  The advent of end-to-end encrypted (E2EE) messaging and backup services has
brought new challenges for usable authentication. Compared to regular web
services, the nature of E2EE implies that the provider cannot recover data for
users who have forgotten passwords or lost devices. Therefore, new forms of
robustness and recoverability are required, leading to a plethora of solutions
ranging from randomly-generated recovery codes to threshold-based social
verification. These implications also spread to new forms of authentication and
legacy web services: passwordless authentication ("passkeys") has become a
promising candidate to replace passwords altogether, but are inherently
device-bound. However, users expect that they can login from multiple devices
and recover their passwords in case of device loss--prompting providers to sync
credentials to cloud storage using E2EE, resulting in the very same
authentication challenges of regular E2EE services. Hence, E2EE authentication
quickly becomes relevant not only for a niche group of dedicated E2EE
enthusiasts but for the general public using the passwordless authentication
techniques promoted by their device vendors. In this paper we systematize
existing research literature and industry practice relating to security,
privacy, usability, and recoverability of E2EE authentication. We investigate
authentication and recovery schemes in all widely-used E2EE web services and
survey passwordless authentication deployment in the top-200 most popular
websites. Finally, we present concrete research directions based on observed
gaps between industry deployment and academic literature.
