with non-adaptive zero-knowledge to one with adaptive zero-knowledge. Though this can be shown by carefully combining known results, to the best of our knowledge, no explicit proof of this generic conversion has been presented.

2023

TCC

Publicly Verifiable Deletion from Minimal Assumptions
Abstract

We present a general compiler to add the publicly verifiable deletion property for various cryptographic primitives including public key encryption, attribute-based encryption, and quantum fully homomorphic encryption. Our compiler only uses one-way functions, or more generally hard quantum planted problems for NP, which are implied by one-way functions.
It relies on minimal assumptions and enables us to add the publicly verifiable deletion property with no additional assumption for the above primitives. Previously, such a compiler needs additional assumptions such as injective trapdoor one-way functions or pseudorandom group actions [Bartusek-Khurana-Poremba, CRYPTO 2023]. Technically, we upgrade an existing compiler for privately verifiable deletion [Bartusek-Khurana, CRYPTO 2023] to achieve publicly verifiable deletion by using digital signatures.

2023

TCC

One-out-of-Many Unclonable Cryptography: Definitions, Constructions, and More
Abstract

The no-cloning principle of quantum mechanics enables us to achieve amazing unclonable cryptographic primitives, which is impossible in classical cryptography. However, the security definitions for unclonable cryptography are tricky. Achieving desirable security notions for unclonability is a challenging task. In particular, there is no indistinguishable-secure unclonable encryption and quantum copy-protection for single-bit output point functions in the standard model. To tackle this problem, we introduce and study relaxed but meaningful security notions for unclonable cryptography in this work. We call the new security notion one-out-of-many unclonable security.
We obtain the following results.
- We show that one-time strong anti-piracy secure secret key single-decryptor encryption (SDE) implies one-out-of-many indistinguishable-secure unclonable encryption.
- We construct a one-time strong anti-piracy secure secret key SDE scheme in the standard model from the LWE assumption. This scheme can encrypt multi-bit messages.
- We construct one-out-of-many copy-protection for single-bit output point functions from one-out-of-many indistinguishable-secure unclonable encryption and the LWE assumption.
- We construct one-out-of-many unclonable predicate encryption (PE) from one-out-of-many indistinguishable-secure unclonable encryption and the LWE assumption.
Thus, we obtain one-out-of-many indistinguishable-secure unclonable encryption, one-out-of-many copy-protection for single-bit output point functions, and one-out-of-many unclonable PE in the standard model from the LWE assumption. In addition, our one-time SDE scheme is the first multi-bit SDE scheme that does not rely on any oracle heuristics and strong assumptions such as indistinguishability obfuscation and witness encryption.

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.

2022

ASIACRYPT

Functional Encryption with Secure Key Leasing
📺
Abstract

Secure software leasing is a quantum cryptographic primitive that enables us to lease software to a user by encoding it into a quantum state. Secure software leasing has a mechanism that verifies whether a returned software is valid or not. The security notion guarantees that once a user returns a software in a valid form, the user no longer uses the software.
In this work, we introduce the notion of secret-key functional encryption (SKFE) with secure key leasing, where a decryption key can be securely leased in the sense of secure software leasing. We also instantiate it with standard cryptographic assumptions. More specifically, our contribution is as follows.
- We define the syntax and security definitions for SKFE with secure key leasing.
- We achieve a transformation from standard SKFE into SKFE with secure key leasing without using additional assumptions. Especially, we obtain bounded collusion-resistant SKFE for P/poly with secure key leasing based on post-quantum one-way functions since we can instantiate bounded collusion-resistant SKFE for P/poly with the assumption.
Some previous secure software leasing schemes capture only pirate software that runs on an honest evaluation algorithm (on a legitimate platform). However, our secure key leasing notion captures arbitrary attack strategies and does not have such a limitation.
As an additional contribution, we introduce the notion of single-decryptor FE (SDFE), where each functional decryption key is copy-protected. Since copy-protection is a stronger primitive than secure software leasing, this notion can be seen as a stronger cryptographic primitive than FE with secure key leasing. More specifically, our additional contribution is as follows.
- We define the syntax and security definitions for SDFE.
- We achieve collusion-resistant single-decryptor PKFE for P/poly from post-quantum indistinguishability obfuscation and quantum hardness of the learning with errors problem.

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

JOFC

Obfustopia Built on Secret-Key Functional Encryption
Abstract

We show that indistinguishability obfuscation (IO) for all circuits can be constructed solely from secret-key functional encryption (SKFE). In the construction, SKFE needs to be secure against an unbounded number of functional key queries, that is, collusion-resistant. Our strategy is to replace public-key functional encryption (PKFE) in the construction of IO proposed by Bitansky and Vaikuntanathan (FOCS 2015) with puncturable SKFE . Bitansky and Vaikuntanathan introduced the notion of puncturable SKFE and observed that the strategy works. However, it has not been clear whether we can construct puncturable SKFE without assuming PKFE. In particular, it has not been known whether puncturable SKFE can be constructed from standard SKFE. In this work, we show that a relaxed variant of puncturable SKFE can be constructed from collusion-resistant SKFE. Moreover, we show that the relaxed variant of puncturable SKFE is sufficient for constructing IO. Ananth and Jain (CRYPTO 2015) also proposed an IO construction from PKFE. However, their strategy is different from that of Biransky and Vaikuntanathan. In addition, we also study the relation of collusion-resistance and succinctness for SKFE. Functional encryption is said to be weakly succinct if the size of its encryption circuit is sub-linear in the size of functions. We show that collusion-resistant SKFE can be constructed from weakly succinct SKFE supporting only one functional key. By combining the above two results, we show that IO for all circuits can be constructed from weakly succinct SKFE supporting only one functional key.

2022

JOFC

CCA Security and Trapdoor Functions via Key-Dependent-Message Security
Abstract

We study the relationship among public-key encryption (PKE) satisfying indistinguishability against chosen plaintext attacks (IND-CPA security), that against chosen ciphertext attacks (IND-CCA security), and trapdoor functions (TDF). Specifically, we aim at finding a unified approach and some additional requirement to realize IND-CCA secure PKE and TDF based on IND-CPA secure PKE, and show the following two main results. As the first main result, we show how to achieve IND-CCA security via a weak form of key-dependent-message (KDM) security. More specifically, we construct an IND-CCA secure PKE scheme based on an IND-CPA secure PKE scheme and a secret-key encryption (SKE) scheme satisfying one-time KDM security with respect to projection functions (projection-KDM security). Projection functions are elementary functions with respect to which KDM security has been widely studied. Since the existence of projection-KDM secure PKE implies that of the above two building blocks, as a corollary of this result, we see that the existence of IND-CCA secure PKE is implied by that of projection-KDM secure PKE. As the second main result, we extend the above construction of IND-CCA secure PKE into that of TDF by additionally requiring a mild requirement for each building block. Our TDF satisfies adaptive one-wayness. We can instantiate our TDF based on a wide variety of computational assumptions. Especially, we obtain the first TDF (with adaptive one-wayness) based on the sub-exponential hardness of the constant-noise learning-parity-with-noise (LPN) problem. In addition, we show that by extending the above constructions, we can obtain PKE schemes satisfying advanced security notions under CCA, that is, optimal rate leakage-resilience under CCA and selective-opening security under CCA. As a result, we obtain the first PKE schemes satisfying these security notions based on the computational Diffie–Hellman (CDH) assumption or the low-noise LPN assumption.

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

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

TCC

NIZK from SNARG
📺
Abstract

We give a construction of a non-interactive zero-knowledge (NIZK) argument for all NP languages based on a succinct non-interactive argument (SNARG) for all NP languages and a one-way function. The succinctness requirement for the SNARG is rather mild: We only require that the proof size be $|\pi|=\mathsf{poly}(\lambda)(|x|+|w|)^c$ for some constant $c<1/2$, where $|x|$ is the statement length, $|w|$ is the witness length, and $\lambda$ is the security parameter. Especially, we do not require anything about the efficiency of the verification.
Based on this result, we also give a generic conversion from a SNARG to a zero-knowledge SNARG assuming the existence of CPA secure public-key encryption. For this conversion, we require a SNARG to have efficient verification, i.e., the computational complexity of the verification algorithm is $\mathsf{poly}(\lambda)(|x|+|w|)^{o(1)}$. Before this work, such a conversion was only known if we additionally assume the existence of a NIZK.
Along the way of obtaining our result, we give a generic compiler to upgrade a NIZK for all NP languages with non-adaptive zero-knowledge to one with adaptive zero-knowledge. Though this can be shown by carefully combining known results, to the best of our knowledge, no explicit proof of this generic conversion has been presented.

2020

ASIACRYPT

Non-Committing Encryption with Constant Ciphertext Expansion from Standard Assumptions
📺
Abstract

Non-committing encryption (NCE) introduced by Canetti et al. (STOC '96) is a central tool to achieve multi-party computation protocols secure in the adaptive setting. Recently, Yoshida et al. (ASIACRYPT '19) proposed an NCE scheme based on the hardness of the DDH problem, which has ciphertext expansion $\mathcal{O}(\log\lambda)$ and public-key expansion $\mathcal{O}(\lambda^2)$.
In this work, we improve their result and propose a methodology to construct an NCE scheme that achieves \emph{constant} ciphertext expansion. Our methodology can be instantiated from the DDH assumption and the LWE assumption. When instantiated from the LWE assumption, the public-key expansion is $\lambda\cdot\mathsf{poly}(\log\lambda)$. They are the first NCE schemes satisfying constant ciphertext expansion without using iO or common reference strings.
Along the way, we define a weak notion of NCE, which satisfies only weak forms of correctness and security. We show how to amplify such a weak NCE scheme into a full-fledged one using wiretap codes with a new security property.

2020

ASIACRYPT

Circular Security Is Complete for KDM Security
📺
Abstract

Circular security is the most elementary form of key-dependent message (KDM) security, which allows us to securely encrypt only a copy of secret key bits. In this work, we show that circular security is complete for KDM security in the sense that an encryption scheme satisfying this security notion can be transformed into one satisfying KDM security with respect to all functions computable by a-priori bounded-size circuits (bounded-KDM security). This result holds in the presence of any number of keys and in any of secret-key/public-key and CPA/CCA settings. Such a completeness result was previously shown by Applebaum (EUROCRYPT 2011) for KDM security with respect to projection functions (projection-KDM security) that allows us to securely encrypt both a copy and a negation of secret key bits.
Besides amplifying the strength of KDM security, our transformation in fact can start from an encryption scheme satisfying circular security against CPA attacks and results in one satisfying bounded-KDM security against CCA attacks. This result improves the recent result by Kitagawa and Matsuda (TCC 2019) showing a CPA-to-CCA transformation for KDM secure public-key encryption schemes.

2019

CRYPTO

CCA Security and Trapdoor Functions via Key-Dependent-Message Security
📺
Abstract

We study the relationship among public-key encryption (PKE) satisfying indistinguishability against chosen plaintext attacks (IND-CPA security), that against chosen ciphertext attacks (IND-CCA security), and trapdoor functions (TDF). Specifically, we aim at finding a unified approach and some additional requirement to realize IND-CCA secure PKE and TDF based on IND-CPA secure PKE, and show the following two main results.As the first main result, we show how to achieve IND-CCA security via a weak form of key-dependent-message (KDM) security. More specifically, we construct an IND-CCA secure PKE scheme based on an IND-CPA secure PKE scheme and a secret-key encryption (SKE) scheme satisfying one-time KDM security with respect to projection functions (projection-KDM security). Projection functions are very simple functions with respect to which KDM security has been widely studied. Since the existence of projection-KDM secure PKE implies that of the above two building blocks, as a corollary of this result, we see that the existence of IND-CCA secure PKE is implied by that of projection-KDM secure PKE.As the second main result, we extend the above construction of IND-CCA secure PKE into that of TDF by additionally requiring a mild requirement for each building block. Our TDF satisfies adaptive one-wayness. We can instantiate our TDF based on a wide variety of computational assumptions. Especially, we obtain the first TDF (with adaptive one-wayness) based on the sub-exponential hardness of the constant-noise learning-parity-with-noise (LPN) problem.

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

TCC

CPA-to-CCA Transformation for KDM Security
Abstract

We show that chosen plaintext attacks (CPA) security is equivalent to chosen ciphertext attacks (CCA) security for key-dependent message (KDM) security. Concretely, we show how to construct a public-key encryption (PKE) scheme that is KDM-CCA secure with respect to all functions computable by circuits of a-priori bounded size, based only on a PKE scheme that is KDM-CPA secure with respect to projection functions. Our construction works for KDM security in the single user setting.Our main result is achieved by combining the following two steps. First, we observe that by combining the results and techniques from the recent works by Lombardi et al. (CRYPTO 2019), and by Kitagawa et al. (CRYPTO 2019), we can construct a reusable designated-verifier non-interactive zero-knowledge (DV-NIZK) argument system based on an IND-CPA secure PKE scheme and a secret-key encryption (SKE) scheme satisfying one-time KDM security with respect to projection functions. This observation leads to the first reusable DV-NIZK argument system under the learning-parity-with-noise (LPN) assumption. Then, as the second and main technical step, we show a generic construction of a KDM-CCA secure PKE scheme using an IND-CPA secure PKE scheme, a reusable DV-NIZK argument system, and an SKE scheme satisfying one-time KDM security with respect to projection functions. Since the classical Naor-Yung paradigm (STOC 1990) with a DV-NIZK argument system does not work for proving KDM security, we propose a new construction methodology to achieve this generic construction.Moreover, we show how to extend our generic construction and achieve KDM-CCA security in the multi-user setting, by additionally requiring the underlying SKE scheme in our generic construction to satisfy a weak form of KDM security against related-key attacks (RKA-KDM security) instead of one-time KDM security. From this extension, we obtain the first KDM-CCA secure PKE schemes in the multi-user setting under the CDH or LPN assumption.

2019

ASIACRYPT

Simple and Efficient KDM-CCA Secure Public Key Encryption
Abstract

We propose two efficient public key encryption (PKE) schemes satisfying key dependent message security against chosen ciphertext attacks (KDM-CCA security). The first one is KDM-CCA secure with respect to affine functions. The other one is KDM-CCA secure with respect to polynomial functions. Both of our schemes are based on the KDM-CPA secure PKE schemes proposed by Malkin, Teranishi, and Yung (EUROCRYPT 2011). Although our schemes satisfy KDM-CCA security, their efficiency overheads compared to Malkin et al.’s schemes are very small. Thus, efficiency of our schemes is drastically improved compared to the existing KDM-CCA secure schemes.We achieve our results by extending the construction technique by Kitagawa and Tanaka (ASIACRYPT 2018). Our schemes are obtained via semi-generic constructions using an IND-CCA secure PKE scheme as a building block. We prove the KDM-CCA security of our schemes based on the decisional composite residuosity (DCR) assumption and the IND-CCA security of the building block PKE scheme.Moreover, our security proofs are tight if the IND-CCA security of the building block PKE scheme is tightly reduced to its underlying computational assumption. By instantiating our schemes using existing tightly IND-CCA secure PKE schemes, we obtain the first tightly KDM-CCA secure PKE schemes whose ciphertext consists only of a constant number of group elements.

2019

ASIACRYPT

Non-Committing Encryption with Quasi-Optimal Ciphertext-Rate Based on the DDH Problem
Abstract

Non-committing encryption (NCE) was introduced by Canetti et al. (STOC ’96). Informally, an encryption scheme is non-committing if it can generate a dummy ciphertext that is indistinguishable from a real one. The dummy ciphertext can be opened to any message later by producing a secret key and an encryption random coin which “explain” the ciphertext as an encryption of the message. Canetti et al. showed that NCE is a central tool to achieve multi-party computation protocols secure in the adaptive setting. An important measure of the efficiently of NCE is the ciphertext rate, that is the ciphertext length divided by the message length, and previous works studying NCE have focused on constructing NCE schemes with better ciphertext rates.We propose an NCE scheme satisfying the ciphertext rate based on the decisional Diffie-Hellman (DDH) problem, where is the security parameter. The proposed construction achieves the best ciphertext rate among existing constructions proposed in the plain model, that is, the model without using common reference strings. Previously to our work, an NCE scheme with the best ciphertext rate based on the DDH problem was the one proposed by Choi et al. (ASIACRYPT ’09) that has ciphertext rate . Our construction of NCE is similar in spirit to that of the recent construction of the trapdoor function proposed by Garg and Hajiabadi (CRYPTO ’18).

2018

PKC

Key Dependent Message Security and Receiver Selective Opening Security for Identity-Based Encryption
Abstract

We construct two identity-based encryption (IBE) schemes. The first one is IBE satisfying key dependent message (KDM) security for user secret keys. The second one is IBE satisfying simulation-based receiver selective opening (RSO) security. Both schemes are secure against adaptive-ID attacks and do not have any a-priori bound on the number of challenge identities queried by adversaries in the security games. They are the first constructions of IBE satisfying such levels of security.Our constructions of IBE are very simple. We construct KDM secure IBE by transforming KDM secure secret-key encryption using IBE satisfying only ordinary indistinguishability against adaptive-ID attacks (IND-ID-CPA security). Our simulation-based RSO secure IBE is based only on IND-ID-CPA secure IBE.We also demonstrate that our construction technique for KDM secure IBE is used to construct KDM secure public-key encryption. More precisely, we show how to construct KDM secure public-key encryption from KDM secure secret-key encryption and public-key encryption satisfying only ordinary indistinguishability against chosen plaintext attacks.

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.

2018

ASIACRYPT

A Framework for Achieving KDM-CCA Secure Public-Key Encryption
Abstract

We propose a framework for achieving a public-key encryption (PKE) scheme that satisfies key dependent message security against chosen ciphertext attacks (KDM-CCA security) based on projective hash function. Our framework can be instantiated under the decisional diffie-hellman (DDH), quadratic residuosity (QR), and decisional composite residuosity (DCR) assumptions. The constructed schemes are KDM-CCA secure with respect to affine functions and compatible with the amplification method shown by Applebaum (EUROCRYPT 2011). Thus, they lead to PKE schemes satisfying KDM-CCA security for all functions computable by a-priori bounded size circuits. They are the first PKE schemes satisfying such a security notion in the standard model using neither non-interactive zero knowledge proof nor bilinear pairing. The above framework based on projective hash function captures only KDM-CCA security in the single user setting. However, we can prove the KDM-CCA security in the multi user setting of our concrete instantiations by using their algebraic structures explicitly. Especially, we prove that our DDH based scheme satisfies KDM-CCA security in the multi user setting with the same parameter setting as in the single user setting.

#### Program Committees

- Asiacrypt 2024
- PKC 2022

#### Coauthors

- Shweta Agrawal (2)
- Goichiro Hanaoka (1)
- Taiga Hiroka (1)
- Fuyuki Kitagawa (27)
- Takahiro Matsuda (8)
- Anuja Modi (1)
- Tomoyuki Morimae (2)
- Ryo Nishimaki (15)
- Tapas Pal (1)
- Keisuke Tanaka (13)
- Keita Xagawa (1)
- Shota Yamada (2)
- Takashi Yamakawa (9)
- Yusuke Yoshida (2)