Abstract or Summary:
One recent thread of academic and commercial research into web authentication has focused on schemes where users scan a visual code with their smartphone, which is a convenient alternative to password-based login. We find that many schemes in the literature (including, previously, our own) are, unfortunately, vulnerable to relay attacks. We explain the inherent reasons for this vulnerability and offer an architectural fix, evaluating its trade-offs and discussing why it has never been proposed by other authors.