International Association
for Cryptologic Research

The Fast IDentity Online (FIDO) Alliance has developed the widely adopted FIDO2 protocol suite that allows for passwordless online authentication. Cryptographic keys stored on a user’s device (e.g. their smartphone) are used as credentials to authenticate to services by performing a challenge-response protocol. Yet, this approach leaves users unable to access their accounts in case their authenticator is lost. The device manufacturer Yubico thus proposed a FIDO2-compliant mech- anism that allows to easily create backup authenticators. Frymann et al. (CCS 2020) have first analyzed the cryptographic core of this pro- posal by introducing the new primitive of Asynchronous Remote Key Generation (ARKG) and accompanying security definitions. Later works instantiated ARKG both from classical and post-quantum assumptions (ACNS 2023, EuroS&P 2023). As we will point out in this paper, the security definitions put forward and used in these papers do not adequately capture the desired security requirements in FIDO2-based authentication and recovery. This issue was also identified in independent and concurrent work by Stebila and Wilson (AsiaCCS 2024), who proposed a new framework for the analy- sis of account recovery mechanisms, along with a secure post-quantum instantiation from KEMs and key-blinding signature schemes. In this work, we propose alternative security definitions for the primitive ARKG when used inside an account recovery mechanism in FIDO2. We give a secure instantiation from KEMs and standard signature schemes, which may in particular provide post-quantum security. Our solution strikes a middle ground between the compact, but (for this particular use case) inadequate security notions put forward by Frymann et al., and the secure, but more involved and highly tailored model introduced by Stebila and Wilson.

The key exchange protocol that establishes initial shared secrets in the handshake of the Signal end-to-end encrypted messaging protocol has several important characteristics: (1) it runs asynchronously (without both parties needing to be simultaneously online), (2) it provides implicit mutual authentication while retaining deniability (transcripts cannot be used to prove either party participated in the protocol), and (3) it retains security even if some keys are compromised (forward secrecy and beyond). All of these properties emerge from clever use of the highly flexible Diffie--Hellman protocol. While quantum-resistant key encapsulation mechanisms (KEMs) can replace Diffie--Hellman key exchange in some settings, there is no KEM-based replacement for the Signal handshake that achieves all three aforementioned properties, in part due to the inherent asymmetry of KEM operations. In this paper, we show how to construct asynchronous deniable key exchange by combining KEMs and designated verifier signature (DVS) schemes. There are several candidates for post-quantum DVS schemes, either direct constructions or via ring signatures. This yields a template for an efficient post-quantum realization of the Signal handshake with the same asynchronicity and security properties as the original Signal protocol.

The Signal protocol for end-to-end encrypted messaging provides a range of desirable security properties: asynchronicity, offline deniability, mutual implicit authentication, forward secrecy, and post-compromise security. Transitioning Signal to a post-quantum secure version with the same guarantees proves tricky, however. This is due to the fact that post-quantum key encapsulation mechanisms cannot be used as a drop-in replacement for the clever use of the Diffie--Hellman protocol in Signal's initial key exchange X3DH. In this talk, we elaborate on this obstacle, which may arise in further high-level protocols with subtle security guarantees, and show how to achieve asynchronous deniable key exchange from key encapsulation mechanisms and designated verifier signatures. In particular, we present a provably-secure construction for the post-quantum Signal initial key agreement which achieves the same security guarantees as the currently used X3DH.