International Association
for Cryptologic Research

We construct the first decentralized multi-authority attribute-based encryption (????????-????????????) scheme for a non-trivial class of access policies whose security is based (in the random oracle model) solely on the Learning With Errors (LWE) assumption. The supported access policies are ones described by ???????????? formulas. All previous constructions of ????????-???????????? schemes supporting any non-trivial class of access policies were proven secure (in the random oracle model) assuming various assumptions on bilinear maps. In our system, any party can become an authority and there is no requirement for any global coordination other than the creation of an initial set of common reference parameters. A party can simply act as a standard ABE authority by creating a public key and issuing private keys to different users that reflect their attributes. A user can encrypt data in terms of any ???????????? formulas over attributes issued from any chosen set of authorities. Finally, our system does not require any central authority. In terms of efficiency, when instantiating the scheme with a global bound ???? on the size of access policies, the sizes of public keys, secret keys, and ciphertexts, all grow with ????. Technically, we develop new tools for building ciphertext-policy ABE (????????-????????????) schemes using LWE. Along the way, we construct the first provably secure ????????-???????????? scheme supporting access policies in ????????^1 under the LWE assumption that avoids the generic universal-circuit-based key-policy to ciphertext-policy transformation. In particular, our construction relies on linear secret sharing schemes with new properties and in some sense is more similar to ????????-???????????? schemes that rely on bilinear maps. While our ????????-???????????? construction is not more efficient than existing ones, it is conceptually intriguing and further we show how to extend it to get the ????????-???????????? scheme described above.

There has been recent exciting progress in building non-interactive non-malleable commitments from judicious assumptions. All proposed approaches proceed in two steps. First, obtain simple “base” commitment schemes for very small tag/identity spaces based on a various sub-exponential hardness assumptions. Next, assuming sub-exponential non-interactive witness indistinguishable proofs (NIWIs), and variants of keyless collision-resistant hash functions, construct non-interactive compilers that convert tag-based non-malleable commitments for a small tag space into tag-based non-malleable commitments for a larger tag space. We propose the first black-box construction of non-interactive non-malleable commitments. Our key technical contribution is a novel implementation of the non-interactive proof of consistency required for tag amplification. Prior to our work, the only known approach to tag amplification without setup and with black-box use of the base scheme (Goyal, Lee, Ostrovsky, and Visconti, FOCS 2012) added multiple rounds of interaction. Our construction satisfies the strongest known definition of non-malleability, i.e., CCA (chosen commitment attack) security. In addition to being black-box, our approach dispenses with the need for sub-exponential NIWIs, that was common to all prior work. Instead of NIWIs, we rely on sub-exponential hinting PRGs which can be obtained based on a broad set of assumptions such as sub-exponential CDH or LWE.

In this work, we put forth the notion of compatibility of any key generation or setup algorithm. We focus on the specific case of encryption, and say that a key generation algorithm KeyGen is X-compatible (for X \in {CPA, CCA1, CCA2}) if there exist encryption and decryption algorithms that together with KeyGen, result in an X-secure public-key encryption scheme. We study the following question: Is every CPA-compatible key generation algorithm also CCA-compatible? We obtain the following answers: - Every sub-exponentially CPA-compatible KeyGen algorithm is CCA1-compatible, assuming the existence of hinting PRGs and sub-exponentially secure keyless collision resistant hash functions. - Every sub-exponentially CPA-compatible KeyGen algorithm is also CCA2-compatible, assuming the existence of non-interactive CCA2 secure commitments, in addition to sub-exponential security of the assumptions listed in the previous bullet. Here, sub-exponentially CPA-compatible KeyGen refers to any key generation algorithm for which there exist encryption and decryption algorithms that result in a CPA-secure public-key encryption scheme {\em against sub-exponential adversaries}. This gives a way to perform CCA secure encryption given any public key infrastructure that has been established with only (sub-exponential) CPA security in mind. The resulting CCA encryption makes black-box use of the CPA scheme and all other underlying primitives.

Lossy trapdoor functions, introduced by Peikert and Waters (STOC '08), can be initialized in one of two indistinguishable modes: in injective mode, the function preserves all information about its input, and can be efficiently inverted given a trapdoor, while in lossy mode, the function loses some information about its input. Such functions have found countless applications in cryptography, and can be constructed from a variety of Cryptomania assumptions. In this work, we introduce \emph{targeted lossy functions (TLFs)}, which relax lossy trapdoor functions along two orthogonal dimensions. Firstly, they do not require an inversion trapdoor in injective mode. Secondly, the lossy mode of the function is initialized with some target input, and the function is only required to lose information about this particular target. The injective and lossy modes should be indistinguishable even given the target. We construct TLFs from Minicrypt assumptions, namely, injective pseudorandom generators, or even one-way functions under a natural relaxation of injectivity. We then generalize TLFs to incorporate \emph{branches}, and construct \emph{all-injective-but-one} and \emph{all-lossy-but-one} variants. We show a wide variety of applications of targeted lossy functions. In several cases, we get the first Minicrypt constructions of primitives that were previously only known under Cryptomania assumptions. Our applications include: \begin{itemize} \item \emph{Pseudo-entropy functions} from one-way functions. \item Deterministic leakage-resilient message-authentication codes and improved leakage-resilient symmetric-key encryption from one-way functions. \item Extractors for \emph{extractor-dependent sources} from one-way functions. \item Selective-opening secure symmetric-key encryption from one-way functions. \item A new construction of CCA PKE from (exponentially secure) trapdoor functions and injective pseudorandom generators. \end{itemize} We also discuss a fascinating connection to distributed point functions.

We give an attribute-based encryption system for Turing Machines that is provably secure assuming only the existence of identity- based encryption (IBE) for large identity spaces. Currently, IBE is known to be realizable from most mainstream number theoretic assumptions that imply public key cryptography including factoring, the search Diffie-Hellman assumption, and the Learning with Errors assumption. Our core construction provides security against an attacker that makes a single key query for a machine T before declaring a challenge string w∗ that is associated with the challenge ciphertext. We build our construction by leveraging a Garbled RAM construction of Gentry, Halevi, Raykova and Wichs; however, to prove security we need to introduce a new notion of security called iterated simulation security. We then show how to transform our core construction into one that is secure for an a-priori bounded number q = q(\lambda) of key queries that can occur either before or after the challenge ciphertext. We do this by first showing how one can use a special type of non-committing encryption to transform a system that is secure only if a single key is chosen before the challenge ciphertext is declared into one where the single key can be requested either before or after the challenge ciphertext. We give a simple construction of this non-committing encryption from public key encryption in the Random Oracle Model. Next, one can apply standard combinatorial techniques to lift from single-key adaptive security to q-key adaptive security.

One of the primary research challenges in Attribute-Based Encryption (ABE) is constructing and proving cryptosystems that are adaptively secure. To date the main paradigm for achieving adaptive security in ABE is dual system encryption. However, almost all such solutions in bilinear groups rely on (variants of) either the subgroup decision problem over composite order groups or the decision linear assumption. Both of these assumptions are decisional rather than search assumptions and the target of the assumption is a source or bilinear group element. This is in contrast to earlier selectively secure ABE systems which can be proven secure from either the decisional or search Bilinear Diffie-Hellman assumption. In this work we make progress on closing this gap by giving a new ABE construction for the subset functionality and prove security under the Search Bilinear Diffie-Hellman assumption. We first provide a framework for proving adaptive security in Attribute-Based Encryption systems. We introduce a concept of ABE with deletable attributes where any party can take a ciphertext encrypted under the attribute string x in {0, 1}^n and modify it into a ciphertext encrypted under any string x' in {0, 1, bot}^n where x' is derived by replacing any bits of x with bot symbols (i.e. ``deleting" attributes of x). The semantics of the system are that any private key for a circuit C can be used to decrypt a ciphertext associated with x' if none of the input bits read by circuit C are bot symbols and C(x') = 1. We show a pathway for combining ABE with deletable attributes with constrained pseudorandom functions to obtain adaptively secure ABE building upon the recent work of [Tsabary19]. Our new ABE system will be adaptively secure and be a ciphertext-policy ABE that supports the same functionality as the underlying constrained PRF as long as the PRF is ``deletion conforming". Here we also provide a simple constrained PRF construction that gives subset functionality. Our approach enables us to access a broader array of Attribute-Based Encryption schemes support deletion of attributes. For example, we show that both the [GPSW06] and [Boyen13] ABE schemes can trivially handle a deletion operation. And, by using a hardcore bit variant of GPSW scheme we obtain an adaptively secure ABE scheme under the Search Bilinear Diffie-Hellman assumption in addition to pseudo random functions in NC1. This gives the first adaptively secure ABE from a search assumption as all prior work relied on decision assumptions over source group elements.

Software watermarking schemes allow a user to embed an identifier into a piece of code such that the resulting program is nearly functionally-equivalent to the original program, and yet, it is difficult to remove the identifier without destroying the functionality of the program. Such schemes are often considered for proving software ownership or for digital rights management. Existing constructions of watermarking have focused primarily on watermarking pseudorandom functions (PRFs). In this work, we revisit the definitional foundations of watermarking, and begin by highlighting a major flaw in existing security notions. Existing security notions for watermarking only require that the identifier be successfully extracted from programs that preserve the exact input/output behavior of the original program. In the context of PRFs, this means that an adversary that constructs a program which computes a quarter of the output bits of the PRF or that is able to distinguish the outputs of the PRF from random are considered to be outside the threat model. However, in any application (e.g., watermarking a decryption device or an authentication token) that relies on PRF security, an adversary that manages to predict a quarter of the bits or distinguishes the PRF outputs from random would be considered to have defeated the scheme. Thus, existing watermarking schemes provide very little security guarantee against realistic adversaries. None of the existing constructions of watermarkable PRFs would be able to extract the identifier from a program that only outputs a quarter of the bits of the PRF or one that perfectly distinguishes. To address the shortcomings in existing watermarkable PRF definitions, we introduce a new primitive called a traceable PRF. Our definitions are inspired by similar definitions from public-key traitor tracing, and aim to capture a very robust set of adversaries: namely, any adversary that produces a useful distinguisher (i.e., a program that can break PRF security), can be traced to a specific identifier. We provide a general framework for constructing traceable PRFs via an intermediate primitive called private linear constrained PRFs. Finally, we show how to construct traceable PRFs from a similar set of assumptions previously used to realize software watermarking. Namely, we obtain a single-key traceable PRF from standard lattice assumptions and a fully collusion-resistant traceable PRF from indistinguishability obfuscation (together with injective one-way functions).

We provide a construction of chosen ciphertext secure public-key encryption from (injective) trapdoor functions. Our construction is black box and assumes no special properties (e.g. ``lossy'', ``correlated product secure'') of the trapdoor function.

Over the last few years, there has been a surge of new cryptographic results, including laconic oblivious transfer, (anonymous/ hierarchical) identity-based encryption, trapdoor functions, chosen-ciphertext security transformations, designated-verifier zero-knowledge proofs, due to a beautiful framework recently introduced in the works of Cho et al. (Crypto 2017), and Dottling and Garg (Crypto 2017). The primitive of one-way function with encryption (OWFE) and its relatives (chameleon encryption, one-time signatures with encryption, hinting PRGs, trapdoor hash encryption, batch encryption) has been a centerpiece in all these results. While there exist multiple realizations of OWFE (and its relatives) from a variety of assumptions such as CDH, Factoring, and LWE, all such constructions fall under the same general ``missing block" framework. Although this framework has been instrumental in opening up a new pathway towards various cryptographic functionalities via the abstraction of OWFE (and its relatives), it has been accompanied by undesirable inefficiencies that might inhibit a much wider adoption in many practical scenarios. Motivated by the surging importance of the OWFE abstraction (and its relatives), a natural question to ask is whether the existing approaches can be diversified to not only obtain more constructions from different assumptions, but also in developing newer frameworks. We believe answering this question will eventually lead to important and previously unexplored performance trade-offs in the overarching applications of this novel cryptographic paradigm. In this work, we propose a new accumulation-style framework for building a new class of OWFE as well as hinting PRG constructions with a special focus on achieving shorter ciphertext size and shorter public parameter size (respectively). Such performance improvements parlay into shorter parameters in their corresponding applications. Briefly, we explore the following performance trade-offs --- (1) for OWFE, our constructions outperform in terms of ciphertext size as well as encryption time, but this comes at the cost of larger evaluation and setup times, (2) for hinting PRGs, our constructions provide a rather dramatic trade-off between evaluation time versus parameter size, with our construction leading to significantly shorter public parameter size. The trade-off enabled by our hinting PRG construction also leads to interesting implications in the CPA-to-CCA transformation provided in. We also provide concrete performance measurements for our constructions and compare them with existing approaches. We believe highlighting such trade-offs will lead to wider adoption of these abstractions in a practical sense.

In a lockable obfuscation scheme a party takes as input a program P, a lock value alpha, a message msg, and produces an obfuscated program P'. The obfuscated program can be evaluated on an input x to learn the message msg if P(x)= alpha. The security of such schemes states that if alpha is randomly chosen (independent of P and msg), then one cannot distinguish an obfuscation of $P$ from a dummy obfuscation. Existing constructions of lockable obfuscation achieve provable security under the Learning with Errors assumption. One limitation of these constructions is that they achieve only statistical correctness and allow for a possible one-sided error where the obfuscated program could output the msg on some value x where P(x) \neq alpha. In this work we motivate the problem of studying perfect correctness in lockable obfuscation for the case where the party performing the obfuscation might wish to inject a backdoor or hole in the correctness. We begin by studying the existing constructions and identify two components that are susceptible to imperfect correctness. The first is in the LWE-based pseudo-random generators (PRGs) that are non-injective, while the second is in the last level testing procedure of the core constructions. We address each in turn. First, we build upon previous work to design injective PRGs that are provably secure from the LWE assumption. Next, we design an alternative last level testing procedure that has an additional structure to prevent correctness errors. We then provide surgical proof of security (to avoid redundancy) that connects our construction to the construction by Goyal, Koppula, and Waters (GKW). Specifically, we show how for a random value alpha an obfuscation under our new construction is indistinguishable from an obfuscation under the existing GKW construction.

A proof of replication system is a cryptographic primitive that allows a server (or group of servers) to prove to a client that it is dedicated to storing multiple copies or replicas of a file. Until recently, all such protocols required fined-grained timing assumptions on the amount of time it takes for a server to produce such replicas. Damgard, Ganesh, and Orlandi [DGO19] proposed a novel notion that we will call proof of replication with client setup. Here, a client first operates with secret coins to generate the replicas for a file. Such systems do not inherently have to require fine-grained timing assumptions. At the core of their solution to building proofs of replication with client setup is an abstraction called replica encodings. Briefly, these comprise a private coin scheme where a client algorithm given a file m can produce an encoding \sigma. The encodings have the property that, given any encoding \sigma, one can decode and retrieve the original file m. Secondly, if a server has significantly less than n·|m| bit of storage, it cannot reproduce n encodings. The authors give a construction of encodings from ideal permutations and trapdoor functions. In this work, we make three central contributions: 1) Our first contribution is that we discover and demonstrate that the security argument put forth by [DGO19] is fundamentally flawed. Briefly, the security argument makes assumptions on the attacker's storage behavior that does not capture general attacker strategies. We demonstrate this issue by constructing a trapdoor permutation which is secure assuming indistinguishability obfuscation, serves as a counterexample to their claim (for the parameterization stated). 2) In our second contribution we show that the DGO construction is actually secure in the ideal permutation model from any trapdoor permutation when parameterized correctly. In particular, when the number of rounds in the construction is equal to \lambda·n·b where \lambda is the security parameter, n is the number of replicas and b is the number of blocks. To do so we build up a proof approach from the ground up that accounts for general attacker storage behavior where we create an analysis technique that we call "sequence-then-switch". 3) Finally, we show a new construction that is provably secure in the random oracle (or random function) model. Thus requiring less structure on the ideal function.

We present the first attribute-based encryption (ABE) scheme for deterministic finite automaton (DFA) based on static assumptions in bilinear groups; this resolves an open problem posed by Waters (CRYPTO 2012). Our main construction achieves selective security against unbounded collusions under the standard k-linear assumption in prime-order bilinear groups, whereas previous constructions all rely on q-type assumptions.

An emerging trend is for researchers to identify cryptography primitives for which feasibility was first established under obfuscation and then move the realization to a different setting. In this work we explore a new such avenue—to move obfuscation-based cryptography to the assumption of (positional) witness encryption. Our goal is to develop techniques and tools, which we will dub “witness encryption friendly” primitives and use these to develop a methodology for building advanced cryptography from positional witness encryption.We take a bottom up approach and pursue our general agenda by attacking the specific problem of building collusion-resistant broadcast systems with tracing from positional witness encryption. We achieve a system where the size of ciphertexts, public key and private key are polynomial in the security parameter $$\lambda $$ and independent of the number of users N in the broadcast system. Currently, systems with such parameters are only known from indistinguishability obfuscation.

We provide generic and black box transformations from any chosen plaintext secure Attribute-Based Encryption (ABE) or One-sided Predicate Encryption system into a chosen ciphertext secure system. Our transformation requires only the IND-CPA security of the original ABE scheme coupled with a pseudorandom generator (PRG) with a special security property.In particular, we consider a PRG with an n bit input $$s \in \{0,1\}^n$$ and $$n \cdot \ell $$ bit output $$y_1, \ldots , y_n$$ where each $$y_i$$ is an $$\ell $$ bit string. Then for a randomly chosen s the following two distributions should be computationally indistinguishable. In the first distribution $$r_{s_i, i} = y_i$$ and $$r_{\bar{s}_i, i}$$ is chosen randomly for $$i \in [n]$$. In the second distribution all $$r_{b, i}$$ are chosen randomly for $$i \in [n], b \in \{0,1\}$$.We show that such PRGs can be built from either the computational Diffie-Hellman assumption (in non-bilinear groups) or the Learning with Errors (LWE) assumption (and potentially other assumptions). Thus, one can transform any IND-CPA secure system into a chosen ciphertext secure one by adding either assumption. (Or by simply assuming an existing PRG is hinting secure.) In addition, our work provides a new approach and perspective for obtaining chosen ciphertext security in the basic case of public key encryption.

A software watermarking scheme enables users to embed a message or mark within a program while preserving its functionality. Moreover, it is difficult for an adversary to remove a watermark from a marked program without corrupting its behavior. Existing constructions of software watermarking from standard assumptions have focused exclusively on watermarking pseudorandom functions (PRFs).In this work, we study watermarking public-key primitives such as the signing key of a digital signature scheme or the decryption key of a public-key (predicate) encryption scheme. While watermarking public-key primitives might intuitively seem more challenging than watermarking PRFs, our constructions only rely on simple assumptions. Our watermarkable signature scheme can be built from the minimal assumption of one-way functions while our watermarkable public-key encryption scheme can be built from most standard algebraic assumptions that imply public-key encryption (e.g., factoring, discrete log, or lattice assumptions). Our schemes also satisfy a number of appealing properties: public marking, public mark-extraction, and collusion resistance. Our schemes are the first to simultaneously achieve all of these properties.The key enabler of our new constructions is a relaxed notion of functionality-preserving. While traditionally, we require that a marked program (approximately) preserve the input/output behavior of the original program, in the public-key setting, preserving the “functionality” does not necessarily require preserving the exact input/output behavior. For instance, if we want to mark a signing algorithm, it suffices that the marked algorithm still output valid signatures (even if those signatures might be different from the ones output by the unmarked algorithm). Similarly, if we want to mark a decryption algorithm, it suffices that the marked algorithm correctly decrypt all valid ciphertexts (but may behave differently from the unmarked algorithm on invalid or malformed ciphertexts). Our relaxed notion of functionality-preserving captures the essence of watermarking and still supports the traditional applications, but provides additional flexibility to enable new and simple realizations of this powerful cryptographic notion.

We construct a broadcast and trace scheme (also known as trace and revoke or broadcast, trace and revoke) with N users, where the ciphertext size can be made as low as $$O(N^\varepsilon )$$ , for any arbitrarily small constant $$\varepsilon >0$$ . This improves on the prior best construction of broadcast and trace under standard assumptions by Boneh and Waters (CCS ‘06), which had ciphertext size $$O(N^{1/2})$$ . While that construction relied on bilinear maps, ours uses a combination of the learning with errors (LWE) assumption and bilinear maps.Recall that, in both broadcast encryption and traitor-tracing schemes, there is a collection of N users, each of which gets a different secret key $${\mathsf {sk}}_i$$ . In broadcast encryption, it is possible to create ciphertexts targeted to a subset $$S \subseteq [N]$$ of the users such that only those users can decrypt it correctly. In a traitor tracing scheme, if a subset of users gets together and creates a decoder box D that is capable of decrypting ciphertexts, then it is possible to trace at least one of the users responsible for creating D. A broadcast and trace scheme intertwines the two properties, in a way that results in more than just their union. In particular, it ensures that if a decoder D is able to decrypt ciphertexts targeted toward a set S of users, then it should be possible to trace one of the users in the set S responsible for creating D, even if other users outside of S also participated. As of recently, we have essentially optimal broadcast encryption (Boneh, Gentry, Waters CRYPTO ’05) under bilinear maps and traitor tracing (Goyal, Koppula, Waters STOC ’18) under LWE, where the ciphertext size is at most poly-logarithmic in N. The main contribution of our paper is to carefully combine LWE and bilinear-map based components, and get them to interact with each other, to achieve broadcast and trace.

In a traitor tracing (TT) system for n users, every user has his/her own secret key. Content providers can encrypt messages using a public key, and each user can decrypt the ciphertext using his/her secret key. Suppose some of the n users collude to construct a pirate decoding box. Then the tracing scheme has a special algorithm, called $$\mathsf {Trace}$$, which can identify at least one of the secret keys used to construct the pirate decoding box.Traditionally, the trace algorithm output only the ‘index’ associated with the traitors. As a result, to use such systems, either a central master authority must map the indices to actual identities, or there should be a public mapping of indices to identities. Both these options are problematic, especially if we need public tracing with anonymity of users. Nishimaki, Wichs, and Zhandry (NWZ) [Eurocrypt 2016] addressed this problem by constructing a traitor tracing scheme where the identities of users are embedded in the secret keys, and the trace algorithm, given a decoding box D, can recover the entire identities of the traitors. We call such schemes ‘Embedded Identity Traitor Tracing’ schemes. NWZ constructed such schemes based on adaptively secure functional encryption (FE). Currently, the only known constructions of FE schemes are based on nonstandard assumptions such as multilinear maps and iO.In this work, we study the problem of embedded identities TT based on standard assumptions. We provide a range of constructions based on different assumptions such as public key encryption (PKE), bilinear maps and the Learning with Errors (LWE) assumption. The different constructions have different efficiency trade offs. In our PKE based construction, the ciphertext size grows linearly with the number of users; the bilinear maps based construction has sub-linear ($$\sqrt{n}$$) sized ciphertexts. Both these schemes have public tracing. The LWE based scheme is a private tracing scheme with optimal ciphertexts (i.e., $$\log (n)$$). Finally, we also present other notions of traitor tracing, and discuss how they can be build in a generic manner from our base embedded identity TT scheme.

In this work, we study the fascinating notion of output-compressing randomized encodings for Turing Machines, in a shared randomness model. In this model, the encoder and decoder have access to a shared random string, and the efficiency requirement is, the size of the encoding must be independent of the running time and output length of the Turing Machine on the given input, while the length of the shared random string is allowed to grow with the length of the output. We show how to construct output-compressing randomized encodings for Turing machines in the shared randomness model, assuming iO for circuits and any assumption in the set $$\{$$ LWE, DDH, N $$^{th}$$ Residuosity $$\}$$ .We then show interesting implications of the above result to basic feasibility questions in the areas of secure multiparty computation (MPC) and indistinguishability obfuscation (iO): 1.Compact MPC for Turing Machines in the Random Oracle Model. In the context of MPC, we consider the following basic feasibility question: does there exist a malicious-secure MPC protocol for Turing Machines whose communication complexity is independent of the running time and output length of the Turing Machine when executed on the combined inputs of all parties? We call such a protocol as a compact MPC protocol. Hubácek and Wichs [HW15] showed via an incompressibility argument, that, even for the restricted setting of circuits, it is impossible to construct a malicious secure two party computation protocol in the plain model where the communication complexity is independent of the output length. In this work, we show how to evade this impossibility by compiling any (non-compact) MPC protocol in the plain model to a compact MPC protocol for Turing Machines in the Random Oracle Model, assuming output-compressing randomized encodings in the shared randomness model.2.Succinct iO for Turing Machines in the Shared Randomness Model. In all existing constructions of iO for Turing Machines, the size of the obfuscated program grows with a bound on the input length. In this work, we show how to construct an iO scheme for Turing Machines in the shared randomness model where the size of the obfuscated program is independent of a bound on the input length, assuming iO for circuits and any assumption in the set $$\{$$ LWE, DDH, N $$^{th}$$ Residuosity $$\}$$ .

In this work we seek to construct collusion-resistant traitor tracing systems with small ciphertexts from standard assumptions that also move toward practical efficiency. In our approach we will hold steadfast to the principle of collusion resistance, but relax the requirement on catching a traitor from a successful decoding algorithm. We define a f-risky traitor tracing system as one where the probability of identifying a traitor is $$f(\lambda ,n)$$f(λ,n) times the probability a successful box is produced. We then go on to show how to build such systems from prime order bilinear groups with assumptions close to those used in prior works. Our core system achieves, for any $$k > 0$$k>0, $$f(\lambda ,n) \approx \frac{k}{n + k - 1}$$f(λ,n)≈kn+k-1 where ciphertexts consists of $$(k + 4)$$(k+4) group elements and decryption requires $$(k + 3)$$(k+3) pairing operations.At first glance the utility of such a system might seem questionable since the f we achieve for short ciphertexts is relatively small. Indeed an attacker in such a system can more likely than not get away with producing a decoding box. However, we believe this approach to be viable for four reasons:1.A risky traitor tracing system will provide deterrence against risk averse attackers. In some settings the consequences of being caught might bear a high cost and an attacker will have to weigh his utility of producing a decryption D box against the expected cost of being caught.2.Consider a broadcast system where we want to support low overhead broadcast encrypted communications, but will periodically allow for a more expensive key refresh operation. We refer to an adversary produced algorithm that maintains the ability to decrypt across key refreshes as a persistent decoder. We show how if we employ a risky traitor tracing systems in this setting, even for a small f, we can amplify the chances of catching such a “persistent decoder” to be negligibly close to 1.3.In certain resource constrained settings risky traitor tracing provides a best tracing effort where there are no other collusion-resistant alternatives. For instance, suppose we had to support 100 K users over a radio link that had just 10 KB of additional resources for extra ciphertext overhead. None of the existing $$\sqrt{N}$$N bilinear map systems can fit in these constraints. On the other hand a risky traitor tracing system provides a spectrum of tracing probability versus overhead tradeoffs and can be configured to at least give some deterrence in this setting.4.Finally, we can capture impossibility results for differential privacy from $$\frac{1}{n}$$1n-risky traitor tracing. Since our ciphertexts are short ($$O(\lambda )$$O(λ)), we get the negative result which matches what one would get plugging in the obfuscation based tracing system Boneh-Zhandry [9] solution into the prior impossibility result of Dwork et al. [14].

The notion of Functional Encryption (FE) has recently emerged as a strong primitive with several exciting applications. In this work, we initiate the study of the following question: Can existing public key encryption schemes be “upgraded” to Functional Encryption schemes without changing their public keys or the encryption algorithm? We call a public-key encryption scheme with this property to be FE-compatible. Indeed, assuming ideal obfuscation, it is easy to see that every CCA-secure public-key encryption scheme is FE-compatible. Despite the recent success in using indistinguishability obfuscation to replace ideal obfuscation for many applications, we show that this phenomenon most likely will not apply here. We show that assuming fully homomorphic encryption and the learning with errors (LWE) assumption, there exists a CCA-secure encryption scheme that is provably not FE-compatible. We also show that a large class of natural CCA-secure encryption schemes proven secure in the random oracle model are not FE-compatible in the random oracle model.Nevertheless, we identify a key structure that, if present, is sufficient to provide FE-compatibility. Specifically, we show that assuming sub-exponentially secure iO and sub-exponentially secure one way functions, there exists a class of public key encryption schemes which we call Special-CCA secure encryption schemes that are in fact, FE-compatible. In particular, each of the following popular CCA secure encryption schemes (some of which existed even before the notion of FE was introduced) fall into the class of Special-CCA secure encryption schemes and are thus FE-compatible:1.[CHK04] when instantiated with the IBE scheme of [BB04].2.[CHK04] when instantiated with any Hierarchical IBE scheme.3.[PW08] when instantiated with any Lossy Trapdoor Function.

In this work we study the feasibility of achieving simulation security in functional encryption (FE) in the random oracle model. Our main result is negative in that we give a functionality for which it is impossible to achieve simulation security even with the aid of random oracles.We begin by giving a formal definition of simulation security that explicitly incorporates the random oracles. Next, we show a particular functionality for which it is impossible to achieve simulation security. Here messages are interpreted as seeds to a (weak) pseudorandom function family F and private keys are ascribed to points in the domain of the function. On a message s and private key x one can learn F(s, x). We show that there exists an attacker that makes a polynomial number of private key queries followed by a single ciphertext query for which there exists no simulator.Our functionality and attacker access pattern closely matches the standard model impossibility result of Agrawal, Gorbunov, Vaikuntanathan and Wee (CRYPTO 2013). The crux of their argument is that no simulator can succinctly program in the outputs of an unbounded number of evaluations of a pseudorandom function family into a fixed size ciphertext. However, their argument does not apply in the random oracle setting since the oracle acts as an additional conduit of information which the simulator can program. We overcome this barrier by proposing an attacker who decrypts the challenge ciphertext with the secret keys issued earlier without using the random oracle, even though the decryption algorithm may require it. This involves collecting most of the useful random oracle queries in advance, without giving the simulator too many opportunities to program.On the flip side, we demonstrate the utility of the random oracle in simulation security. Given only public key encryption and low-depth PRGs we show how to build an FE system that is simulation secure for any poly-time attacker that makes an unbounded number of message queries, but an a-priori bounded number of key queries. This bests what is possible in the standard model where it is only feasible to achieve security for an attacker that is bounded both in the number of key and message queries it makes. We achieve this by creating a system that leverages the random oracle to get one-key security and then adapt previously known techniques to boost the system to resist up to q queries.Finally, we ask whether it is possible to achieve simulation security for an unbounded number of messages and keys, but where all key queries are made after the message queries. We show this too is impossible to achieve using a different twist on our first impossibility result.

A traitor tracing scheme is a public key encryption scheme for which there are many secret decryption keys. Any of these keys can decrypt a ciphertext; moreover, even if a coalition of users collude, put together their decryption keys and attempt to create a new decryption key, there is an efficient algorithm to trace the new key to at least one the colluders.Recently, Goyal, Koppula and Waters (GKW, STOC 18) provided the first traitor tracing scheme from LWE with ciphertext and secret key sizes that grow polynomially in $$\log n$$, where n is the number of users. The main technical building block in their construction is a strengthening of (bounded collusion secure) secret-key functional encryption which they refer to as mixed functional encryption (FE).In this work, we improve upon and extend the GKW traitor tracing scheme:We provide simpler constructions of mixed FE schemes based on the LWE assumption. Our constructions improve upon the GKW construction in terms of expressiveness, modularity, and security.We provide a construction of attribute-based traitor tracing for all circuits based on the LWE assumption.

In a proof-of-retrievability system, a data storage center must prove to a verifier that he is actually storing all of a client’s data. The central challenge is to build systems that are both efficient and provably secure—that is, it should be possible to extract the client’s data from any prover that passes a verification check. In this paper, we give the first proof-of-retrievability schemes with full proofs of security against arbitrary adversaries in the strongest model, that of Juels and Kaliski.Our first scheme, built from BLS signatures and secure in the random oracle model, features a proof-of-retrievability protocol in which the client’s query and server’s response are both extremely short. This scheme allows public verifiability: anyone can act as a verifier, not just the file owner. Our second scheme, which builds on pseudorandom functions (PRFs) and is secure in the standard model, allows only private verification. It features a proof-of-retrievability protocol with an even shorter server’s response than our first scheme, but the client’s query is long. Both schemes rely on homomorphic properties to aggregate a proof into one small authenticator value.