Chapter 5. Topics

This chapter covers miscellaneous topics that do not fit elsewhere in the user manual. As topics are added, they may be collected and factored into chapters that stand on their own. If you would like to see a particular topic covered, please suggest it. Even better, volunteer to write a first draft covering your suggested topic!

Writing user interfaces

Alma Whitten and Doug Tygar have done a study on NAI's PGP 5.0 user interface and came to the conclusion that novice users find PGP confusing and frustrating. In their human factors study, only four out of twelve test subjects managed to correctly send encrypted email to their team members, and three out of twelve emailed the secret without encryption. Furthermore, half of the test subjects had a technical background.

These results are not surprising. PGP 5.0 has a nice user interface that is excellent if you already understand how public-key encryption works and are familiar with the web-of-trust key management model specified by OpenPGP. Unfortunately, novice users understand neither public-key encryption nor key management, and the user interface does little to help.

You should certainly read Whitten and Tygar's report if you are writing a user interface. It gives specific comments from each of the test subjects, and those details are enlightening. For example, it would appear that many of subjects believed that a message being sent to other people should be encrypted to the test subject's own public key. Consider it for a minute, and you will see that it is an easy mistake to make. In general, novice users have difficulty understanding the different roles of the public key and private key when using GnuPG. As a user interface designer, you should try to make it clear at all times when one of the two keys is being used. You could also use wizards or other common GUI techniques for guiding the user through common tasks such as key generation where extra steps such as generating a key revocation certification and making a backup are all but essential for using GnuPG correctly. Other comments from the paper include the following.

Designing an effective user interface for key management is even more difficult. The OpenPGP web-of-trust model is unfortunately quite obtuse. For example, the specification imposes three arbitrary trust levels onto the user: none, marginal, and complete. All degrees of trust felt by the user must be fit into one of those three cubby holes. The key validation algorithm is also difficult for non-computer scientists to understand, particularly the notions of ``marginals needed'' and ``completes needed''. Since the web-of-trust model is well-specified and cannot be changed, you will have to do your best and design a user interface that helps to clarify it for the user. A definite improvement, for example, would be to generate a diagram of how a key was validated when requested by the user. Relevant comments from the paper include the following.