Found in 3 comments on Hacker News
daxelrod · 2016-11-30 · Original thread
I haven't read it, but "PGP: Source Code and Internals", by PGP's author Philip Zimmerman is worth a try. I've read his other book "Official PGP User's Guide" and learned quite a bit.

· 2016-02-15 · Original thread

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