Keyringless GnuPG

This article was discussed on Hacker News.

My favorite music player is Audacious. It follows the Winamp Classic tradition of not trying to manage my music library. Instead it waits patiently for me to throw files and directories at it. These selections will be informally grouped into transient, disposable playlists of whatever I fancy that day.

This matters to me because my music collection is the result of around 25 years of hoarding music files from various sources including CD rips, Napster P2P sharing, and, most recently, YouTube downloads. It’s not well-organized, but it’s organized well enough. Each album has its own directory, and related albums are sometimes grouped together under a directory for a particular artist.

Over the years I’ve tried various music players, and some have either wanted to manage this library or hide the underlying file-organized nature of my collection. Both situations are annoying because I really don’t want or need that abstraction. I’m going just fine thinking of my music library in terms of files, thank you very much. Same goes for ebooks.

GnuPG is like a media player that wants to manage your whole music library. Rather than MP3s, it’s crypto keys on a keyring. Nearly every operation requires keys that have been imported into the keyring. Until GnuPG 2.2.8 (June 2018), which added the --show-keys command, you couldn’t even be sure what you were importing until after it was already imported. Hopefully it wasn’t garbage.

GnuPG does has a pretty good excuse. It’s oriented around the Web of Trust model, and it can’t follow this model effectively without having all the keys at once. However, even if you don’t buy into the Web of Trust, the GnuPG interface still requires you to play by its rules. Sometimes I’ve got a message, a signature, and a public key and I just want to verify that they’re all consistent with each other, damnit.

$ gpg --import foo.asc
gpg: key 1A719EF63AEB2CFE: public key "foo" imported
gpg: Total number processed: 1
gpg:               imported: 1
$ gpg --verify --trust-model always message.txt.sig message.txt
gpg: Signature made Fri 09 Aug 2019 05:44:43 PM EDT
gpg:                using EDDSA key ...1A719EF63AEB2CFE
gpg: Good signature from "foo" [unknown]
gpg: WARNING: Using untrusted key!
$ gpg --batch --yes --delete-key 1A719EF63AEB2CFE

Three commands and seven lines of output when one of each would do. Plus there’s a false warning: Wouldn’t an “always” trust model mean that this key is indeed trusted?

Signify

Compare this to OpenBSD’s signify (also). There’s no keyring, and it’s up to the user — or the program shelling out to signify — to supply the appropriate key. It’s like the music player that just plays whatever I give it. Here’s a simplified usage overview:

signify -G [-c comment] -p pubkey -s seckey
signify -S [-x sigfile] -s seckey -m message
signify -V [-x sigfile] -p pubkey -m message

When generating a new keypair (-G), the user must choose the destination files for the public and secret keys. When signing a message (a file), the user must supply the secret key and the message. When verifying a file, the user must supply the public key and the message. This is a popular enough model that other, compatible implementations with the same interface have been developed.

Signify is deliberately incompatible with OpenPGP and uses its own simpler, and less featureful, format. Wouldn’t it be nice to have a similar interface to verify OpenPGP signatures?

SimpleGPG

Well, I thought so. So I put together a shell script that wraps GnuPG and provides such an interface:

SimpleGPG

The interface is nearly identical to signify, and the GnuPG keyring is hidden away as if it didn’t exist. The main difference is that the keys and signatures produced and consumed by this tool are fully compatible with OpenPGP. You could use this script without requiring anyone else to adopt something new or different.

To avoid touching your real keyring, the script creates a temporary keyring directory each time it’s run. The GnuPG option --homedir instructs it to use this temporary keyring and ignore the usual one. The temporary keyring is destroyed when the script exits. This is kind of clunky, but there’s no way around it.

Verification looks roughly like this in the script:

$ tmp=$(mktemp -d simplegpg-XXXXXX)
$ gpg --homedir $tmp
$ gpg --homedir $tmp --import foo.asc
$ gpg --homedir $tmp --verify message.txt.sig message.txt
$ rm -rf $tmp

Generating a key is trivial, and there’s only a prompt for the protection passphrase. Like signify, it will generate an Ed25519 key and all outputs are ASCII-armored.

$ simplegpg -G -p keyname.asc -s keyname.pgp
passphrase:
passphrase (confirm):

Since signify doesn’t have a concept of a user ID for a key, just an “untrusted comment”, the user ID is not emphasized here. The default user ID will be “simplegpg key”, so, if you plan to share the key with regular GnuPG users who will need to import it into a keyring, you probably want to use -c to give it a more informative name.

Unfortunately due GnuPG’s very limited, keyring-oriented interface, key generation is about three times slower than it should be. That’s because the protection key is run though the String-to-Key (S2K) algorithm three times:

  1. Immediately after the key is generated, the passphrase is converted to a key, the key is encrypted, and it’s put onto the temporary keyring.

  2. When exporting, the key passphrase is again run through the S2K to get the protection key to decrypt it.

  3. The export format uses a slightly different S2K algorithm, so this export S2K is now used to create yet another protection key.

Technically the second could be avoided since gpg-agent, which is always required, could be holding the secret key material. As far as I can tell, gpg-agent simply does not learn freshly-generated keys. I do not know why this is the case.

This is related to another issue. If you’re accustomed to GnuPG, you may notice that the passphrase prompt didn’t come from pinentry, a program specialized for passphrase prompts. GnuPG normally uses it for this. Instead, the script handles the passphrase prompt and passes the passphrase to GnuPG (via a file descriptor). This would not be necessary if gpg-agent did its job. Without this part of the script, users are prompted three times, via pinentry, for their passphrase when generating a key.

When signing messages, the passphrase prompt comes from pinentry since it’s initiated by GnuPG.

$ simplegpg -S -s keyname.pgp -m message.txt
passphrase:

This will produce message.txt.sig with an OpenPGP detached signature.

The passphrase prompt is for --import, not --detach-sign. As with key generation, the S2K is run more than necessary: twice instead of once. First to generate the decryption key, then a second time to generate a different encryption key for the keyring since the export format and keyring use different algorithms. Ugh.

But at least gpg-agent does its job this time, so only one passphrase prompt is necessary. In general, a downside of these temporary keyrings is that gpg-agent treats each as different keys, and you will need to enter your passphrase once for each message signed. Just like signify.

Verification, of course, requires no prompting and no S2K.

$ simplegpg -V -p keyname.asc -m message.txt

That’s all there is to keyringless OpenPGP signatures. Since I’m not interested in the Web of Trust or keyservers, I wish GnuPG was more friendly to this model of operation.

passphrase2pgp

I mentioned that SimpleGPG is fully compatible with other OpenPGP systems. This includes my own passphrase2pgp, where your secret key is stored only in your brain. No need for a secret key file. In the time since I first wrote about it, passphrase2pgp has gained the ability to produce signatures itself!

I’ve got my environment set up — $REALNAME, $EMAIL, and $KEYID per the README — so I don’t need to supply a user ID argument, nor will I be prompted to confirm my passphrase since it’s checked against a known fingerprint. Generating the public key, for sharing, looks like this:

$ passphrase2pgp -K --armor --public >keyname.asc

Or just:

$ passphrase2pgp -ap >keyname.asc

Like with signify and SimplePGP, to sign a message I’m prompted for my passphrase. It takes longer since the “S2K” here is much stronger by necessity. The passphrase is used to generate the secret key, then from that the signature on the message:

$ passphrase2pgp -S message.txt

For the SimpleGPG user on the other side it all looks the same as before:

$ simplegpg -V -p keyname.asc -m message.txt

I’m probably going to start signing my open source software releases, and this is how I intend to do it.

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Chris Wellons

(PGP)