  This paper proposes a frictionless authentication system, provides a
comprehensive security analysis of and proposes potential solutions for this
system. It first presents a system that allows users to authenticate to
services in a frictionless manner, i.e., without the need to perform any
particular authentication-related actions. Based on this system model, the
paper analyses security problems and potential privacy threats imposed on
users, leading to the specification of a set of security and privacy
requirements. These requirements can be used as a guidance on designing secure
and privacy-friendly frictionless authentication systems. The paper also
sketches three potential solutions for such systems and highlights their
advantages and disadvantages.
