International Association for Cryptologic Research

International Association
for Cryptologic Research


The Pre-Shared Key Modes of HPKE

Joël Alwen , AWS Wickr
Jonas Janneck , Ruhr-Universität Bochum
Eike Kiltz , Ruhr-Universität Bochum
Benjamin Lipp , Max Planck Institute for Security and Privacy
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Presentation: Slides
Conference: ASIACRYPT 2023
Abstract: The Hybrid Public Key Encryption (HPKE) standard was recently published as RFC 9180 by the Crypto Forum Research Group (CFRG) of the Internet Research Task Force (IRTF). The RFC specifies an efficient public key encryption scheme, combining asymmetric and symmetric cryptographic building blocks. Out of HPKE’s four modes, two have already been formally analyzed by Alwen et al. (EUROCRYPT 2021). This work considers the remaining two modes: HPKE_PSK and HPKE_AuthPSK. Both of them are “pre-shared key” modes that assume the sender and receiver hold a symmetric pre- shared key. We capture the schemes with two new primitives which we call pre-shared key public-key encryption (pskPKE) and pre-shared key authenticated public-key encryption (pskAPKE). We provide formal secu- rity models for pskPKE and pskAPKE and prove (via general composition theorems) that the two modes HPKE_PSK and HPKE_AuthPSK offer active security (in the sense of insider privacy and outsider authenticity) under the Gap Diffie-Hellman assumption. We furthermore explore possible post-quantum secure instantiations of the HPKE standard and propose new solutions based on lattices and isogenies. Moreover, we show how HPKE’s basic HPKE_PSK and HPKEAuth_PSK modes can be used black-box in a simple way to build actively secure post- quantum/classic-hybrid (authenticated) encryption schemes. Our hybrid constructions provide a cheap and easy path towards a practical post- quantum secure drop-in replacement for the basic HPKE modes HPKE_Base and HPKE_Auth.
  title={The Pre-Shared Key Modes of HPKE},
  author={Joël Alwen and Jonas Janneck and Eike Kiltz and Benjamin Lipp},