International Association
for Cryptologic Research

Broadcast encryption allows a user to encrypt a message to N recipients with a ciphertext whose size scales sublinearly with N. The natural security notion for broadcast encryption is adaptive security which allows an adversary to choose the set of recipients after seeing the public parameters. Achieving adaptive security in broadcast encryption is challenging, and in the plain model, the primary technique is the celebrated dual-systems approach, which can be implemented over groups with bilinear maps. Unfortunately, it has been challenging to replicate the dual-systems approach in other settings (e.g., with lattices or witness encryption). Moreover, even if we focus on pairing-based constructions, the dual-systems framework relies critically on decisional (and source-group) assumptions. We do not have constructions of adaptively-secure broadcast encryption from search (or target-group) assumptions assumptions in the plain model. Gentry and Waters (EUROCRYPT 2009) described a compiler that takes any semi-statically-secure broadcast encryption scheme and transforms it into an adaptively-secure scheme in the random oracle model. While semi-static security is easier to achieve and constructions are known from witness encryption as well as search (and target-group) assumptions on pairing groups, the transformed scheme relies on random oracles. In this work, we show that using publicly-sampleable projective PRGs, we can achieve adaptive security in the plain model. We then show how to build publicly-sampleable projective PRGs from many standard number-theoretic assumptions (e.g., CDH, LWE, RSA). Our compiler yields the first adaptively-secure broadcast encryption schemes from search assumptions as well as the first adaptively-secure scheme from witness encryption (which can in turn be based on evasive LWE) in the plain model. We also obtain the first adaptively-secure pairing-based scheme with linear-size public keys and constant-size ciphertexts. Previous adaptively-secure pairing-based schemes with constant-size ciphertexts had quadratic-size public keys.

We present a general framework for constructing attribute-based encryption (ABE) schemes for arbitrary function class based on lattices from two ingredients, i) a noisy linear secret sharing scheme for the class and ii) a new type of inner-product functional encryption (IPFE) scheme, termed *evasive* IPFE, which we introduce in this work. We propose lattice-based evasive IPFE schemes and establish their security under simple conditions based on variants of evasive learning with errors (LWE) assumption recently proposed by Wee [EUROCRYPT '22] and Tsabary [CRYPTO '22]. Our general framework is modular and conceptually simple, reducing the task of constructing ABE to that of constructing noisy linear secret sharing schemes, a more lightweight primitive. The versatility of our framework is demonstrated by three new ABE schemes based on variants of the evasive LWE assumption. - We obtain two ciphertext-policy ABE schemes for all polynomial-size circuits with a predetermined depth bound. One of these schemes has *succinct* ciphertexts and secret keys, of size polynomial in the depth bound, rather than the circuit size. This eliminates the need for tensor LWE, another new assumption, from the previous state-of-the-art construction by Wee [EUROCRYPT '22]. - We develop ciphertext-policy and key-policy ABE schemes for deterministic finite automata (DFA) and logspace Turing machines (L). They are the first lattice-based public-key ABE schemes supporting uniform models of computation. Previous lattice-based schemes for uniform computation were limited to the secret-key setting or offered only weaker security against bounded collusion. Lastly, the new primitive of evasive IPFE serves as the lattice-based counterpart of pairing-based IPFE, enabling the application of techniques developed in pairing-based ABE constructions to lattice-based constructions. We believe it is of independent interest and may find other applications.

Time-lock puzzles wrap a solution s inside a puzzle P in such a way that “solving” P to find s requires significantly more time than generating the pair (s, P), even if the adversary has access to parallel computing; hence it can be thought of as sending a message s to the future. It is known [Mahmoody, Moran, Vadhan, Crypto’11] that when the source of hardness is only a random oracle, then any puzzle generator with n queries can be (efficiently) broken by an adversary in O(n) rounds of queries to the oracle. In this work, we revisit time-lock puzzles in a quantum world by allowing the parties to use quantum computing and, in particular, access the random oracle in quantum superposition. An interesting setting is when the puzzle generator is efficient and classical, while the solver (who might be an entity developed in the future) is quantum-powered and is supposed to need a long sequential time to succeed. We prove that in this setting there is no construction of time-lock puzzles solely from quantum (accessible) random oracles. In particular, for any n-query classical puzzle generator, our attack only asks O(n) (also classical) queries to the random oracle, even though it does indeed run in quantum polynomial time if the honest puzzle solver needs quantum computing. Assuming perfect completeness, we also show how to make the above attack run in exactly n rounds while asking a total of m · n queries where m is the query complexity of the puzzle solver. This is indeed tight in the round complexity, as we also prove that a classical puzzle scheme of Mahmoody et al. is also secure against quantum solvers who ask n−1 rounds of queries. In fact, even for the fully classical case, our attack quantitatively improves the total queries of the attack of Mahmoody et al. for the case of perfect completeness from O(mn log n) to mn. Finally, assuming perfect completeness, we present an attack in the “dual” setting in which the puzzle generator is quantum while the solver is classical. We then ask whether one can extend our classical-query attack to the fully quantum setting, in which both the puzzle generator and the solver could be quantum. We show a barrier for proving such results unconditionally. In particular, we show that if the folklore simulation conjecture, first formally stated by Aaronson and Ambainis [arXiv’2009] is false, then there is indeed a time-lock puzzle in the quantum random oracle model that cannot be broken by classical adversaries. This result improves the previous barrier of Austrin et. al [Crypto’22] about key agreements (that can have interactions in both directions) to time-lock puzzles (that only include unidirectional communication).