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There is an inescapable engineering tradeoff between security and usability. While it is easy to make a system that is not very secure or usable it is impossible to make a system that gets top marks in both categories.

This is simply because the more security a system requires, the more authentication evidence is required. A 4-digit PIN is easy to remember and easy to input, but also easy to guess so it provides just a few bits of Bayesian evidence that the user is in fact who they claim to be.

Kinds of evidence that can increase the system’s authentication confidence include:

Obviously each layer of added security reduces usability because it requires more effort from the user or restricts the flexibility of the use cases.


There is no “right” answer here, just a cost associated with a given preference represented by the shade of grey (lighter has lower costs, darker has higher costs, but black is unattainable no matter the cost).