Words of Wisdom: Security vs. Usability

"At the heart of the apparent conflict between security and usability is the idea that security makes operations harder, yet usability makes operations easier. Although this is usually true, it’s imprecise. Security isn’t about making all operations difficult; it’s about restricting access to operations with undesirable effects. Usability isn’t about making all operations easy, either; it’s about improving access to operations with desirable effects. Tension between the two arises to the extent that a system is unable to determine whether a particular result is desirable. Security and usability come into harmony when a system correctly interprets the user’s desires.”

Yee, Ka-Ping. "Guidelines and strategies for secure interaction design." Security and Usability: Designing Secure Systems That People Can Use (2005): 247-273
http://labs.toolness.com/temp/sid/ch13yee.pdf