## CryptoDB

### Junichi Tomida

#### ORCID: 0000-0001-7995-877X

#### Publications

**Year**

**Venue**

**Title**

2024

CRYPTO

A Modular Approach to Registered ABE for Unbounded Predicates
Abstract

Registered attribute-based encryption (Reg-ABE), introduced by Hohenberger et.al. (Eurocrypt'23), emerges as a pivotal extension of attribute-based encryption (ABE), aimed at mitigating the key-escrow problem. Although several Reg-ABE schemes with black-box use of cryptography have been proposed so far, there remains a significant gap in the class of achievable predicates between vanilla ABE and Reg-ABE. To narrow this gap, we propose a modular framework for constructing Reg-ABE schemes for a broader class of predicates. Our framework is a Reg-ABE analog of the predicate transformation framework for ABE introduced by Attrapadung (Eurocrypt'19) and later refined by Attrapadung and Tomida (Asiacrypt'20) to function under the standard MDDH assumption.
As immediate applications, our framework implies the following new Reg-ABE schemes under the standard MDDH assumption:
\begin{itemize}
\item the first Reg-ABE scheme for (non-)monotone span programs with the traditional completely unbounded property.
\item the first Reg-ABE scheme for general non-monotone span programs (also with the completely unbounded property) as defined in the case of vanilla ABE by Attrapadung and Tomida (Asiacrypt'20).
\end{itemize}
Here, the term ``completely unbounded'' signifies the absence of restrictions on attribute sets for users and policies associated with ciphertexts.
From a technical standpoint, we first substantially modify pair encoding schemes (PES), originally devised for vanilla ABE by Attrapadung (Eurocrypt'14), to make them compatible with Reg-ABE. Subsequently, we present a series of predicate transformations through which we can construct complex predicates, particularly those with an ``unbounded'' characteristic, starting from simple ones. Finally, we define new properties of PES necessary for constructing Reg-ABE schemes and prove that these properties are preserved through the transformations. This immediately implies that we can obtain Reg-ABE schemes for any predicates derived via predicate transformations.

2023

EUROCRYPT

Unbounded Quadratic Functional Encryption and More from Pairings
Abstract

We propose the first unbounded functional encryption (FE) scheme for quadratic functions and its extension, in which the sizes of messages to be encrypted are not a priori bounded.
Prior to our work, all FE schemes for quadratic functions are bounded, meaning that the message length is fixed at the setup.
In the first scheme, encryption takes $\{x_{i}\}_{i \in S_{c}}$, key generation takes $\{c_{i,j}\}_{i,j \in S_{k}}$, and decryption outputs $\sum_{i,j \in S_{k}} c_{i,j}x_{i}x_{j}$ if and only if $S_{k} \subseteq S_{c}$, where the sizes of $S_{c}$ and $S_{k}$ can be arbitrary.
Our second scheme is the extension of the first scheme to partially-hiding FE that computes an arithmetic branching program on a public input and a quadratic function on a private input.
Concretely, encryption takes a public input $\vec{u}$ in addition to $\{x_{i}\}_{i \in S_{c}}$, a secret key is associated with arithmetic branching programs $\{f_{i,j}\}_{i,j \in S_{k}}$, and decryption yields $\sum_{i,j \in S_{k}} f_{i,j}(\vec{u})x_{i}x_{j}$ if and only if $S_{k} \subseteq S_{c}$.
Both our schemes are based on pairings and secure in the simulation-based model under the standard MDDH assumption.

2023

CRYPTO

Attribute-Based Multi-Input FE (and more) for Attribute-Weighted Sums
Abstract

Recently, Abdalla, Gong and Wee (Crypto 2020) provided the first functional encryption scheme for attribute-weighted sums (AWS), where encryption takes as input $N$ (unbounded) attribute-value pairs $\Set{x_i, z_i}_{i \in [N]}$ where $x_i$ is public and $z_i$ is private, the secret key is associated with an arithmetic branching programs $f$, and decryption returns the weighted sum ${\sum}_{{i \in [N]}} f(x_i)^\top z_i$, leaking no additional information about the $z_i$'s.
We extend FE for AWS to the significantly more challenging multi-party setting and provide the first construction for {\it attribute-based} multi-input FE (MIFE) supporting AWS. For $i \in [n]$, encryptor $i$ can choose an attribute $y_i$ together with AWS input $\Set{x_{i,j}, z_{i,j}}$ where $j \in [N_i]$ and $N_i$ is unbounded, the key generator can choose an access control policy $g_i$ along with its AWS function $h_i$ for each $i \in [n]$, and the decryptor can compute
$$\sum_{i \in [n]} \sum_{j \in [N_{i}]}h_{i}(x_{i,j})^\top z_{i,j} \text{ iff } g_{i}(y_{i}) =0 \text{ for all } i \in [n]$$
Previously, the only known attribute based MIFE was for the inner product functionality (Abdalla \etal~Asiacrypt 2020), where additionally, $y_i$ had to be fixed during setup and must remain the same for all ciphertexts in a given slot.
Our attribute based MIFE implies the notion of multi-input attribute based encryption (MIABE) recently studied by Agrawal, Yadav and Yamada (Crypto 2022) and Francati, Friolo, Malavolta and Venturi (Eurocrypt 2023), for a conjunction of predicates represented as arithmetic branching programs (ABP).
Along the way, we also provide the first constructions of multi-client FE (MCFE) and dynamic decentralized FE (DDFE) for the AWS functionality. Previously, the best known MCFE and DDFE schemes were for inner products (Chotard \etal~ePrint 2018, Abdalla, Benhamouda and Gay, Asiacrypt 2019, and Chotard \etal~Crypto 2020).

2022

TCC

Multi-Input Quadratic Functional Encryption: Stronger Security, Broader Functionality
Abstract

Multi-input functional encryption, MIFE, is a powerful generalization of functional encryption that allows computation on encrypted data coming from multiple different data sources. In a recent work, Agrawal, Goyal, and Tomida (CRYPTO 2021) constructed MIFE for the class of quadratic functions. This was the first MIFE construction from bilinear maps that went beyond inner product computation. We advance the state-of-the-art in MIFE, and propose new constructions with stronger security and broader functionality.
• Stronger Security: In the typical formulation of MIFE security, an attacker is allowed to either corrupt all or none of the users who can encrypt the data. In this work, we study MIFE security in a stronger and more natural model where we allow an attacker to corrupt any subset of the users, instead of only permitting all-or-nothing corruption. We formalize the model by providing each user a unique encryption key, and letting the attacker corrupt all non-trivial subsets of the encryption keys, while still maintaining the MIFE security for ciphertexts generated using honest keys. We construct a secure MIFE system for quadratic functions in this fine-grained corruption model from bilinear maps. Our construction departs significantly from the existing MIFE schemes as we need to tackle a more general class of attackers.
• Broader Functionality: The notion of multi-client functional encryption, MCFE, is a useful extension of MIFE. In MCFE, each encryptor can additionally tag each ciphertext with appropriate metadata such that ciphertexts with only matching metadata can be decrypted together. In more detail, each ciphertext is now annotated with a unique label such that ciphertexts encrypted for different slots can now only be combined together during decryption as long as the associated labels are an exact match for all individual ciphertexts. In this work, we upgrade our MIFE scheme to also support ciphertext labelling. While the functionality of our scheme matches that of MCFE for quadratic functions, our security guarantee falls short of the general corruption model studied for MCFE. In our model, all encryptors share a secret key, therefore this yields a secret-key version of quadratic MCFE, which we denote by SK-MCFE. We leave the problem of proving security in the general corruption model as an important open problem.

2021

CRYPTO

Multi-Input Quadratic Functional Encryption from Pairings
📺
Abstract

We construct the first multi-input functional encryption (MIFE) scheme for quadratic functions from pairings. Our construction supports polynomial number of users, where user $i$, for $i \in [n]$, encrypts input $\bfx_i \in \mbZ^m$ to obtain ciphertext $\ct_i$, the key generator provides a key $\sk_\bfc$ for vector $\bfc \in \mbZ^{({mn})^2}$ and decryption, given $\ct_1,\ldots,\ct_n$ and $\sk_\bfc$, recovers $\ip{\bfc}{\bfx \otimes \bfx}$ and nothing else. We achieve indistinguishability-based (selective) security against unbounded collusions under the standard bilateral matrix Diffie-Hellman assumption. All previous MIFE schemes either support only inner products (linear functions) or rely on strong cryptographic assumptions such as indistinguishability obfuscation or multi-linear maps.

2021

TCC

Multi-Party Functional Encryption
📺
Abstract

We initiate the study of multi-party functional encryption (MPFE) which unifies and abstracts out various notions of functional encryption which support distributed ciphertexts or secret keys, such as multi-input FE, multi-client FE, decentralized multi-client FE, multi-authority FE, dynamic decentralized FE, adhoc multi-input FE and such others. Using our framework, we identify several gaps in the literature and provide some constructions to fill these:
1. Multi-Authority ABE with Inner Product Computation. The recent work of Abdalla et al. (ASIACRYPT’20) constructed a novel “composition” of Attribute Based Encryption (ABE) and Inner Product Functional Encryption (IPFE), namely functional encryption schemes that combine the access control functionality of attribute based encryption with the possibility of performing linear operations on the encrypted data. In this work, we extend the access control component to support the much more challenging multi-authority setting, i.e. “lift” the primitive of ABE in their construction to multi-authority ABE for the same class of access control policies (LSSS structures). This yields the first construction of a nontrivial multi-authority FE beyond ABE from simple assumptions on pairings to the best of our knowledge.
Our techniques can also be used to generalize the decentralized attribute based encryption scheme of Michalevsky and Joye (ESORICS’18) to support inner product computation on the message. While this scheme only supports inner product predicates which is less general than those supported by the Lewko-Waters (EUROCRYPT’11) construction, it supports policy hiding which the latter does not. Our extension inherits these features and is secure based on the k-linear assumption, in the random oracle model.
2. Function Hiding DDFE. The novel primitive of dynamic decentralized functional encryption (DDFE) was recently introduced by Chotard et al. (CRYPTO’20), where they also provided the first construction for inner products. However, the primitive of DDFE does not support function hiding, which is a significant limitation for several applications. In this work, we provide a new construction for inner product DDFE which supports function hiding. To achieve our final result, we define and construct the first function hiding multi-client functional encryption (MCFE) scheme for inner products, which may be of independent interest.
3. Distributed Ciphertext-Policy ABE. We provide a distributed variant of the recent ciphertext- policy attribute based encryption scheme, constructed by Agrawal and Yamada (EUROCRYPT’20). Our construction supports NC1 access policies, and is secure based on “Learning With Errors” and relies on the generic bilinear group model as well as the random oracle model.
Our new MPFE abstraction predicts meaningful new variants of functional encryption as useful targets for future work.

2020

PKC

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

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

2020

ASIACRYPT

Unbounded Dynamic Predicate Compositions in ABE from Standard Assumptions
📺
Abstract

At Eurocrypt'19, Attrapadung presented several transformations that dynamically compose a set of attribute-based encryption (ABE) schemes for simpler predicates into a new ABE scheme for more expressive predicates. Due to the powerful unbounded and modular nature of his compositions, many new ABE schemes can be obtained in a systematic manner (including those that resolved some open problems at the time). However, his approach heavily relies on so-called $q$-type assumptions, which are not standard. Devising such powerful compositions from standard assumptions was left as an important open problem.
In this paper, we present a new framework for constructing ABE schemes that allow unbounded and dynamic predicate compositions among them, and show that the adaptive security of these composed ABE will be preserved by relying only on the standard matrix Diffie-Hellman (MDDH) assumption. This thus resolves the open problem posed by Attrapadung.
As for applications, we obtain various ABEs that are the first such instantiations of their kinds from standard assumptions. These include the following adaptively secure \emph{large-universe} ABEs for Boolean formulae under MDDH:
- The first completely unbounded monotone key-policy (KP)/ciphertext-policy (CP) ABE. Previously, such ABE has been only recently proposed, but only for the KP and \emph{small-universe} flavor (Kowalczyk and Wee, Eurocrypt'19).
- The first completely unbounded non-monotone KP/CP-ABE. Especially, our ABEs support a new type of non-monotonicity that subsumes previous two types of non-monotonicity, namely, by Ostrovsky et al. (CCS'07) and by Okamoto and Takashima (CRYPTO'10).
- The first non-monotone KP and CP-ABE with constant-size ciphertexts and secret keys, respectively.
- The first monotone KP and CP-ABE with constant-size secret keys and ciphertexts, respectively.
At the core of our framework lies a new \emph{partially symmetric} design of the core 1-key 1-ciphertext oracle component called Key Encoding Indistinguishability, which exploits the symmetry so as to obtain compositions.

2019

ASIACRYPT

Tightly Secure Inner Product Functional Encryption: Multi-input and Function-Hiding Constructions
Abstract

Tightly secure cryptographic schemes have been extensively studied in the fields of chosen-ciphertext secure public-key encryption, identity-based encryption, signatures and more. We extend tightly secure cryptography to inner product functional encryption (IPFE) and present the first tightly secure schemes related to IPFE.We first construct a new IPFE scheme that is tightly secure in the multi-user and multi-challenge setting. In other words, the security of our scheme does not degrade even if an adversary obtains many ciphertexts generated by many users. Our scheme is constructible on a pairing-free group and secure under the matrix decisional Diffie-Hellman (MDDH) assumption, which is the generalization of the decisional Diffie-Hellman (DDH) assumption. Applying the known conversions by Lin (CRYPTO 2017) and Abdalla et al. (CRYPTO 2018) to our scheme, we can obtain the first tightly secure function-hiding IPFE scheme and multi-input IPFE (MIPFE) scheme respectively.Our second main contribution is the proposal of a new generic conversion from function-hiding IPFE to function-hiding MIPFE, which was left as an open problem by Abdalla et al. (CRYPTO 2018). We obtain the first tightly secure function-hiding MIPFE scheme by applying our conversion to the tightly secure function-hiding IPFE scheme described above.Finally, the security reductions of all our schemes are fully tight, which means that the security of our schemes is reduced to the MDDH assumption with a constant security loss.

2018

PKC

Full-Hiding (Unbounded) Multi-input Inner Product Functional Encryption from the k-Linear Assumption
Abstract

This paper presents two non-generic and practically efficient private key multi-input functional encryption (MIFE) schemes for the multi-input version of the inner product functionality that are the first to achieve simultaneous message and function privacy, namely, the full-hiding security for a non-trivial multi-input functionality under well-studied cryptographic assumptions. Our MIFE schemes are built in bilinear groups of prime order, and their security is based on the standard k-Linear (k-LIN) assumption (along with the existence of semantically secure symmetric key encryption and pseudorandom functions). Our constructions support polynomial number of encryption slots (inputs) without incurring any super-polynomial loss in the security reduction. While the number of encryption slots in our first scheme is apriori bounded, our second scheme can withstand an arbitrary number of encryption slots. Prior to our work, there was no known MIFE scheme for a non-trivial functionality, even without function privacy, that can support an unbounded number of encryption slots without relying on any heavy-duty building block or little-understood cryptographic assumption.

2018

ASIACRYPT

Unbounded Inner Product Functional Encryption from Bilinear Maps
Abstract

Inner product functional encryption (IPFE), introduced by Abdalla et al. (PKC2015), is a kind of functional encryption supporting only inner product functionality. All previous IPFE schemes are bounded schemes, meaning that the vector length that can be handled in the scheme is fixed in the setup phase. In this paper, we propose the first unbounded IPFE schemes, in which we do not have to fix the lengths of vectors in the setup phase and can handle (a priori) unbounded polynomial lengths of vectors. Our first scheme is private-key based and fully function hiding. That is, secret keys hide the information of the associated function. Our second scheme is public-key based and provides adaptive security in the indistinguishability based security definition. Both our schemes are based on SXDH, which is a well-studied standard assumption, and secure in the standard model. Furthermore, our schemes are quite efficient, incurring an efficiency loss by only a small constant factor from previous bounded function hiding schemes.

#### Program Committees

- Asiacrypt 2024

#### Coauthors

- Shweta Agrawal (4)
- Nuttapong Attrapadung (2)
- Pratish Datta (1)
- Rishab Goyal (3)
- Yuto Kawahara (1)
- Ryo Nishimaki (1)
- Tatsuaki Okamoto (1)
- Katsuyuki Takashima (1)
- Junichi Tomida (11)
- Anshu Yadav (1)