Universally Composable Password-Hardened Encryption

Behzad Abdolmaleki; Ruben Baecker; Paul Gerhart; Mike Graf; Mojtaba Khalili; Daniel Rausch; Dominique Schröder

Paper 2025/1647

Universally Composable Password-Hardened Encryption

Behzad Abdolmaleki

, University of Sheffield

Ruben Baecker

, Friedrich-Alexander-Universität Erlangen-Nürnberg

Paul Gerhart

, TU Wien

Mike Graf

, University of Stuttgart

Mojtaba Khalili

, Isfahan University of Technology

Daniel Rausch

, University of Stuttgart

Dominique Schröder

, TU Wien

Abstract

Password-Hardened Encryption (PHE) protects against offline brute-force attacks by involving an external ratelimiter that enforces rate-limited decryption without learning passwords or keys. Threshold Password-Hardened Encryption (TPHE), introduced by Brost et al. (CCS’20), distributes this trust among multiple ratelimiters. Despite its promise, the security foundations of TPHE remain unclear. We make three contributions: (1) We uncover a flaw in the proof of Brost et al.’s TPHE scheme, which invalidates its claimed security and leaves the guarantees of existing constructions uncertain; (2) We provide the first universal composability (UC) formalization of PHE and TPHE, unifying previous fragmented models and supporting key rotation, an essential feature for long-term security and related primitives such as updatable encryption; (3) We present the first provably secure TPHE scheme, which is both round-optimal and UC-secure, thus composable in real-world settings; and we implement and evaluate our protocol, demonstrating practical efficiency that outperforms prior work in realistic WAN scenarios.

Metadata

Available format(s): PDF
Category: Cryptographic protocols
Publication info: Published by the IACR in ASIACRYPT 2025
Keywords: Password-Hardened Encryption Threshold Cryptography Universal Composability
Contact author(s): behzad abdolmaleki @ sheffield ac uk
ruben baecker @ fau de
paul gerhart @ tuwien ac at
mike graf @ sec uni-stuttgart de
mojtabakhalyly @ gmail com
daniel rausch @ sec uni-stuttgart de
dominique schroeder @ tuwien ac at
History: 2025-09-12: approved; 2025-09-11: received; See all versions
Short URL: https://ia.cr/2025/1647
License: CC BY

BibTeX

@misc{cryptoeprint:2025/1647,
      author = {Behzad Abdolmaleki and Ruben Baecker and Paul Gerhart and Mike Graf and Mojtaba Khalili and Daniel Rausch and Dominique Schröder},
      title = {Universally Composable Password-Hardened Encryption},
      howpublished = {Cryptology {ePrint} Archive, Paper 2025/1647},
      year = {2025},
      url = {https://eprint.iacr.org/2025/1647}
}