Registered attribute-based encryption (ABE) is a generalization of public-key encryption that enables fine-grained access control to encrypted data (like standard ABE), but without needing a central trusted authority. In a key-policy registered ABE scheme, users choose their own public and private keys and then register their public keys together with a decryption policy with an (untrusted) key curator. The key curator aggregates all of the individual public keys into a short master public...

Traceable Receipt-free Encryption (TREnc) has recently been introduced as a verifiable public-key encryption primitive endowed with a unique security model. In a nutshell, TREnc allows randomizing ciphertexts in transit in order to remove any subliminal information up to a public trace that ensures the non-malleability of the underlying plaintext. A remarkable property of TREnc is the indistinguishability of the randomization of chosen ciphertexts against traceable chosen-ciphertext attacks...

Multi-input functional encryption is a primitive that allows for the evaluation of an $\ell$-ary function over multiple ciphertexts, without learning any information about the underlying plaintexts. This type of computation is useful in many cases where one has to compute over encrypted data, such as privacy-preserving cloud services, federated learning, or more generally delegation of computation from multiple clients. It has recently been shown by Alborch et al. in PETS '24 to be useful to...

Quantum information allows us to build quantum money schemes, where a bank can issue banknotes in the form of authenticatable quantum states that cannot be cloned or counterfeited: a user in possession of k banknotes cannot produce k +1 banknotes. Similar to paper banknotes, in existing quantum money schemes, a banknote consists of an unclonable quantum state and a classical serial number, signed by bank. Thus, they lack one of the most fundamental properties cryptographers look for in a...

We introduce the notion of pseudorandom obfuscation (PRO), a way to obfuscate (keyed) pseudorandom functions $f_K$ in an average-case sense. We introduce several variants of pseudorandom obfuscation and show constructions and applications. For some of our applications that can be achieved using full-fledged indistinguishability obfuscation (iO), we show constructions using lattice-based assumptions alone; the other applications we enable using PRO are simply not known even assuming iO. We...

We provide the first construction of compact Functional Encryption (FE) for pseudorandom functionalities from the evasive LWE and LWE assumptions. Intuitively, a pseudorandom functionality means that the output of the circuit is indistinguishable from uniform for every input seen by the adversary. This yields the first compact FE for a nontrivial class of functions which does not rely on pairings. We demonstrate the power of our new tool by using it to achieve optimal parameters for both...

In a functional encryption (FE) scheme, a user that holds a ciphertext and a function-key can learn the result of applying the function to the plaintext message. Security requires that the user does not learn anything beyond the function evaluation. On the other hand, unclonable encryption (UE) is a uniquely quantum primitive, which ensures that an adversary cannot duplicate a ciphertext to decrypt the same message multiple times. In this work we introduce unclonable quantum...

As an emerging primitive, Registered Functional Encryption (RFE) eliminates the key-escrow issue that threatens numerous works for functional encryption, by replacing the trusted authority with a transparent key curator and allowing each user to sample their decryption keys locally. In this work, we present a new black-box approach to construct RFE for all polynomial-sized circuits. It considers adaptive simulation-based security in the bounded collusion model (Gorbunov et al. - CRYPTO'12),...

In this paper, we introduce the notion of relaxed lattice-based programmable hash function (RPHF), which is a novel variant of lattice-based programmable hash functions (PHFs). Lattice-based PHFs, together with preimage trapdoor functions (TDFs), have been widely utilized (implicitly or explicitly) in the construction of adaptively secure identity-based encryption (IBE) schemes. The preimage length and the output length of the underlying PHF and TDF together determine the user secret key and...

Signature-based witness encryption (SWE) is a recently proposed notion that allows to encrypt a message with respect to a tag $T$ and a set of signature verification keys. The resulting ciphertext can only be decrypted by a party who holds at least $k$ different valid signatures w.r.t. $T$ and $k$ different verification keys out of the $n$ keys specified at encryption time. Natural applications of this primitive involve distributed settings (e.g., blockchains), where multiple parties sign...

A broadcast encryption scheme allows a user to encrypt a message to $N$ recipients with a ciphertext whose size scales sublinearly with $N$. While broadcast encryption enables succinct encrypted broadcasts, it also introduces a strong trust assumption and a single point of failure; namely, there is a central authority who generates the decryption keys for all users in the system. Distributed broadcast encryption offers an appealing alternative where there is a one-time (trusted) setup...

Traceable Receipt-free Encryption (TREnc) is a verifiable public-key encryption primitive introduced at Asiacrypt 2022. A TREnc allows randomizing ciphertexts in transit in order to remove any subliminal information up to a public trace that ensures the non-malleability of the underlying plaintext. A remarkable property of TREnc is the indistinguishability of the randomization of chosen ciphertexts against traceable chosen-ciphertext attacks (TCCA). This property can support applications...

Strike-lists are a common technique for rollback and replay prevention in protocols that require that clients remain anonymous or that their current position in a state machine remain confidential. Strike-lists are heavily used in anonymous credentials, e-cash schemes, and trusted execution environments, and are widely deployed on the web in the form of Privacy Pass (PoPETS '18) and Google Private State Tokens. In such protocols, clients submit pseudorandom tokens associated with each...

Internet of Medical Things (IoMT) has gained significant research focus in both academic and medical institutions. Nevertheless, the sensitive data involved in IoMT raises concerns regarding user validation and data privacy. To address these concerns, certificateless signcryption (CLSC) has emerged as a promising solution, offering authenticity, confidentiality, and unforgeability. Unfortunately, most existing CLSC schemes are impractical for IoMT due to their heavy computational and storage...

In this research, we introduce MIND-Crypt, a novel attack framework that uses deep learning (DL) and transfer learning (TL) to challenge the indistinguishability of block ciphers, specifically SPECK32/64 encryption algorithm in CBC mode (Cipher Block Chaining) against Known Plaintext Attacks (KPA). Our methodology includes training a DL model with ciphertexts of two messages encrypted using the same key. The selected messages have the same byte-length and differ by only one bit at the binary...

This paper introduces the first adaptively secure streaming functional encryption (sFE) scheme for P/Poly. sFE stands as an evolved variant of traditional functional encryption (FE), catering specifically to contexts with vast and/or dynamically evolving data sets. sFE is designed for applications where data arrives in a streaming fashion and is computed on in an iterative manner as the stream arrives. Unlike standard FE, in sFE: (1) encryption is possible without knowledge of the full data...

We build a concretely efficient threshold encryption scheme where the joint public key of a set of parties is computed as a deterministic function of their locally computed public keys, enabling a silent setup phase. By eliminating interaction from the setup phase, our scheme immediately enjoys several highly desirable features such as asynchronous setup, multiverse support, and dynamic threshold. Prior to our work, the only known constructions of threshold encryption with silent setup...

We focus on the problem of constructing fully homomorphic encryption (FHE) schemes that achieve some meaningful notion of adaptive chosen-ciphertext security beyond CCA1. Towards this, we propose a new notion, called security against verified chosen-ciphertext attack (vCCA). The idea behind it is to ascertain integrity of the ciphertext by imposing a strong control on the evaluation algorithm. Essentially, we require that a ciphertext obtained by the use of homomorphic evaluation must be...

Traitor-tracing systems allow identifying the users who contributed to building a rogue decoder in a broadcast environment. In a traditional traitor-tracing system, a key authority is responsible for generating the global public parameters and issuing secret keys to users. All security is lost if the \emph{key authority itself} is corrupt. This raises the question: Can we construct a traitor-tracing scheme, without a trusted authority? In this work, we propose a new model for...

Although we have known about fully homomorphic encryption (FHE) from circular security assumptions for over a decade [Gentry, STOC '09; Brakerski–Vaikuntanathan, FOCS '11], there is still a significant gap in understanding related homomorphic primitives supporting all *unrestricted* polynomial-size computations. One prominent example is attribute-based encryption (ABE). The state-of-the-art constructions, relying on the hardness of learning with errors (LWE) [Gorbunov–Vaikuntanathan–Wee,...

Circuit privacy is an important notion in Fully Homomorphic Encryption (FHE), well-illustrated by the Machine Learning-as-a-Service scenario. A scheme is circuit private (first defined in Gentry’s PhD Thesis) if an adversary cannot learn the circuit evaluated on a ciphertext from the computation result. In this work, we first show that the BGV FHE scheme by Brakerski, Gentry and Vaikuntanathan (ITCS’12) is computationally circuit private in a semi-honest context, and then present an extended...

We revisit OCAKE (ACNS 23), a generic recipe that constructs password-based authenticated key exchange (PAKE) from key encapsulation mechanisms (KEMs), to allow instantiations with post-quantums KEM like KYBER. The ACNS23 paper left as an open problem to argue security against quantum attackers, with its security proof being in the universal composability (UC) framework. This is common for PAKE, however, at the time of this submission’s writing, it was not known how to prove (computational)...

Incompressibility is a popular security notion for white-box cryptography and captures that a large encryption program cannot be compressed without losing functionality. Fouque, Karpman, Kirchner and Minaud (FKKM) defined strong incompressibility, where a compressed program should not even help to distinguish encryptions of two messages of equal length. Equivalently, the notion can be phrased as indistinguishability under chosen-plaintext attacks and key-leakage (LK-IND-CPA), where the...

Distributed broadcast encryption (DBE) improves on the traditional notion of broadcast encryption by eliminating the key-escrow problem: In a DBE system, users generate their own secret keys non- interactively without the help of a trusted party. Then anyone can broadcast a message for a subset S of the users, in such a way that the resulting ciphertext size is sublinear in (and, ideally, independent of) |S|. Unfortunately, the only known constructions of DBE requires heavy cryptographic...

Confidentiality, authentication, and anonymity are the basic security requirements in broadcast communication, that can be achieved by Digital Signature (DS), encryption, and pseudo-identity (PID) techniques. Signcryption offers both DS and encryption more efficiently than "sign-then-encrypt,". However, compared to hybrid signcryption, it has higher computational and communication costs. Our paper proposes an Anonymous Multi-receiver Certificateless Hybrid Signcryption (AMCLHS) for secure...

Deniable encryption (Canetti et al. CRYPTO ’97) is an intriguing primitive, which provides security guarantee against coercion by allowing a sender to convincingly open the ciphertext into a fake message. Despite the notable result by Sahai and Waters STOC ’14 and other efforts in functionality extension, all the deniable public key encryption (DPKE) schemes suffer from intolerable overhead due to the heavy building blocks, e.g., translucent sets or indistinguishability obfuscation. Besides,...

Laconic function evaluation (LFE) allows Alice to compress a large circuit $\mathbf{C}$ into a small digest $\mathsf{d}$. Given Alice's digest, Bob can encrypt some input $x$ under $\mathsf{d}$ in a way that enables Alice to recover $\mathbf{C}(x)$, without learning anything beyond that. The scheme is said to be $laconic$ if the size of $\mathsf{d}$, the runtime of the encryption algorithm, and the size of the ciphertext are all sublinear in the size of $\mathbf{C}$. Until now, all...

Predicate inner product functional encryption (P-IPFE) is essentially attribute-based IPFE (AB-IPFE) which additionally hides attributes associated to ciphertexts. In a P-IPFE, a message x is encrypted under an attribute w and a secret key is generated for a pair (y, v) such that recovery of ⟨x, y⟩ requires the vectors w, v to satisfy a linear relation. We call a P-IPFE unbounded if it can encrypt unbounded length attributes and message vectors. • zero predicate IPFE. We construct the first...

Non-malleability is one of the basic security goals for encryption schemes which ensures the resistance of the scheme against ciphertext modifications in the sense that any adversary, given a ciphertext of a plaintext, cannot generate another ciphertext whose underlying plaintext is meaningfully related to the initial one. There are multiple formulations of non-malleable encryption schemes, depending on whether they are based on simulation or comparison, or whether they impose valid...

This paper introduces the first registered functional encryption RFE scheme tailored for linear functions. Distinctly different from classical functional encryption (FE), RFE addresses the key-escrow issue and negates the master key exfiltration attack. Instead of relying on a centralized trusted authority, it introduces a “key curator” - a fully transparent entity that does not retain secrets. In an RFE framework, users independently generate secret keys and subsequently register their...

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...

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...

The learning with errors (LWE) assumption is a powerful tool for building encryption schemes with useful properties, such as plausible resistance to quantum computers, or support for homomorphic computations. Despite this, essentially the only method of achieving threshold decryption in schemes based on LWE requires a modulus that is superpolynomial in the security parameter, leading to a large overhead in ciphertext sizes and computation time. In this work, we propose a (fully...

Authenticated encryption with associated data (AEAD) forms the core of much of symmetric cryptography, yet the standard techniques for modeling AEAD assume recipients have no ambiguity about what secret key to use for decryption. This is divorced from what occurs in practice, such as in key management services, where a message recipient can store numerous keys and must identify the correct key before decrypting. To date there has been no formal investigation of their security properties or...

We propose and analyze LowMS, a new rank-based key encapsulation mechanism (KEM). The acronym stands for Loidreau with Multiple Syndromes, since our work combines the cryptosystem of Loidreau (presented at PQCrypto 2017) together with the multiple syndrome approach, that allows to reduce parameters by sending several syndromes with the same error support in one ciphertext. Our scheme is designed without using ideal structures. Considering cryptosystems without such an ideal structure,...

This paper presents the first functional encryption (FE) scheme for the attribute-weighted sum (AWS) functionality that supports the uniform model of computation. In such an FE scheme, encryption takes as input a pair of attributes (x,z) where the attribute x is public while the attribute z is private. A secret key corresponds to some weight function f, and decryption recovers the weighted sum f(x)z. This is an important functionality with a wide range of potential real life applications,...

Extending work leveraging program obfuscation to instantiate random-oracle-based transforms (e.g., Hohenberger et al., EUROCRYPT 2014, Kalai et al., CRYPTO 2017), we show that, using obfuscation and other assumptions, there exist standard-model hash functions that suffice to instantiate the classical RO-model encryption transforms OAEP (Bellare and Rogaway, EUROCRYPT 1994) and Fujisaki-Okamoto (CRYPTO 1999, J. Cryptology 2013) for specific public-key encryption (PKE) schemes to achieve...

Fully Homomorphic Encryption (FHE) promises to secure our data on the untrusted cloud, while allowing arbitrary computations. Recent research has shown two side channel attacks on the client side running a popular HE library. However, no side channel attacks have yet been reported on the server side in existing literature. The current paper shows that it is possible for adversaries to inject perturbations in the ciphertexts stored in the cloud to result in decryption errors. Most...

Attribute-based encryption (ABE) generalizes public-key encryption and enables fine-grained control to encrypted data. However, ABE upends the traditional trust model of public-key encryption by requiring a single trusted authority to issue decryption keys. If an adversary compromises the central authority and exfiltrates its secret key, then the adversary can decrypt every ciphertext in the system. This work introduces registered ABE, a primitive that allows users to generate secret keys...

A 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...

We propose a novel variant of functional encryption which supports ciphertext updates, dubbed ciphertext-updatable functional encryption (CUFE). Such a feature further broadens the practical applicability of the functional-encryption paradigm and allows for fine-grained access control even after a ciphertext is generated. Updating ciphertexts is carried out via so-called update tokens which a dedicated party can use to convert ciphertexts. However, allowing update tokens requires some care...

We say a public-key encryption is plaintext-extractable in the random oracle model if there exists an algorithm that given access to all inputs/outputs queries to the random oracles can simulate the decryption oracle. We argue that plaintext-extractability is enough to show the indistinguishably under chosen ciphertext attack (IND-CCA) of OAEP+ transform (Shoup, Crypto 2001) when the underlying trapdoor permutation is one-way. We extend the result to the quantum random oracle model...

We introduce a new notion of public key encryption, knowledge encryption, for which its ciphertexts can be reduced to the public-key, i.e., any algorithm that can break the ciphertext indistinguishability can be used to extract the (partial) secret key. We show that knowledge encryption can be built solely on any two-round oblivious transfer with game-based security, which are known based on various standard (polynomial-hardness) assumptions, such as the DDH, the Quadratic($N^{th}$)...

The qINDqCPA security notion for public-key encryption schemes by Gagliardoni et al. (PQCrypto'21) models security against adversaries which are able to obtain ciphertexts in superposition. Defining this security notion requires a special type of quantum operator. Known constructions differ in which keys are necessary to construct this operator, depending on properties of the encryption scheme. We argue—for the typical setting of securing communication between Alice and Bob—that in order to...

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. Formalizing Security: We provide definitions for ${\sf miABE}$ and ${\sf miPE}$ in the {symmetric} key setting and formalize security in the standard indistinguishability (IND) paradigm, against unbounded collusions. 2. Two-input ${\sf ABE}$ for ${\sf NC}_1$...

In this paper, we formalize the plaintext-awareness notion in the superposition access model in which a quantum adversary may implement the encryption oracle in a quantum device and make superposition queries to the decryption oracle. Due to various possible ways an adversary can access the decryption oracles, we present six security definitions to capture the plaintext-awareness notion with respect to each way of access. We study the relationships between these definitions and present...

Recent work of Li and Micciancio (Eurocrypt 2021) has shown that the traditional formulation of indistinguishability under chosen plaintext attack (INDCPA) is not adequate to capture the security of approximate homomorphic encryption against passive adversaries, and identified a stronger INDCPA^D security definition (INDCPA with decryption oracles) as the appropriate security target for approximate encryption schemes. We show how to any approximate homomorphic encryption scheme achieving...

Incompressible encryption, recently proposed by Guan, Wichs and Zhandry (EUROCRYPT'22), is a novel encryption paradigm geared towards providing strong long-term security guarantees against adversaries with bounded long-term memory. Given that the adversary forgets just a small fraction of a ciphertext, this notion provides strong security for the message encrypted therein, even if, at some point in the future, the entire secret key is exposed. This comes at the price of having potentially...

Selective opening (SO) security is one of the most important security notions of public key encryption (PKE) in a multi-user setting. Even though messages and random coins used in some ciphertexts are leaked, SO security guarantees the confidentiality of the other ciphertexts. Actually, it is shown that there exist PKE schemes which meet the standard security such as indistinguishability against chosen ciphertext attacks (IND-CCA security) but do not meet SO security against chosen...

We consider threshold Computational Secret Sharing Schemes, i.e., such that the secret can be recovered from any $t+1$ out of $n$ shares, and such that no computationally bounded adversary can distinguish between $t$ shares of a chosen secret and a uniform string. We say that such a scheme has Constant Size (CSSS) if, in the asymptotic regime of many shares of small size the security parameter, then the total size of shares reaches the minimum, which is the size of an erasures-correction...

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...

Updatable Encryption (UE) and Proxy Re-encryption (PRE) allow re-encrypting a ciphertext from one key to another in the symmetric-key and public-key settings, respectively, without decryption. A longstanding open question has been the following: do unidirectional UE and PRE schemes (where ciphertext re-encryption is permitted in only one direction) necessarily require stronger/more structured assumptions as compared to their bidirectional counterparts? Known constructions of UE and PRE seem...

In this work, we study the security of sponge-based authenticated encryption schemes against quantum attackers. In particular, we analyse the sponge-based authenticated encryption scheme SLAE as put forward by Degabriele et al. (ASIACRYPT'19). We show that the scheme achieves security in the post-quantum (QS1) setting in the quantum random oracle model by using the one-way to hiding lemma. Furthermore, we analyse the scheme in a fully-quantum (QS2) setting. There we provide a set of attacks...

(Fully) homomorphic encryption ((F)HE) allows users to publicly evaluate circuits on encrypted data. Although publicly homomorphic evaluation property has various applications, (F)HE cannot achieve security against chosen ciphertext attacks (CCA2) due to its nature. To achieve both the CCA2 security and homomorphic evaluation property, Emura et al. (PKC 2013) introduced keyed-homomorphic public key encryption (KH-PKE) and formalized its security denoted by $\mathsf{KH\textup{-}CCA}$...

Incompressible encryption allows us to make the ciphertext size flexibly large and ensures that an adversary learns nothing about the encrypted data, even if the decryption key later leaks, unless she stores essentially the entire ciphertext. Incompressible signatures can be made arbitrarily large and ensure that an adversary cannot produce a signature on any message, even one she has seen signed before, unless she stores one of the signatures essentially in its entirety. In this work, we...

This paper presents the first adaptively simulation secure functional encryption (FE) schemes for attribute-weighted sums. In such an FE scheme, encryption takes as input N pairs of attribute {(x_i, z_i )}_{i \in [N]} for some N \in \mathbb{N} where the attributes {x_i}_{i \in [N]} are public while the attributes {z_i}_{i \in [N]} are private. The indices i \in [N] are referred to as the slots. A secret key corresponds to some weight function f, and decryption recovers the weighted sum...

In the last decades, several signcryption schemes have been proposed for different privacy-enhancing purposes. In this paper, we propose a new privacy-enhancing group signcryption scheme that provides: unforgeability, confidentiality, ciphertext and sender anonymity, traceability, unlinkability, exculpability, coalition-resistance, and unforgeable tracing verification. It is important to notice that the proposed scheme allows a signer to anonymously signcryt a message on the group's behalf...

With the rapid development of cloud computing, an increasing number of companies are adopting cloud storage technology to reduce overhead. However, to ensure the privacy of sensitive data, the uploaded data need to be encrypted before being outsourced to the cloud. The concept of public-key encryption with keyword search (PEKS) was introduced by Boneh \textit{et al.} to provide flexible usage of the encrypted data. Unfortunately, most of the PEKS schemes are not secure against inside...

Code-based public key cryptosystems are one of the main techniques available in the area of Post-Quantum Cryptography. This work aims to propose a key encapsulation mechanism (KEM) with short ciphertext and secret key. Our goal is achieved in two steps. We first present a public key encryption (PKE) scheme, basicPKE, using a parity check matrix of Maximum Distance Separable (MDS) code as the public key matrix. In our construction, we exploit the structure of a companion matrix to obtain an...

Homomorphic encryption (HE) is a useful variant of public key encryption (PKE), but it has a drawback that HE cannot fully achieve IND-CCA2 security, which is a standard security notion for PKE. Beyond existing HE schemes achieving weaker IND-CCA1 security, Emura et al.\ (PKC 2013) proposed the notion of \lq\lq keyed\rq\rq{} version of HE, called KH-PKE, which introduces an evaluation key controlling the functionality of homomorphic operations and achieves security stronger than IND-CCA1...

Broadbent and Islam (TCC '20) proposed a quantum cryptographic primitive called quantum encryption with certified deletion. In this primitive, a receiver in possession of a quantum ciphertext can generate a classical certificate that the encrypted message is deleted. Although their construction is information-theoretically secure, it is limited to the setting of one-time symmetric key encryption (SKE), where a sender and receiver have to share a common key in advance and the key can be used...

Functional encryption generates sophisticated keys for users so that they can learn specific functions of the encrypted message. We provide a generic construction of chosen ciphertext attacks (CCA) secure public-key functional encryption (PKFE) for all polynomial-size circuits. Our PKFE produces succinct ciphertexts that are independent of the size and depth of the circuit class under consideration. We accomplish our goal in two steps. First, we define a new cryptographic tool called...

Anonymous routing is one of the most fundamental online privacy problems and has been studied extensively for decades. Almost all known approaches for anonymous routing (e.g., mix-nets, DC-nets, and others) rely on multiple servers or routers to engage in some {\it interactive} protocol; and anonymity is guaranteed in the {\it threshold} model, i.e., if one or more of the servers/routers behave honestly. Departing from all prior approaches, we propose a novel {\it non-interactive}...

Unclonable encryption, introduced by Broadbent and Lord (TQC'20), is an encryption scheme with the following attractive feature: given a ciphertext, an adversary cannot create two ciphertexts both of which decrypt to the same message as the original ciphertext. We revisit this notion and show the following: - Reusability: The constructions proposed by Broadbent and Lord have the disadvantage that they either guarantee one-time security (that is, the encryption key can only be used once to...

We study uncloneable quantum encryption schemes for classical messages as recently proposed by Broadbent and Lord. We focus on the information-theoretic setting and give several limitations on the structure and security of these schemes: Concretely, 1) We give an explicit cloning-indistinguishable attack that succeeds with probability $\frac12 + \mu/16$ where $\mu$ is related to the largest eigenvalue of the resulting quantum ciphertexts. 2) For a uniform message distribution, we partially...

Broadbent and Islam (TCC '20) proposed a quantum cryptographic primitive called quantum encryption with certified deletion. In this primitive, a receiver in possession of a quantum ciphertext can generate a classical certificate that the encrypted message is deleted. Though they proved that their construction is information theoretically secure, a drawback is that the construction is limited to the setting of one-time symmetric key encryption (SKE) where a sender and receiver have to share...

In this paper, we show that OAEP transform is indistinguishable under chosen ciphertext attack in the quantum random oracle model if the underlying trapdoor permutation is quantum partial-domain one-way. The existing post-quantum security of OAEP (TCC 2016-B ) requires a modification to the OAEP transform using an extra hash function. We prove the security of the OAEP transform without any modification and this answers an open question in one of the finalists of NIST competition, NTRU...

The notion of functional encryption (FE) was proposed as a generalization of plain public-key encryption to enable a much more fine-grained handling of encrypted data, with advanced applications such as cloud computing, multi-party computations, obfuscating circuits or Turing machines. While FE for general circuits or Turing machines gives a natural instantiation of the many cryptographic primitives, existing FE schemes are based on indistinguishability obfuscation or multilinear maps which...

Logic access control enforces who can read and write data; the enforcement is typically performed by a fully trusted entity. At TCC 2016, Damg\aa rd et al. proposed Access Control Encryption (ACE) schemes where a predicate function decides whether or not users can read (decrypt) and write (encrypt) data, while the message secrecy and the users' anonymity are preserved against malicious parties. Subsequently, several ACE constructions with an arbitrary identity-based access policy have been...

We define and construct Deniable Fully Homomorphic Encryption based on the Learning With Errors (LWE) polynomial hardness assumption. Deniable FHE enables storing encrypted data in the cloud to be processed securely without decryption, maintaining deniability of the encrypted data, as well the prevention of vote-buying in electronic voting schemes where encrypted votes can be tallied without decryption. Our constructions achieve compactness independently of the level of deniability- both...

In a witness encryption scheme, to decrypt a ciphertext associated with an NP statement, the decrypter takes as input a witness testifying that the statement is in the language. When the statement is not in the language, then the message is hidden. Thus far, the only provably secure constructions assume the existence of indistinguishability obfuscation (iO) and multilinear maps (MMaps). We make progress towards building polynomially efficient witness encryption for NP without resorting to...

The celebrated work of Gorbunov, Vaikuntanathan and Wee 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...

A multi-identity pure fully homomorphic encryption (MIFHE) enables a server to perform arbitrary computation on the ciphertexts that are encrypted under different identities. In case of multi-attribute pure FHE (MAFHE), the ciphertexts are associated with different attributes. Clear and McGoldrick (CANS 2014) gave the first chosen-plaintext attack secure MIFHE and MAFHE based on indistinguishability obfuscation. In this study, we focus on building MIFHE and MAFHE which are se- cure under...

Ciphertext indistinguishability under chosen plaintext attacks is a standard security notion for public key encryption. It crucially relies on the usage of good randomness and is trivially unachievable if the randomness is known by the adversary. Yilek (CT-RSA'10) defined security against resetting attacks, where randomness might be reused but remains unknown to the adversary. Furthermore, Yilek claimed that security against adversaries making a single query to the challenge oracle implies...

We construct the first multi-input functional encryption \allowbreak(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 ...

Boneh et al. proposed the cryptographic primitive public key encryption with keyword search (PEKS) to search on encrypted data without exposing the privacy of the keyword. Most standard PEKS schemes are vulnerable to inside keyword guessing attacks (KGA), i.e., a malicious server may generate a ciphertext by its own and then to guess the keyword of the trapdoor by testing. Huang et al. solved this problem by proposing the public-key authenticated encryption with keyword search (PAEKS)...

Public key encryption with keyword search (PEKS) is first introduced by Boneh et al. enabling a cloud server to search on encrypted data without leaking any information of the keyword. In almost all PEKS schemes, the privacy of trapdoor is vulnerable to inside keyword guessing attacks (KGA), i.e., the server can generate the ciphertext by its own and then run the test algorithm to guess the keyword contained in the trapdoor. To sole this problem, Huang et al. proposed the public-key...

Non-committing encryption (NCE) is a type of public key encryption which comes with the ability to equivocate ciphertexts to encryptions of arbitrary messages, i.e., it allows one to find coins for key generation and encryption which ``explain'' a given ciphertext as an encryption of any message. NCE is the cornerstone to construct adaptively secure multiparty computation [Canetti et al. STOC'96] and can be seen as the quintessential notion of security for public key encryption to realize...

This paper presents the first functional encryption schemes for quadratic functions (or degree-2 polynomials) achieving simulation-based security in the semi-adaptive model with constant-size secret key. The unique prior construction with the same security guarantee by Gay [PKC 20] has secret keys of size linear in the message size. They also enjoy shorter ciphertexts: - our first scheme is based on bilateral DLIN (decisional linear) assumption as Gay's scheme and the ciphertext is 15%...

In this work, we study the question of what set of simple-to-state assumptions suffice for constructing functional encryption and indistinguishability obfuscation (iO), supporting all functions describable by polynomial-size circuits. Our work improves over the state-of-the-art work of Jain, Lin, Matt, and Sahai (Eurocrypt 2019) in multiple dimensions. New Assumption: Previous to our work, all constructions of iO from simple assumptions required novel pseudorandomness generators involving...

Recently, Huang et al. proposed a concept of public key encryption with filtered equality test (PKE-FET) that allows a tester who has a warrant for the selected message set to check equality of messages in ciphertexts that belong to that set (Journal of Computer and System Sciences, 2017). They also presented an instantiation of the PKE-FET that was asserted to achieve the indistinguishability against adaptive chosen ciphertext attacks (IND-CCA2) in the standard model. In this note, we show...

In this work, we introduce the notion of puncturable witness pseudorandom function (pWPRF) which is a stronger variant of WPRF proposed by Zhandry, TCC 2016. The punctured technique is similar to what we have seen for puncturable PRFs and is capable of extending the applications of WPRF. Specifically, we construct a semi-adaptively secure offline witness encryption (OWE) scheme using a pWPRF, an indistinguishability obfuscation (iO) and a symmetric-key encryption (SKE), which enables us to...

We propose a new approach to construct general-purpose indistinguishability obfuscation (iO). Our construction is obtained via a new intermediate primitive that we call split fully-homomorphic encryption (split FHE), which we show to be sufficient for constructing iO. Specifically, split FHE is FHE where decryption takes the following two-step syntactic form: (i) A secret decryption step uses the secret key and produces a hint which is (asymptotically) shorter than the length of the...

We present a new general framework for constructing compact and adaptively secure attribute-based encryption (ABE) schemes from $k$-Lin in asymmetric bilinear pairing groups. Previously, the only construction [Kowalczyk and Wee, Eurocrypt '19] that simultaneously achieves compactness and adaptive security from static assumptions supports policies represented by Boolean formulae. Our framework enables supporting more expressive policies represented by arithmetic branching programs. Our...

Indistinguishability against adaptive chosen-ciphertext attacks (IND-CCA2) is usually considered the most desirable security notion for classical encryption. In this work, we investigate its adaptation in the quantum world, when an adversary can perform superposition queries. The security of quantum-secure classical encryption has first been studied by Boneh and Zhandry (CRYPTO'13), but they restricted the adversary to classical challenge queries, which makes the indistinguishability only...

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. However, his approach heavily relies on $q$-type assumptions, which are not standard. Devising such powerful compositions from standard assumptions...

Inner product functional encryption (IPFE) [1] is a popular primitive which enables inner product computations on encrypted data. In IPFE, the ciphertext is associated with a vector x, the secret key is associated with a vector y and decryption reveals the inner product <x,y>. Previously, it was known how to achieve adaptive indistinguishability (IND) based security for IPFE from the DDH, DCR and LWE assumptions [8]. However, in the stronger simulation (SIM) based security game, it was only...

We propose a candidate ciphertext-policy attribute-based encryption (CP-ABE) scheme for circuits, where the ciphertext size depends only on the depth of the policy circuit (and not its size). This, in particular, gives us a Broadcast Encryption (BE) scheme where the size of the keys and ciphertexts have a poly-logarithmic dependence on the number of users. This goal was previously only known to be achievable assuming ideal multilinear maps (Boneh, Waters and Zhandry, Crypto 2014) or...

We give the first public-key functional encryption that supports the generation of functional decryption keys for degree-2 polynomials, with succinct ciphertexts, whose semi-adaptive simulation-based security is proven under standard assumptions. At the heart of our new paradigm lies a so-called partially function-hiding functional encryption scheme for inner products, which admits public-key instances, and that is sufficient to build functional encryption for degree-2 polynomials. Doing so,...

Characterizing the decoding failure rate of iteratively decoded Low- and Moderate-Density Parity Check (LDPC/MDPC) codes is paramount to build cryptosystems based on them, able to achieve indistinguishability under adaptive chosen ciphertext attacks. In this paper, we provide a statistical worst-case analysis of our proposed iterative decoder obtained through a simple modification of the classic in-place bit-flipping decoder. This worst case analysis allows both to derive the worst-case...

The existence of secure indistinguishability obfuscators ($i\mathcal{O}$) has far-reaching implications, significantly expanding the scope of problems amenable to cryptographic study. A recent line of work [Ananth, Jain, and Sahai, 2018; Aggrawal, 2018; Lin and Matt, 2018; Jain, Lin, Matt, and Sahai, 2019] has developed a new theory for building $i\mathcal{O}$~from simpler building blocks, and represents the state of the art in constructing $i\mathcal{O}$~from succinct and...

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...

In this work, we introduce and construct $D$-restricted Functional Encryption (FE) for any constant $D \ge 3$, based only on the SXDH assumption over bilinear groups. This generalizes the notion of $3$-restricted FE recently introduced and constructed by Ananth et al. (ePrint 2018) in the generic bilinear group model. A $D=(d+2)$-restricted FE scheme is a secret key FE scheme that allows an encryptor to efficiently encrypt a message of the form $M=(\vec{x},\vec{y},\vec{z})$. Here,...

Highly efficient encryption and authentication of short messages is an essential requirement for enabling security in constrained scenarios such as the CAN FD in automotive systems (max. message size 64 bytes), massive IoT, critical communication domains of 5G, and Narrowband IoT, to mention a few. In addition, one of the NIST lightweight cryptography project requirements is that AEAD schemes shall be “optimized to be efficient for short messages (e.g., as short as 8 bytes)”. In this work...

After the work of Impagliazzo and Rudich (STOC, 1989), the black box framework has become one of the main research domain of cryptography. However black box techniques say nothing about non-black box techniques such as making use of zero-knowledge proofs. Brakerski et al. introduced a new black box framework named augmented black box framework, in which they gave a zero-knowledge proof oracle in addition to a base primitive oracle (TCC, 2011). They showed a construction of a non-interactive...

In this paper we put forth new efficient one-message proof systems for several practical applications, like proving that an El Gamal ciphertext (over a multiplicative group) decrypts to a given value and correctness of a shuffle. Our proof systems are built from multiplicative groups of hidden order, are not based on any setup/trust assumption like the RO or the common reference string model and are perfectly sound, that is they are written proofs in the sense of mathematics. Our proof...

We show how to combine a fully-homomorphic encryption scheme with linear decryption and a linearly-homomorphic encryption schemes to obtain constructions with new properties. Specifically, we present the following new results. (1) Rate-1 Fully-Homomorphic Encryption: We construct the first scheme with message-to-ciphertext length ratio (i.e., rate) $1-\sigma$ for $\sigma = o(1)$. Our scheme is based on the hardness of the Learning with Errors (LWE) problem and $\sigma$ is proportional to...

We initiate the study of fully homomorphic encryption for RAMs (RAM-FHE). This is a public-key encryption scheme where, given an encryption of a large database $D$, anybody can efficiently compute an encryption of $P(D)$ for an arbitrary RAM program $P$. The running time over the encrypted data should be as close as possible to the worst case running time of $P$, which may be sub-linear in the data size. A central difficulty in constructing a RAM-FHE scheme is hiding the sequence of memory...

We construct private-key and public-key functional encryption schemes secure against adversaries that corrupt an a-priori bounded number of users and obtain their functional keys, from minimal assumptions. For a collusion bound of $Q=Q(\lambda)$ (where $\lambda$ is the security parameter), our public-key (resp. private-key) functional encryption scheme (a) supports the class of all polynomial-size circuits; (b) can be built solely from a vanilla public-key (resp. private-key) encryption...

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...