Why Johnny can't encrypt: A usability evaluation of PGP 5.0

A. Whitten and J. D. Tygar. “Why Johnny can’t encrypt: A usability evaluation of PGP 5.0.” In Security and Usability: Designing Secure Systems that People Can Use, eds. L. Cranor and G. Simson. O'Reilly, 2005, pp. 679-702.