For your purposes, I think that your choice of PEM is probably the best. As you note, it is logically equivalent to the s-expression (it is typed, not a bag of

For your purposes, I think that your choice of PEM is probably the best. As you note, it is logically equivalent to the s-expression (it is typed, not a bag of bytes), and it has the advantage of being a well-known format. If æsthetic concerns are a higher priority than interoperability (say, for a personal hobby project), then I do recommend folks adopt the format I gave an example of. It has some history behind it, dating back to the 1997–1999 timeframe and the IETF’s SPKI Working Group (e.g. https://theworld.com/~cme/examples.txt https://theworld.com/~cme/examples.txt) which resulted in RFCs 2692 & 2693, as well as some great drafts (https://people.csail.mit.edu/rivest/Sexp.txt https://people.csail.mit.edu/rivest/Sexp.txt & https://theworld.com/~cme/spki.txt https://theworld.com/~cme/spki.txt) which never progressed because Reasons™. I still think that the SPKI project got PKI right. Oh well, what might have been!