If you have time and are fine with it being a bit dry, you can read RFC4880 [0], the RFC for OpenPGP.

This is something I have done some work on (I wrote a basic implementation in an attempt to understand a while ago [1]), but I don't have a nice writeup.

An OpenPGP file, whether it is a public key or encrypted file, consists of a list of packets. Generally it is a binary file, but an armored file consists of this binary in base64 and then a checksum. You can get these packets with gpg --list-packets <file>

Example output from a signed and encrypted file

  gpg: encrypted with 2048-bit RSA key, ID 09FBFEF359DD186F, created 2016-11-30
        "asdfas <sdfasdfasd@asdfasd.asdf>"
  # off=0 ctb=85 tag=1 hlen=3 plen=268
  :pubkey enc packet: version 3, algo 1, keyid 09FBFEF359DD186F
	  data: [2047 bits]
  # off=271 ctb=d2 tag=18 hlen=3 plen=377 new-ctb
  :encrypted data packet:
	  length: 377
	  mdc_method: 2
  # off=293 ctb=a3 tag=8 hlen=1 plen=0 indeterminate
  :compressed packet: algo=2
  # off=295 ctb=90 tag=4 hlen=2 plen=13
  :onepass_sig packet: keyid 0D3B106118D1EFBE
  	version 3, sigclass 0x00, digest 8, pubkey 1, last=1
  # off=310 ctb=ac tag=11 hlen=2 plen=19
  :literal data packet:
  	mode b (62), created 1480523012, name="file.txt",
  	raw data: 5 bytes
  # off=331 ctb=89 tag=2 hlen=3 plen=284
  :signature packet: algo 1, keyid 0D3B106118D1EFBE
  	version 4, created 1480523012, md5len 0, sigclass 0x00
  	digest algo 8, begin of digest 05 c4
  	hashed subpkt 2 len 4 (sig created 2016-11-30)
  	subpkt 16 len 8 (issuer key ID 0D3B106118D1EFBE)
  	data: [2046 bits]

The pubkey encrypted packets contain a key used to encrypt the data. The encrypted data packet includes that symmetrically encrypted data.

When I have more time, I may do a more useful writeup on my site, but currently I am too busy.

[0] https://www.ietf.org/rfc/rfc4880.txt [1] All I could find was my file parsing code, I dumped it at https://github.com/artemist/mupg

user:	artemist
created:	November 19, 2016
karma:	40