Survey of Simple Signature Schemes and KEMs?

I want to do a survey of simple asymmetric signature schemes and KEM or PKI schemes.

For instance schnorr signatures are very simple and can be explained on a business card. And elgamal and some matrix schemes are comparitively simple compared to schemes based upon curves and lattices. I would like to learn about other simple schemes that are not popular or that use simple yet arcane maths. The simpler a scheme is the more interest I will have in studying it. Please follow up post some suggested schemes, especially arcane ones that don't have a lot of testing history and are not known to be broken.

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