Signed GPG keys and keyservers

Recently, a couple of coworkers and I had a little key signing party in order for everyone on the team to be able to send and receive credentials and other sensitive data via insecure channels like Slack or e-mail.

Once we had all our keys generated and started signing them I noticed something weird. After checking with a collegue I uploaded his key that I had just signed onto keys.openpgp.org so that (1) he and (2) all the other people in the room could get the newly signed version easily. There was a problem, though: Turns out, the key that we all downloaded again from the keyserver didn’t contain my signature. In fact, it didn’t contain any signature except for the original one.

Looking through the FAQ I didn’t immediate saw it but Vincent Breitmoser, the admin of keys.openpgp.org, quickly pointed me to this little section in the FAQ detailing their handling of so-called “third-party signatures”:

Do you distribute “third party signatures”?

Short answer: No.

A “third party signature” is a signature on a key that was made by some other key. Most commonly, those are the signatures produced when “signing someone’s key”, which are the basis for the “Web of Trust”. For a number of reasons, those signatures are not currently distributed via keys.openpgp.org.

The killer reason is spam. Third party signatures allow attaching arbitrary data to anyone’s key, and nothing stops a malicious user from attaching so many megabytes of bloat to a key that it becomes practically unusable. Even worse, they could attach offensive or illegal content.

There are ideas to resolve this issue. For example, signatures could be distributed with the signer, rather than the signee. Alternatively, we could require cross-signing by the signee before distribution to support a caff-style workflow. If there is enough interest, we are open to working with other OpenPGP projects on a solution.

How is another popular keyserver, keyserver.ubuntu.com, handling that? To try that out, I exported my own signed key from my keychain, uploaded it there, and inspected what the server returns:

$ curl --silent https://keyserver.ubuntu.com/pks/lookup\?op\=get\&search\=0xc4e02ba840483a5d6b7616076f203f0d220f8e98 | gpg -v                                                                                                                                                                [k:taa-master.conf // minikube]
gpg: WARNING: no command supplied.  Trying to guess what you mean ...
pub   rsa4096 2017-01-15 [SC]
      C4E02BA840483A5D6B7616076F203F0D220F8E98
uid           Horst Gutmann <horst@zerokspot.com>
sig        6F203F0D220F8E98 2017-01-15   [selfsig]
sig        6F203F0D220F8E98 2017-01-15   [selfsig]
sig        6F203F0D220F8E98 2018-12-20   [selfsig]
sig        7CAF0EEF0B8DBFB6 2019-07-24   Robert van der Stel <robert@vanderstel.at>
sig        E8620BE896A1864F 2019-09-05   Maximilian Zollneritsch <m.zollneritsch@netconomy.net>
uid           Horst Gutmann <horst.gutmann@gmail.com>
sig        6F203F0D220F8E98 2017-01-15   [selfsig]
sig        6F203F0D220F8E98 2018-12-20   [selfsig]
sig        7CAF0EEF0B8DBFB6 2019-07-24   Robert van der Stel <robert@vanderstel.at>
sig        E8620BE896A1864F 2019-09-05   Maximilian Zollneritsch <m.zollneritsch@netconomy.net>
uid           Horst Gutmann <zerok@zerokspot.com>
sig        6F203F0D220F8E98 2017-01-15   [selfsig]
sig        6F203F0D220F8E98 2018-12-20   [selfsig]
sig        7CAF0EEF0B8DBFB6 2019-07-24   Robert van der Stel <robert@vanderstel.at>
sig        E8620BE896A1864F 2019-09-05   Maximilian Zollneritsch <m.zollneritsch@netconomy.net>
uid           Horst Gutmann <h.gutmann@netconomy.net>
sig        6F203F0D220F8E98 2017-01-15   [selfsig]
sig        6F203F0D220F8E98 2018-12-20   [selfsig]
sig        7CAF0EEF0B8DBFB6 2019-07-24   Robert van der Stel <robert@vanderstel.at>
sig        E8620BE896A1864F 2019-09-05   Maximilian Zollneritsch <m.zollneritsch@netconomy.net>
sub   rsa4096 2018-06-17 [A]
sig        6F203F0D220F8E98 2018-06-17   [keybind]
sub   rsa4096 2017-01-15 [E]
sig        6F203F0D220F8E98 2017-01-15   [keybind]
sig        6F203F0D220F8E98 2017-01-15   [selfsig]
sig        6F203F0D220F8E98 2018-12-20   [keybind]
sig        6F203F0D220F8E98 2018-12-20   [keybind]

OK, so that keyserver allows third-party signatures. What about pgp.mit.edu? Despite running into lots of timeouts and proxy errors I eventually managed to run the same test there with a similar result. The MIT server also returns third-party signatures.

So while the MIT and the Ubuntu servers “work”, I can absolutely get behind the reasoning behind openpgp.org’s decision not to support that feature. So for now I will simply request from anyone signing my key that they send me the signed key instead of uploading it somewhere.

That being said, I’m looking forward to a future where the spam problem has been fixed somehow…

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