Statement-Oblivious Threshold Witness Encryption
Sebastian Faust, Carmit Hazay, David Kretzler, Benjamin Schlosser · 2023
The notion of witness encryption introduced by Garg et al. (STOC'13) allows to encrypt a message under a statement$x$from some NP-language$\mathcal{L}$with associated relation$(x, w)\in \mathcal{R}$, where decryption can be carried out with the corresponding witness$w$. Unfortunately, known constructions for general-purpose witness encryption rely on strong assumptions, and are mostly of theoretical interest. To address these short-comings, Goyal et al. (PKC'22) recently introduced a blockchain-based alternative, where a committee decrypts ciphertexts when provided with a valid witness$w$. Blockchain-based committee solutions have recently gained broad interest to offer security against more powerful adversaries and construct new cryptographic primitives. We follow this line of work, and propose a new notion of statement-oblivious threshold witness encryption. Our new notion offers the functionality of committee-based witness encryption while additionally hiding the statement used for encryption. We present two ways to build statement-oblivious threshold witness encryption, one generic transformation based on anonymous threshold identity-based encryption (A-TIBE) and one direct construction based on bilinear maps. Due to the lack of efficient A-TIBE schemes, the former mainly constitutes a feasibility result, while the latter yields a concretely efficient scheme.