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Ask HN: Is there a “ground-up” explanation of PGP/GnuPG?

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Re: Ask HN: Is there a “ground-up” explanation of PGP/GnuPG?

#12

Post by kqr »

hannob wrote: post #3

It depends a lot on from which angle you want to understand it. There's a difference between "understanding the variety of command line options" vs. "understanding the meaning of the raw data structures". I learned quite a bit by looking up things in the RFC: https://tools.ietf.org/html/rfc4880

I'm hoping that by understanding the meaning of the raw data structures, I can ask much more educated questions when I am faced with a new operation I want to perform using GnuPG. The idea is that instead of asking myself "what happens if I set a new, future expiration date on a revoked key?" with no clear answer, I could just think in terms of (e.g., I have no idea how it really works) "since the expiration date is set as an optional header extension to the key data structure, and the revocation bit is maintained in the mandatory header, the revocation bit takes precedence over any other extension headers, including the new expiry date I accidentally set."

This way, knowing the raw data structures makes it easier for me to figure out which command line arguments I want, if you will.

Re: Ask HN: Is there a “ground-up” explanation of PGP/GnuPG?

#14

Post by happimess »

avmich wrote: post #13

If you have some pointers to Git internals explanation, similar to what you're looking for PGP/GnuPG, can you provide them? That would be useful and illustrative :) .

I'm not OP, but I found this to be useful.

http://eagain.net/articles/git-for-computer-scientists/

Re: Ask HN: Is there a “ground-up” explanation of PGP/GnuPG?

#15

Post by haldean »

avmich wrote: post #13

If you have some pointers to Git internals explanation, similar to what you're looking for PGP/GnuPG, can you provide them? That would be useful and illustrative :) .

Git from the Bottom Up is very good. Previous discussion: https://news.ycombinator.com/item?id=10199391

Re: Ask HN: Is there a “ground-up” explanation of PGP/GnuPG?

#16

Post by mdup »

avmich wrote: post #13

If you have some pointers to Git internals explanation, similar to what you're looking for PGP/GnuPG, can you provide them? That would be useful and illustrative :) .

Not the OP, but I have found the following useful:

https://git-scm.com/book/en/v2/ scroll to Chapter 10 "Git Internals"

(Direct link: https://git-scm.com/book/en/v2/Git-Internals-Plumbing-and-Po... but it only shows you the first page among nine.)

Re: Ask HN: Is there a “ground-up” explanation of PGP/GnuPG?

#17

Post by lucb1e »

I don't know of any full explanations where they dissect the data. However you mentioned (in another comment) that you are familiar with RSA already, so assuming a basic code and crypto background, and from from what I know on a high level, PGP messages are something like this:

    print(number of recipients, algorithm used, etc.)
    for each recipient:
        print(RSA_encrypt(symmetric_key, recipient.public_key))
    print(AES_encrypt(message + hash(message), symmetric_key))
Typically if you send an email from a@example.com to b@example.com, it will find the two public keys from both parties and encrypt the symmetric key for both. The sender obviously wrote it, but they might want to read it back so the symmetric key is also encrypted with their public key, as well as the recipient's.

A random symmetric key is chosen to encrypt the message, since it would be silly to encrypt the whole message for each recipient again and again. And even if there's only one recipient, random key generation plus symmetric key encryption is typically faster than encrypting the whole message with asymmetric crypto (unless the message is just a few bytes, in which case it's fast regardless).

File encryption probably works the same way, except you're typically the sole recipient.

Signatures are done by encrypting a hash of the message with your private key, which everyone can decrypt with your public key to verify the hash. Since you're the only person with the private key, you are the only person who could have encrypted that hash, and since hashes are unique, you must have wanted to sign this text. (N.B. Both keys, public and private, can be used for both encryption and decryption, you just can't use the same key to decrypt if it was already used to encrypt and vice versa.) The hash is used rather than the full message for both speed and because it makes your signature a lot shorter.

Did I miss anything, at least from a crypto standpoint (since I don't know details of the file structure)?

Re: Ask HN: Is there a “ground-up” explanation of PGP/GnuPG?

#18

Post by vk23 »

avmich wrote: post #13

If you have some pointers to Git internals explanation, similar to what you're looking for PGP/GnuPG, can you provide them? That would be useful and illustrative :) .

The official git page has very good documentation regarding git internals. (The whole book is worth reading) https://git-scm.com/book/en/v2/Git-Internals-Plumbing-and-Po... gives you a good overview on how git handles things internally.

Re: Ask HN: Is there a “ground-up” explanation of PGP/GnuPG?

#19

Post by peller »

avmich wrote: post #13

If you have some pointers to Git internals explanation, similar to what you're looking for PGP/GnuPG, can you provide them? That would be useful and illustrative :) .

For a good deep-dive, I found this one very helpful: https://codewords.recurse.com/issues/two/git-from-the-inside...

Discussion: https://news.ycombinator.com/item?id=12802949

Re: Ask HN: Is there a “ground-up” explanation of PGP/GnuPG?

#20

Post by artemist »

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

Example output from a signed and encrypted file

  gpg: encrypted with 2048-bit RSA key, ID 09FBFEF359DD186F, created 2016-11-30
        "asdfas "
  # 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

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