Paper 2024/1983 Original: eprint.iacr.org/2024/1983 ↗

UTRA: Universe Token Reusability Attack and Verifiable Delegatable Order-Revealing Encryption

Abstract

As datasets grow, users increasingly rely on \modi{ cloud services for data storage and processing. Consequently, concerns regarding data protection and the practical use of encrypted data have emerged as significant challenges. One promising solution is} order-revealing encryption (ORE), which enables efficient operations on encrypted numerical data. \modi{To support distributed environments with different users, delegatable ORE (DORE) extends this functionality to multi-client settings, enabling order comparisons between ciphertexts encrypted under different secret keys. However, Hahn et al. proposed a token forgery attack against DORE with a threat model and introduced the secure DORE (SEDORE) scheme as a countermeasure. Despite this enhancement, we claim that SEDORE remains vulnerable under the same threat model.} In this paper, we present a novel Universal Token Reusability Attack, which exposes a critical vulnerability in SEDORE with the identical threat model. To mitigate this, we introduce the concept of verifiable delegatable order-revealing encryption (VDORE), along with a formal definition of token unforgeability. Building on this, we design a new scheme, Token Unforgeable DORE ($\mathsf{TUDORE}$), which ensures token unforgeability. Moreover, $\mathsf{TUDORE}$ achieves 1.5× faster token generation than SEDORE with enhanced security.