International Association for Cryptologic Research

International Association
for Cryptologic Research


Anuja Modi


Bounded Functional Encryption for Turing Machines: Adaptive Security from General Assumptions
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 '21] constructed attribute based encryption (ABE) for Turing machines achieving adaptive indistinguishability based security against bounded (static) collusions from IBE, in the random oracle model. In this work, we significantly improve the state of art for dynamic bounded collusion FE and ABE for Turing machines by achieving \emph{adaptive} simulation style security from a broad class of assumptions, in the standard model. In more detail, we obtain the following results: \begin{enumerate} \item We construct an adaptively secure (AD-SIM) FE for Turing machines, supporting dynamic bounded collusion, from sub-exponential LWE. This improves the result of Agrawal et al. which achieved only non-adaptive (NA-SIM) security in the dynamic bounded collusion model. \item Towards achieving the above goal, we construct a \emph{ciphertext policy} FE scheme (CPFE) for circuits of \emph{unbounded} size and depth, which achieves AD-SIM security in the dynamic bounded collusion model from IBE and \emph{laconic oblivious transfer} (LOT). Both IBE and LOT can be instantiated from a large number of mild assumptions such as the computational Diffie-Hellman assumption, the factoring assumption, and polynomial LWE. This improves the construction of Agrawal et al. which could only achieve NA-SIM security for CPFE supporting circuits of unbounded depth from IBE. \item We construct an AD-SIM secure FE for Turing machines, supporting dynamic bounded collusions, from LOT, ABE for NC1 (or NC) and private information retrieval (PIR) schemes which satisfy certain properties. This significantly expands the class of assumptions on which AD-SIM secure FE for Turing machines can be based. In particular, it leads to new constructions of FE for Turing machines including one based on polynomial LWE and one based on the combination of the bilinear decisional Diffie-Hellman assumption and the decisional Diffie-Hellman assumption on some specific groups. In contrast the only prior construction by Agrawal et al. achieved only NA-SIM security and relied on \emph{sub-exponential} LWE. To achieve the above result, we define the notion of CPFE for read only RAM programs and succinct FE for LOT, which may be of independent interest. \item We also construct an \emph{ABE} scheme for Turing machines which achieves AD-IND security in the \emph{standard model} supporting dynamic bounded collusions. Our scheme is based on IBE and LOT. Previously, the only known candidate that achieved AD-IND security from IBE by Goyal et al. relied on the random oracle model. \end{enumerate}