Confused Johnny: when automatic encryption leads to confusion and mistakes

A common approach to designing usable security is to hide as many security details as possible from the user to reduce the amount of information and actions a user must encounter. This paper gives an overview of Pwm (Private Webmail), our secure webmail system that uses security overlays to integrate tightly with existing webmail services like Gmail. Pwm's security is mostly transparent, including automatic key management and automatic encryption. We describe a series of Pwm user studies indicating that while nearly all users can use the system without any prior training, the security details are so transparent that a small percentage of users mistakenly sent out unencrypted messages and some users are unsure whether they should trust Pwm. We then conducted user studies with an alternative prototype to Pwm that uses manual encryption. Surprisingly users were accepting of the extra steps of cutting and pasting ciphertext themselves. They avoided mistakes and had more trust in the system with manual encryption. Our results suggest that designers may want to reconsider manual encryption as a way to reduce transparency and foster greater trust.