International Association
for Cryptologic Research

A universal thresholdizer (UT), constructed from a threshold fully homomorphic encryption by Boneh et. al, Crypto 2018, is a general framework for universally thresholdizing many cryptographic schemes. However, their framework is insufficient to construct strongly secure threshold schemes, such as threshold signatures and threshold public-key encryption, etc. In this paper, we strengthen the security definition for a universal thresholdizer and propose a scheme which satisfies our stronger security notion. Our UT scheme is an improvement of Boneh et. al ’s construction in the level of threshold fully homomorphic encryption using a key homomorphic pseudorandom function. We apply our strongly secure UT scheme to construct strongly secure threshold signatures and threshold public-key encryption.

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.

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).

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.

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.}