International Association
for Cryptologic Research

Policy-compliant signatures (PCS) are a recently introduced primitive by Badertscher et al. [TCC 2021] in which a central authority distributes secret and public keys associated with sets of attributes (e.g., nationality, affiliation with a specific department, or age) to its users. The authority also enforces a policy determining which senders can sign messages for which receivers based on a joint check of their attributes. For example, senders and receivers must have the same nationality, or only senders that are at least 18 years old can send to members of the computer science department. PCS further requires attribute-privacy -- nothing about the users' attributes is revealed from their public keys and signatures apart from whether the attributes satisfy the policy or not. The policy in a PCS scheme is fixed once and for all during the setup. Therefore, a policy update requires a redistribution of all keys. This severely limits the practicality of PCS. In this work, we introduce the notion of updatable policy-compliant signatures (UPCS) extending PCS with a mechanism to efficiently update the policy without redistributing keys to all participants. We define the notion of UPCS and provide the corresponding security definitions. We then provide a generic construction of UPCS based on digital signatures, a NIZK proof system, and a so-called secret-key two-input partially-hiding predicate encryption (2-PHPE) scheme. Unfortunately, the only known way to build the latter for general two-input predicates is using indistinguishability obfuscation. We show that the reliance on the heavy tool of 2-PHPE is inherent to build UPCS by proving that non-interactive UPCS implies 2-PHPE. To circumvent the reliance on 2-PHPE, we consider interactive UPCS, which allows sender and receiver to interact during the message signing. In this setting, we present two UPCS schemes: the first one requires only a digital signature scheme, a NIZK proof system, and secure two-party computation. This scheme works for arbitrary policies, but requires senders and receivers to engage in the two-party computation for each policy update. Our second scheme additionally requires a (single-input) predicate-encryption scheme and only requires the sender and receiver to interact ones independent of the updates. In contrast to 2-PHPE, single-input predicate encryption supporting certain predicate classes are known to exist (e.g., from pairings) under more concrete and well-understood assumptions.

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 traitor-tracing systems where, instead of having a key authority, users could generate and register their own public keys. The public parameters are computed by aggregating all user public keys. Crucially, the aggregation process is \emph{public}, thus eliminating the need of any trusted authority. We present two new traitor-tracing systems in this model based on bilinear pairings. Our first scheme is proven adaptively secure in the generic group model. This scheme features a {\it transparent} setup, ciphertexts consisting of $6\sqrt{L}+4$ group elements, and a public tracing algorithm. Our second scheme supports a bounded collusion of traitors and is proven selectively secure in the standard model. Our main technical ingredients are new registered functional encryption (RFE) schemes for quadratic and linear functions which, prior to this work, were known only from indistinguishability obfuscation. To substantiate the practicality of our approach, we evaluate the performance a proof of concept implementation. For a group of $L = 1024$ users, encryption and decryption take roughly 50ms and 4ms, respectively, whereas a ciphertext is of size 6.7KB.

Registered encryption (Garg et al., TCC'18) is an emerging paradigm that tackles the key-escrow problem associated with identity-based encryption by replacing the private-key generator with a much weaker entity known as the key curator. The key curator holds no secret information, and is responsible to: (i) update the master public key whenever a new user registers its own public key to the system; (ii) provide helper decryption keys to the users already registered in the system, in order to still enable them to decrypt after new users join the system. For practical purposes, tasks (i) and (ii) need to be efficient, in the sense that the size of the public parameters, of the master public key, and of the helper decryption keys, as well as the running times for key generation and user registration, and the number of updates, must be small. In this paper, we generalize the notion of registered encryption to the setting of functional encryption (FE). As our main contribution, we show an efficient construction of registered FE for the special case of (attribute hiding) inner-product predicates, built over asymmetric bilinear groups of prime order. Our scheme supports a large attribute universe and is proven secure in the bilinear generic group model. We also implement our scheme and experimentally demonstrate the efficiency requirements of the registered settings. Our second contribution is a feasibility result where we build registered FE for P/poly based on indistinguishability obfuscation and somewhere statistically binding hash functions.

Adaptor signatures are a novel cryptographic primitive with important applications for cryptocurrencies. They have been used to construct second layer solutions such as payment channels or cross-currency swaps. The basic idea of an adaptor signature scheme is to tie the signing process to the revelation of a secret value in the sense that, much like a regular signature scheme, an adaptor signature scheme can authenticate messages, but simultaneously leaks a secret to certain parties. Recently, Aumayr et al. provide the first formalization of adaptor signature schemes, and present provably secure constructions from ECDSA and Schnorr signatures. Unfortunately, the formalization and constructions given in this work have two limitations: (1) current schemes are limited to ECDSA and Schnorr signatures, and no generic transformation for constructing adaptor signatures is known; (2) they do not offer support for aggregated two-party signing, which can significantly reduce the blockchain footprint in applications of adaptor signatures. In this work, we address these two shortcomings. First, we show that signature schemes that are constructed from identification (ID) schemes, which additionally satisfy certain homomorphic properties, can generically be transformed into adaptor signature schemes. We further provide an impossibility result which proves that unique signature schemes (e.g., the BLS scheme) cannot be transformed into an adaptor signature scheme. In addition, we define two-party adaptor signature schemes with aggregatable public keys and show how to instantiate them via a generic transformation from ID-based signature schemes. Finally, we give instantiations of our generic transformations for the Schnorr, Katz-Wang and Guillou-Quisquater signature schemes.

The classic work of Gorbunov, Vaikuntanathan and Wee (CRYPTO 2012) and follow-ups provided constructions of bounded collusion Functional Encryption (FE) for circuits from mild assumptions. In this work, we improve the state of affairs for bounded collusion FE in several ways: 1. {\it New Security Notion.} We introduce the notion of {\it dynamic} bounded collusion FE, where the declaration of collusion bound is delayed to the time of encryption. This enables the encryptor to dynamically choose the collusion bound for different ciphertexts depending on their individual level of sensitivity. Hence, the ciphertext size grows linearly with its own collusion bound and the public key size is independent of collusion bound. In contrast, all prior constructions have public key and ciphertext size that grow at least linearly with a fixed bound $Q$. 2. {\it CPFE for circuits with Dynamic Bounded Collusion.} We provide the first CPFE schemes for circuits enjoying dynamic bounded collusion security. By assuming identity based encryption (IBE), we construct CPFE for circuits of {\it unbounded} size satisfying {\it non-adaptive} simulation based security. By strengthening the underlying assumption to IBE with receiver selective opening security, we obtain CPFE for circuits of {\it bounded} size enjoying {\it adaptive} simulation based security. Moreover, we show that IBE is a necessary assumption for these primitives. Furthermore, by relying on the Learning With Errors (LWE) assumption, we obtain the first {\it succinct} CPFE for circuits, i.e. supporting circuits with unbounded size, but fixed output length and depth. This scheme achieves {\it adaptive} simulation based security. 3. {\it KPFE for circuits with dynamic bounded collusion.} We provide the first KPFE for circuits of unbounded size, but bounded depth and output length satisfying dynamic bounded collusion security from LWE. Our construction achieves {\it adaptive} simulation security improving security of \cite{GKPVZ13a}. 4. {\it KP and CP FE for TM/NL with dynamic bounded collusion.} We provide the first KPFE and CPFE constructions of bounded collusion functional encryption for Turing machines in the public key setting from LWE. Our constructions achieve non-adaptive simulation based security. Both the input and the machine in our construction can be of {\it unbounded} polynomial length. We provide a variant of the above scheme that satisfies {\it adaptive} security, but at the cost of supporting a smaller class of computation, namely Nondeterministic Logarithmic-space (NL). Since NL contains Nondeterministic Finite Automata (NFA), this result subsumes {\it all} prior work of bounded collusion FE for uniform models from standard assumptions \cite{AMY19,AS17}.

Inner product functional encryption ( $${mathsf {IPFE}}$$ ) [ 1 ] is a popular primitive which enables inner product computations on encrypted data. In $${mathsf {IPFE}}$$ , the ciphertext is associated with a vector $$varvec{x}$$ , the secret key is associated with a vector $$varvec{y}$$ and decryption reveals the inner product $$langle varvec{x},varvec{y} angle $$ . Previously, it was known how to achieve adaptive indistinguishability ( $$mathsf {IND}$$ ) based security for $${mathsf {IPFE}}$$ from the $$mathsf {DDH}$$ , $$mathsf {DCR}$$ and $$mathsf {LWE}$$ assumptions [ 8 ]. However, in the stronger simulation ( $$mathsf {SIM}$$ ) based security game, it was only known how to support a restricted adversary that makes all its key requests either before or after seeing the challenge ciphertext, but not both. In more detail, Wee [ 46 ] showed that the $$mathsf {DDH}$$ -based scheme of Agrawal et al. (Crypto 2016) achieves semi-adaptive simulation-based security, where the adversary must make all its key requests after seeing the challenge ciphertext. On the other hand, O’Neill showed that all $$mathsf {IND}$$ -secure $${mathsf {IPFE}}$$ schemes (which may be based on $$mathsf {DDH}$$ , $$mathsf {DCR}$$ and $$mathsf {LWE}$$ ) satisfy $$mathsf {SIM}$$ based security in the restricted model where the adversary makes all its key requests before seeing the challenge ciphertext. In this work, we resolve the question of $$mathsf {SIM}$$ -based security for $${mathsf {IPFE}}$$ by showing that variants of the $${mathsf {IPFE}}$$ constructions by Agrawal et al. , based on $$mathsf {DDH}$$ , Paillier and $$mathsf {LWE}$$ , satisfy the strongest possible adaptive $$mathsf {SIM}$$ -based security where the adversary can make an unbounded number of key requests both before and after seeing the (single) challenge ciphertext. This establishes optimal security of the $${mathsf {IPFE}}$$ schemes, under all hardness assumptions on which it can (presently) be based.

Constructing Attribute Based Encryption (ABE) [56] for uniform models of computation from standard assumptions, is an important problem, about which very little is known. The only known ABE schemes in this setting that (i) avoid reliance on multilinear maps or indistinguishability obfuscation, (ii) support unbounded length inputs and (iii) permit unbounded key requests to the adversary in the security game, are by Waters from Crypto, 2012 [57] and its variants. Waters provided the first ABE for Deterministic Finite Automata (DFA) satisfying the above properties, from a parametrized or “q-type” assumption over bilinear maps. Generalizing this construction to Nondeterministic Finite Automata (NFA) was left as an explicit open problem in the same work, and has seen no progress to date. Constructions from other assumptions such as more standard pairing based assumptions, or lattice based assumptions has also proved elusive.In this work, we construct the first symmetric key attribute based encryption scheme for nondeterministic finite automata (NFA) from the learning with errors (LWE) assumption. Our scheme supports unbounded length inputs as well as unbounded length machines. In more detail, secret keys in our construction are associated with an NFA M of unbounded length, ciphertexts are associated with a tuple $$(\mathbf {x}, m)$$ where $$\mathbf {x}$$ is a public attribute of unbounded length and m is a secret message bit, and decryption recovers m if and only if $$M(\mathbf {x})=1$$.Further, we leverage our ABE to achieve (restricted notions of) attribute hiding analogous to the circuit setting, obtaining the first predicate encryption and bounded key functional encryption schemes for NFA from LWE. We achieve machine hiding in the single/bounded key setting to obtain the first reusable garbled NFA from standard assumptions. In terms of lower bounds, we show that secret key functional encryption even for DFAs, with security against unbounded key requests implies indistinguishability obfuscation ($$\mathsf {iO}$$) for circuits; this suggests a barrier in achieving full fledged functional encryption for NFA.

Waters [Crypto, 2012] provided the first attribute based encryption scheme ABE for Deterministic Finite Automata (DFA) from a parametrized or “q-type” assumption over bilinear maps. Obtaining a construction from static assumptions has been elusive, despite much progress in the area of ABE.In this work, we construct the first attribute based encryption scheme for DFA from static assumptions on pairings, namely, the $$\mathsf{DLIN}$$ assumption. Our scheme supports unbounded length inputs, unbounded length machines and unbounded key requests. In more detail, secret keys in our construction are associated with a DFA M of unbounded length, ciphertexts are associated with a tuple $$(\mathbf {x}, \mathsf {\mu })$$ where $$\mathbf {x}$$ is a public attribute of unbounded length and $$\mathsf {\mu }$$ is a secret message bit, and decryption recovers $$\mathsf {\mu }$$ if and only if $$M(\mathbf {x})=1$$.Our techniques are at least as interesting as our final result. We present a simple compiler that combines constructions of unbounded ABE schemes for monotone span programs (MSP) in a black box way to construct ABE for DFA. In more detail, we find a way to embed DFA computation into monotone span programs, which lets us compose existing constructions (modified suitably) of unbounded key-policy ABE ($${\mathsf {kpABE}}$$) and unbounded ciphertext-policy ABE ($${\mathsf {cpABE}}$$) for MSP in a simple and modular way to obtain key-policy ABE for DFA. Our construction uses its building blocks in a symmetric way – by swapping the use of the underlying $${\mathsf {kpABE}}$$ and $${\mathsf {cpABE}}$$, we also obtain a construction of ciphertext-policy ABE for DFA.Our work extends techniques developed recently by Agrawal, Maitra and Yamada [Crypto 2019], which show how to construct ABE that support unbounded machines and unbounded inputs by combining ABE schemes that are bounded in one co-ordinate. At the heart of our work is the observation that unbounded, multi-use ABE for MSP already achieve most of what we need to build ABE for DFA.

We construct Indistinguishability Obfuscation ($$\mathsf {iO}$$) and Functional Encryption ($$\mathsf {FE}$$) schemes in the Turing machine model from the minimal assumption of compact $$\mathsf {FE}$$ for circuits ($$\mathsf {CktFE}$$). Our constructions overcome the barrier of sub-exponential loss incurred by all prior work. Our contributions are:1.We construct $$\mathsf {iO}$$ in the Turing machine model from the same assumptions as required in the circuit model, namely, sub-exponentially secure $$\mathsf {FE}$$ for circuits. The previous best constructions [6, 41] require sub-exponentially secure $$\mathsf {iO}$$ for circuits, which in turn requires sub-exponentially secure $$\mathsf {FE}$$ for circuits [5, 15].2.We provide a new construction of single input $$\mathsf {FE}$$ for Turing machines with unbounded length inputs and optimal parameters from polynomially secure, compact $$\mathsf {FE}$$ for circuits. The previously best known construction by Ananth and Sahai [7] relies on $$\mathsf {iO}$$ for circuits, or equivalently, sub-exponentially secure $$\mathsf {FE}$$ for circuits.3.We provide a new construction of multi-input $$\mathsf {FE}$$ for Turing machines. Our construction supports a fixed number of encryptors (say k), who may each encrypt a string $$\mathbf {x}_i$$ of unbounded length. We rely on sub-exponentially secure $$\mathsf {FE}$$ for circuits, while the only previous construction [10] relies on a strong knowledge type assumption, namely, public coin differing inputs obfuscation. Our techniques are new and from first principles, and avoid usage of sophisticated $$\mathsf {iO}$$ specific machinery such as positional accumulators and splittable signatures that were used by all relevant prior work [6, 7, 41].