This is a draft!
Please give feedback, especially where the description is confusing or breaks the mental model of a hotel/apartment setup. As written, this is mean for fellow digisec trainers to leverage in their explanations, and is not (yet) a document to help people learning PGP by themselves.
There are a ton of ways people have described PGP in specific and public key cryptography more generally - from mixing paint (http://gizmodo.com/5888567/how-to-understand-encryption-using-paint-and-clocks, http://maths.straylight.co.uk/archives/108) to magic lock-boxes and Romeo and Juliet (https://www.level-up.cc/leading-trainings/training-curriculum/activity/love-story). I present another, which uses the experience of a hotel (or, alternatively, apartment buildings with front desk staff) as an analogy which can stretch to cover not just cryptography, but also digital signatures. This is written to explain PGP, but could be adapted for S/MIME and other public key crypto approaches.
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