What is a Secure Email?

While security indicators for web browsers have been and still are an active research area, comparable indicators for secure email have received relatively little attention. However, similar questions emerge, for example, how users interpret existing indicators and whether there exist other indicators that are potentially more effective. In our paper, we review existing indicators used in the context of email encryption. Based thereof, we identify and study promising candidates and compare them using a survey instrument with 164 participants. Based on the results, postcards, mail envelopes and a torn envelope warrant further investigation. They offered an intuitive access and consistent interpretation on par with the dominant padlock metaphor.