Security and usability engineering with particular attention to electronic mail

Support for strong electronic mail security is widely available yet only few communicants appear to make use of these features. Apparently, the operational overhead of security outweighs its perceived benefits. Towards increasing the benefits versus overhead ratio we follow an approach that considers security and usability tradeoffs from the outset. We separate key exchange from binding keys to identities. The best effort key exchange and key maintenance scheme that we devise operates transparently for the user. We also describe complementary visualization and interaction techniques that communicate the security state of sent and received mail to users in a non-intrusive fashion. Structured interviews were conducted with 19 users to assess the usability of the metaphors and the complementary visualizations of the security state. Towards a practical assessment of the overheads of binding keys to identities, we conducted a quantitative analysis of 17 users' anonymized mailbox extracts to determine which security mechanisms would be most appropriate for their communication patterns. We argue that for individual non-commercial users, out-of-band verification of keys could be more economical than building trust in public key certificates issued by third parties.

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