A Simple and Generic Approach to Dynamic Collusion Model

Advances in Cryptology – ASIACRYPT 2016, 2016

In light of security challenges that have emerged in a world with complex networks and cloud computing, the notion of functional encryption has recently emerged. In this work, we show that in several applications of functional encryption (even those cited in the earliest works on functional encryption), the formal notion of functional encryption is actually not sufficient to guarantee security. This is essentially because the case of a malicious authority and/or encryptor is not considered. To address this concern, we put forth the concept of verifiable functional encryption, which captures the basic requirement of output correctness: even if the ciphertext is maliciously generated (and even if the setup and key generation is malicious), the decryptor is still guaranteed a meaningful notion of correctness which we show is crucial in several applications. We formalize the notion of verifiable function encryption and, following prior work in the area, put forth a simulation-based and an indistinguishability-based notion of security. We show that simulationbased verifiable functional encryption is unconditionally impossible even in the most basic setting where there may only be a single key and a single ciphertext. We then give general positive results for the indistinguishability setting: a general compiler from any functional encryption scheme into a verifiable functional encryption scheme with the only additional assumption being the Decision Linear Assumption over Bilinear Groups (DLIN). We also give a generic compiler in the secret-key setting for functional encryption which maintains both message privacy and function privacy. Our positive results are general and also apply to other simpler settings such as Identity-Based Encryption, Attribute-Based Encryption and Predicate Encryption. We also give an application of verifiable functional encryption to the recently introduced primitive A.

Advances in Cryptology – EUROCRYPT 2014, 2014

We introduce the problem of Multi-Input Functional Encryption, where a secret key sk f can correspond to an n-ary function f that takes multiple ciphertexts as input. We formulate both indistinguishability-based and simulation-based definitions of security for this notion, and show close connections with indistinguishability and virtual black-box definitions of obfuscation. Assuming indistinguishability obfuscation for circuits, we present constructions achieving indistinguishability security for a large class of settings. We show how to modify this construction to achieve simulationbased security as well, in those settings where simulation security is possible.

Proceedings of the 2014 ACM SIGSAC Conference on Computer and Communications Security, 2014

Motivated by privacy and usability requirements in various scenarios where existing cryptographic tools (like secure multi-party computation and functional encryption) are not adequate, we introduce a new cryptographic tool called Controlled Functional Encryption (C-FE). As in functional encryption, C-FE allows a user (client) to learn only certain functions of encrypted data, using keys obtained from an authority. However, we allow (and require) the client to send a fresh key request to the authority every time it wants to evaluate a function on a ciphertext. We obtain efficient solutions by carefully combining CCA2 secure public-key encryption (or rerandomizable RCCA secure public-key encryption, depending on the nature of security desired) with Yao's garbled circuit. Our main contributions in this work include developing and formally defining the notion of C-FE; designing theoretical and practical constructions of C-FE schemes achieving these definitions for specific and general classes of functions; and evaluating the performance of our constructions on various application scenarios.

Lecture Notes in Computer Science, 2015

Functional Encryption (FE) is an exciting new paradigm that extends the notion of public key encryption. In this work we explore the security of Inner Product Functional Encryption schemes with the goal of achieving the highest security against practically feasible attacks. While there has been substantial research effort in defining meaningful security models for FE, known definitions run into one of the following difficulties-if general and strong, the definition can be shown impossible to achieve, whereas achievable definitions necessarily restrict the usage scenarios in which FE schemes can be deployed. We argue that it is extremely hard to control the nature of usage scenarios that may arise in practice. Any cryptographic scheme may be deployed in an arbitrarily complex environment and it is vital to have meaningful security guarantees for general scenarios. Hence, in this work, we examine whether it is possible to analyze the security of FE in a wider variety of usage scenarios, but with respect to a meaningful class of adversarial attacks known to be possible in practice. Note that known impossibilities necessitate that we must either restrict the usage scenarios (as done in previous works), or the class of attacks (this work). We study real world loss-of-secrecy attacks against Functional Encryption for Inner Product predicates constructed over elliptic curve groups. Our main contributions are as follows: • We capture a large variety of possible usage scenarios that may arise in practice by providing a stronger, more general, intuitive framework that supports function privacy in addition to data privacy, and a separate encryption key in addition to public key and master secret key. These generalizations allow our framework to capture program obfuscation as a special case of functional encryption, and allows for a separation between users that encrypt data, access data and produce secret keys. • We note that the landscape of attacks over pairing-friendly elliptic curves have been the subject of extensive research and there now exist constructions of pairing friendly elliptic curves where the complexity of all known non-generic attacks is (far) greater than the complexity of generic attacks. Thus, by appropriate choice of the underlying elliptic curve, we can capture all known practically feasible attacks on secrecy by restricting our attention to generic attacks. • We construct a new inner product FE scheme using prime order groups and show it secure under our new, hitherto strongest known framework in the generic group model, thus ruling out all generic attacks in arbitrarily complex real world environments. Since our construction is over prime order groups, we rule out factoring attacks that typically force higher security parameters. Our concrete-analysis proofs provide guidance on the size of elliptic curve groups that are needed for explicit complexity bounds on the attacker.

IACR Cryptol. ePrint Arch., 2020

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:

2014

In this paper, we present Functional Encryption (FE) schemes for finite languages from standard static assumption, viz., Decisional Linear (DLIN) assumption. These finite languages are described by Deterministic Finite Automatas (DFAs). Our first scheme is ciphertext-policy functional encryption (CP-FE), where a key SKw is labeled with a string w over a fixed alphabet Σ and a ciphertext CM is associated with a DFA M over the same alphabet Σ. The key SKw can extract the message from the ciphertext CM if the DFA M accepts the string w. This CP-FE scheme is constructed based on attribute-based encryption (ABE) structure of Okamoto-Takashima in Asiacrypt, 2012. To achieve the adaptive security, we put bounds on number of occurrences of any symbol in a string and in the set of transition tuples of a DFA. Due to this restriction, the size of key space (where the keys are indexed with strings) is reduced to finite. Hence, the functional scope of any DFA in our system can capture only finit...

2020 Second IEEE International Conference on Trust, Privacy and Security in Intelligent Systems and Applications (TPS-ISA), 2020

Increasing incidents of security compromises and privacy leakage have raised serious privacy concerns related to cyberspace. Such privacy concerns have been instrumental in the creation of several regulations and acts to restrict the availability and use of privacy-sensitive data. The secure computation problem, initially and formally introduced as secure two-party computation by Andrew Yao in 1986, has been the focus of intense research in academia because of its fundamental role in building many of the existing privacy-preserving approaches. Most of the existing secure computation solutions rely on garbled-circuits and homomorphic encryption techniques to tackle secure computation issues, including efficiency and security guarantees. However, it is still challenging to adopt these secure computation approaches in emerging compute-intensive and data-intensive applications such as emerging machine learning solutions. Recently proposed functional encryption scheme has shown its promise as an underlying secure computation foundation in recent privacy-preserving machine learning approaches proposed. This paper revisits the secure computation problem using emerging and promising functional encryption techniques and presents a comprehensive study. We first briefly summarize existing conventional secure computation approaches built on garbled-circuits, oblivious transfer, and homomorphic encryption techniques. Then, we elaborate on the unique characteristics and challenges of emerging functional encryption based secure computation approaches and outline several research directions.

2020

Functional encryption (FE) gives the power to retain control of sensitive information and is particularly suitable in several practical real-world use cases. Using this primitive, anyone having a specific functional decryption key (derived from some master secret key) could only obtain the evaluation of an authorized function f over a message m, given its encryption. For many scenarios, the data owner is always different from the functionality owner, such that a classical implementation of functional encryption naturally implies an interactive key generation protocol between an entity owning the function f and another one managing the master secret key. We focus on this particular phase and consider the case where the function needs to be secret. In this paper, we introduce the new notion of blind functional encryption in which, during an interactive key generation protocol, the master secret key owner does not learn anything about the function f. Our new notion can be seen as a gen...

Information Security and Privacy, 2021

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 constrained witness pseudorandom function (CW-PRF) which is motivated by combining WPRF of Zhandry (TCC 2016) and constrained PRF of Boneh and Waters (ASIACRYPT 2013). More specifically, CWPRF computes pseudorandom values associated with NP statements and generates constrained keys for boolean functions. We can recompute the pseudorandom value corresponding to a particular statement either using a public evaluation key with a valid witness for the statement or applying a constrained key for a function that satisfies the statement. We construct CWPRF by coupling indistinguishability obfuscation (iO) and CPRF supporting all polynomial-size functions. In the second and main technical step, we show a generic construction of a CCA secure PKFE for all circuits utilizing our CWPRF. It has been observed that obtaining PKFE supporting all circuits is already a complex task and iO-based constructions of PKFEs are only proven to be chosen plaintext attacks (CPA) secure. On the other hand, existing CCA secure functional encryption schemes are designed for specific functions such as equality testing, membership testing, linear function etc. We emphasize that our construction presents the first CCA secure PKFE for all circuits along with succinct ciphertexts.

Lecture Notes in Computer Science, 2011

We propose a lattice-based functional encryption scheme for inner product predicates whose security follows from the difficulty of the learning with errors (LWE) problem. This construction allows us to achieve applications such as range and subset queries, polynomial evaluation, and CNF/DNF formulas on encrypted data. Our scheme supports inner products over small fields, in contrast to earlier works based on bilinear maps.