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Universally Composable Password-Hardened Encryption

Authors:
Behzad Abdolmaleki , University of Sheffield
Ruben Baecker , FAU 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
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Conference: ASIACRYPT 2025
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.
BibTeX
@inproceedings{asiacrypt-2025-36154,
  title={Universally Composable Password-Hardened Encryption},
  publisher={Springer-Verlag},
  author={Behzad Abdolmaleki and Ruben Baecker and Paul Gerhart and Mike Graf and Mojtaba Khalili and Daniel Rausch and Dominique Schröder},
  year=2025
}