### Witness Encryption and Null-IO from Evasive LWE

##### Abstract

Witness encryption (WE) allows us to use an arbitrary NP statement $x$ as a public key to encrypt a message, and the witness $w$ serves as a decryption key. Security ensures that, when the statement $x$ is false, the encrypted message remains computationally hidden. WE appears to be significantly weaker than indistinguishability obfuscation (iO). Indeed, WE is closely related to a highly restricted form of iO that only guarantees security for null circuits (null iO). However, all current approaches towards constructing WE under nice assumptions go through iO. Such constructions are quite complex and are unlikely to lead to practically instantiable schemes. In this work, we revisit a very simple WE and null iO candidate of Chen, Vaikuntanathan and Wee (CRYPTO 2018). We show how to prove its security under a nice and easy-to-state assumption that we refer to as "evasive LWE" following Wee (EUROCRYPT 2022). Roughly speaking, the evasive LWE assumption says the following: assume we have some joint distributions over matrices $\mathbf{P}$, $\mathbf{S}$ and auxiliary information $\mathsf{aux}$ such that $$({\mathbf{S}\mathbf{B} + \mathbf{E}},{\mathbf{S} \mathbf{P} + \mathbf{E}'}, \mathsf{aux}) \approx_c ({\mathbf{U}},{\mathbf{U'}}, \mathsf{aux}),$$ for a uniformly random (and secret) matrix $\mathbf{B}$, where $\mathbf{U}, \mathbf{U}'$ are uniformly random matrices, and $\mathbf{E},\mathbf{E}'$ are chosen from the LWE error distribution with appropriate parameters. Then it must also be the case that: $$\mathbf{S}\mathbf{B} + \mathbf{E}, \mathbf{B}^{-1}(\mathbf{P}),\mathsf{aux}) \approx_c (\mathbf{U}, \mathbf{B}^{-1}(\mathbf{P}),\mathsf{aux}).$$ Essentially the above says that given $\mathbf{S}\mathbf{B} + \mathbf{E}$, getting the additional component $\mathbf{B}^{-1}(\mathbf{P})$ is no more useful than just getting the product $({\mathbf{S}\mathbf{B} + \mathbf{E}})\cdot \mathbf{B}^{-1}(\mathbf{P}) \approx \mathbf{S} \mathbf{P} + \mathbf{E}'$.

##### Metadata
Available format(s)
Category
Public-key cryptography
Publication info
Published by the IACR in ASIACRYPT 2022
Contact author(s)
vinodv @ mit edu
wee @ di ens fr
wichs @ ccs neu edu
History
2022-09-05: approved
2022-08-31: received
See all versions
Short URL
https://ia.cr/2022/1140
License

CC BY

BibTeX

@misc{cryptoeprint:2022/1140,
author = {Vinod Vaikuntanathan and Hoeteck Wee and Daniel Wichs},
title = {Witness Encryption and Null-IO from Evasive LWE},
howpublished = {Cryptology ePrint Archive, Paper 2022/1140},
year = {2022},
note = {\url{https://eprint.iacr.org/2022/1140}},
url = {https://eprint.iacr.org/2022/1140}
}

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