Setting up WKD for self-hosted automatic key discovery

gpg-wkd.md

I just got this working so I figured I'd share what I found, since there's hardly any information about this anywhere online except an RFC, the GPG mailing list and one tutorial from the GnuPG blog.

You can use automatic key discovery with WKD (Web key directory) to make it easy for users to import your key, in GPG since version 2.1.12. Since this feature is fairly new, it isn't yet available in the current LTS release of Ubuntu (16.04; xenial), however it is available in Debian stable (stretch).

I couldn't add a DNS CERT or DANE / OPENPGPKEY record through my email service (which also hosts my nameservers). I tried making the PKA record - a foo._pka.example.com TXT record but GPG doesn't seem to recognize it and fails; I'm still investigating why.

So the last option for self-hosted auto-discovery was WKD.

First thing I had to do was add an email address to my key. My primary UID is just my name so the key represents my identity rather than any particular email address. This was easy enough:

$ gpg --edit-key 0xDEADBEEFCAFEBABE
gpg> adduid
# follow the prompts
gpg> save

I used this to configure a sub-identity using my domain name as the "real name" and an email address (foo@example.com). I suppose most here will already have an email address associated with their uid, or else be familiar with the process of editing keys.

Then I created a directory on my server:

$ ssh example.com
> mkdir -p /var/www/.well-known/openpgpkey/hu

The file you'll put inside this directory needs to be named the same as the WKD hash for your key, to get that run:

gpg --with-wkd-hash --fingerprint foo@example.com

Below each uid line with an email address, you should see 32 random looking characters @yourdomain.tld, for example:

pub   2048R/0xDEADBEEFCAFEBABE 2015-01-25 [C] [expires: 2020-01-25]
      Key fingerprint = ....
uid                   [ultimate] Your Name <foo@example.com>
                      sc8wrug2g3mz8m8jz4tjrlgweilkgcba@example.com

So the WKD hash in this example is sc8wrug2g3mz8m8jz4tjrlgweilkgcba. Let's create the file that we'll be uploading:

gpg --no-armor --export foo@example.com > sc8wrug2g3mz8m8jz4tjrlgweilkgcba

Copy that file into .well-known/openpgpkey/hu directory on your web server. If you have SSH access to your server configured you can use scp:

scp ./sc8wrug2g3mz8m8jz4tjrlgweilkgcba example.com:/var/www/.well-known/openpgpkey/hu/

I found that you do not need to enable directory listings for this well-known directory. To specify the correct content type with Apache, you can create a .htaccess file inside the .well-known/openpgpkey/hu directory with the following content:

<IfModule mod_mime.c>
    ForceType application/pgp-key
</IfModule>

That will force all files within the directory to be served as application/pgp-key.

That's all you need to do.

You can test that it's working correctly:

gpg --no-default-keyring --keyring /tmp/gpg-$$ --auto-key-locate clear,wkd --locate-keys foo@example.com

Example output:

gpg: key DEADBEEFCAFEBABE: public key "Your Name" imported
gpg: Total number processed: 1
gpg:               imported: 1
gpg: automatically retrieved 'foo@example.com' via WKD
...

You can instruct users who wish to import your key to run the command:

gpg --auto-key-locate clear,wkd --locate-keys foo@example.com

Or, to configure GPG to locate keys using wkd by placing this line in their gpg.conf:

auto-key-locate keyserver,dane,cert,pka,wkd,ldap,hkp://keys.gnupg.net

Note - that's just an example, only the wkd option is relevant for this, but the other options are handy too.

@sanmai

gpg --print-pka-records -k <userid>

is deprecated and replaced with:

gpg --export-options export-pka --export <userid>

Thanks for this gist!
If you need to export multiple keys, try this:

#!/usr/bin/env -S bash - 

key_pattern=yourdomain.here

for item in $(gpg --list-secret-keys --with-colons ${key_pattern:- } \
              | awk -F: '($1 == "sec" && $2 == "u") { print $5}') ; do
	wkd_hash=$(gpg --with-wkd-hash --fingerprint "$item" \
             | grep -A 1 '^uid' \
             | tail -n 1 \
             | tr -d '[[:space:]]')
	gpg --export --no-armor "$item" > "${wkd_hash%%@*}"
	printf "%s\n" "Exported key $item to file $( realpath "${wkd_hash%%@*}")"
done

As others have noted the Content-Type should be application/octet-stream, it appears that gpg version 2.2.41 at least does not seem to care.

I tested:

• application/pgp-key (actually this is wrong... the "official" type for .key files appears to be: application/pgp-keys)

  $ grep -rin 'application/pgp-key'  /usr/share/mime/
  /usr/share/mime/packages/freedesktop.org.xml
  1322:  <mime-type type="application/pgp-keys">
  
  /usr/share/mime/application/pgp-keys.xml
  2:<mime-type xmlns="http://www.freedesktop.org/standards/shared-mime-info" type="application/pgp-keys">
  
  $ grep application/pgp-key /etc/mime.types
  application/pgp-keys
  
  $ xdg-mime query filetype /tmp/test.key
  application/pgp-keys
  

• application/pgp-keys

• application/octet-stream: This is correct for WKD servers, according to the specification

GPG happily imported the key via WKD when each Content-Type setting was used in my tests.

Makes sense that the client is more lenient to accept them given:

Source: draft-koch-openpgp-webkey-service-16: Section 3.1-18

The server SHOULD use "application/octet-stream" as the Content-Type for the data but clients SHOULD also accept any other Content-Type.

@trinitronx AFAIK application/pgp-keys is used for ASCII-armored keys only and WKD mandates binary encoding (but as of recently GnuPG will also accept armored files). I guess there's just no MIME format for OpenPGP certificates (which I think is a problem of the spec).

Thank you @kafene for sharing this gist. We've included this in our guide for @forwardemail at https://forwardemail.net/en/faq#do-you-support-openpgpmime-end-to-end-encryption-e2ee-and-web-key-directory-wkd for OpenPGP/WKD support.