Smartphone authentication has received considerable attention of the research community for the past several years. Every year, in diverse set of top conferences, such as, CHI, MobiSyS, MobiCom, UbiComp, USENIX Security, CCS, NDSS, you can find some new alternative authentication mechanism proposal. This is not surprising given how important smartphones have become for people’s daily lives. Smartphone authentication is also of particular interest because there have been also deployments by the industry beyond the usual PINs and passwords. Two prominent examples include the iOS's fingerprint authentication mechanism TouchID and Android’s 3x3 grid-based graphical password. De Luca and Lindqvist have recently summarized related literature and issues for these authentication methods [1].