  Authentication and authorization are critical security layers to protect a
wide range of online systems, services and content. However, the increased
prevalence of wearable and mobile devices, the expectations of a frictionless
experience and the diverse user environments will challenge the way users are
authenticated. Consumers demand secure and privacy-aware access from any
device, whenever and wherever they are, without any obstacles. This paper
reviews emerging trends and challenges with frictionless authentication systems
and identifies opportunities for further research related to the enrollment of
users, the usability of authentication schemes, as well as security and privacy
trade-offs of mobile and wearable continuous authentication systems.
