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2022-11-26 19:00:16 +01:00

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EasyWKS

OpenPGP WKS for Human Beings

What is WKD/WKS?

Due to all the issues involved with the PGP key servers we're using today, GnuPG introduced a feature named Web Key Discovery (WKD): Instead of searching for keys on the usual key servers, WKD is taking a fully decentralized and federated approach, where each mail domain is responsible for hosting its users public keys on an HTTPS web directory. For example, in order to retrieve the key for john.doe@example.org, they key can be located at

https://openpgpkey.example.org/.well-known/openpgpkey/example.org/hu/iy9q119eutrkn8s1mk4r39qejnbu3n5q?l=john.doe

In order to get the keys there, GnuPG developed an email-based protocol named Web Key Service (WKS), which lets users publish their public keys once they have proven ownership of the key using an email-based challenge-responce mechanism.

At the time, WKS and WKD aren't yet part of the OpenPGP standard, but there's an IETF Draft RFC that's being updated from time to time.

Why EasyWKS?

I've experienced the WKS standard to be extremely cumbersome to conform to. The tools shipped with GnuPG, gpg-wks-server and gpg-wks-client are usable well enough as long as you have shell access on a mailserver and pipe the output of gpg-wks-client into sendmail. However, I'm usually doing my email via SMTP using a variety of clients.

No matter which client I tried, I did not manage to get them to send mails in the format required by gpg-wks-server. Even pasting the output of gpg-wks-client into openssl s_client after manually performing SMTP auth proved to be difficult in case any line of the PGP-encrypted message happened to start with Q or R.

So I decided to write a WKS server that's much more lenient regarding the exact format of the mails it receives. Instead of enforcing the strict format mandated by the standard, EasyWKS only requires:

For the initial submission request: An unsigned & encrypted PGP/MIME message anywhere in the MIME tree, and the ASCII-armored PGP-Key anywhere inside the encrypted message.
For the confirmation response: A signed & encrypted PGP/MIME message anywhere in the MIME tree, and the confirmation response anywhere inside the encrypted message.

This makes EasyWKS usable with every mail client, no matter whether WKS support is built-in or not.

EasyWKS aims to be a drop-in replacement for gpg-wks-server; see "Bootstrapping" below to learn how to migrate from gpg-wks-server to EasyWKS.

Requirements

Python 3.6 or newer
PyYAML
bottle.py
PGPy

License

MIT

Setup

Installation

TODO

Create a cronjob, e.g. in /etc/cron.d/easywks

0 3 * * *    webkey    /path/to/easywks clean

Configuration

Configuration is done in /etc/easywks.yml (or any other place as specified by --config)

---
# EasyWKS works inside this directory.  Its PGP keys as well as all
# the submitted and published keys are stored here.
directory: /var/lib/easywks

# Number of seconds after which a pending submission request is
# considered stale and should be removed by easywks clean.
pending_lifetime: 604800

# Some clients (including recent versions of gpg-wks-client follow an
# older version of the WKS standard where signing the confirmation
# response is only recommended, but not required.  Set this option to
# true if you want to accept such unsigned responses.
permit_unsigned_response: false

# Port configuration for the webserver.  Put this behind an
# HTTPS-terminating reverse proxy!
httpd:
  host: 127.0.0.1
  port: 8080
  # Some older HTTP clients omit the ?l=<userid> query suffix.  Set
  # this to false in order to permit such clients to retrieve keys.
  #require_user_urlparam: true

# Defaults to stdout, supported: stdout, smtp
mailing_method: smtp

# Configure smtp client options
smtp:
  # Connect to this SMTP server to send a mail.
  host: localhost
  port: 25
  # if tls=True, starttls is ignored
  tls: false
  starttls: false
  # Omit username/password if authentication is not needed.
  username: webkey
  password: SuperS3curePassword123

# Configure the LMTP server
lmtpd:
  host: "::1"
  port: 8024

# You can override the mail response templates with your own text.
# The following templates can be overridden:
#   - "header": Placed in front of every message.
#   - "footer": Appended to every message.
#   - "confirm": Sent with the confirmation request.
#   - "done": Sent after a key was published.
#   - "error": Sent when an error occurs.
# The following placeholders can be used (enclosed in curly braces):
#   - {domain}: The email domain for with the request is processed.
#   - {sender}: The submitter's mail address.
#   - {submission}: The submission address.
#  When overriding the "error" template, there's an additional
#  placeholder you can use:
#   - {error}:  The error message.
#responses:
#  error: |
#    An error has occurred while processing your request:
#
#    {error}
#
#    If this error persists, please contact admin@example.org for help.

# Every domain served by EasyWKS must be listed here
domains:
  example.org:
    # Users send their requests to this address.  It's up to you to
    # make sure that the mails sent their get handed to EasyWKS.
    submission_address: webkey@example.org
    # If you want the PGP key for this domain to be password-
    # protected, or if you're supplying your own password-protected
    # key, set the passphrase here:
    passphrase: "Correct Horse Battery Staple"
  # Defaults are gpgwks@<domain> and no password protection.
  example.com: {}

Bootstrapping

Run easywks init (as the correct user) to initialize all files and directories. This will also generate a PGP key for each domain, stored in <workdir>/<domain>/key.pgp. If you want to, you can replace this key by a key you generated yourself. If your key is password-protected, you have to supply the passphrase in the config file or remove the password protection.

If you are migrating from gpg-wks-server to EasyWKS, you can point EasyWKS to gpg-wks-server's working directory (usually /var/lib/gnupg/wks). EasyWKS uses the same directory layout and file formats as gpg-wks-server, so it should be able to take over where gpg-wks-server stopped. The only thing you need to do is to export the private keys from the GnuPG keyring and write them to their domain's key.pgp.

Webserver Setup

There are generally two ways to get WKD working:

The gpg-wks-server approach, i.e. regularly copying the hu directories from EasyWKS' working directory to the webroot.
Running the EasyWKS web server behind a Reverse Proxy, e.g. using the follwing systemd unit:

[Unit]
Description=OpenPGP WKS for Human Beings - HTTP Server

[Service]
Type=simple
ExecStart=/path/to/easywks webserver
Restart=on-failure
User=webkey
Group=webkey
WorkingDirectory=/var/lib/easywks

[Install]
WantedBy=multi-user.target

Once this service is up and running, you can configure your webserver to proxy requests to EasyWKS. E.g. with Apache2:

ProxyPass /.well-known/openpgpkey/ http://127.0.0.1:8080/.well-known/openpgpkey/
ProxyPassReverse /.well-known/openpgpkey/ http://127.0.0.1:8080/.well-known/openpgpkey/

Note that the webserver built into EasyWKS validates that the ?l=<uid> matches the uid hash (the last URL component).

MTA Setup

Postfix

You can either connect EasyWKS using a pipe transport or as a LMTP server.

Pipe Transport

Add an entry in /etc/postfix/master.cf:

webkey  unix  -       n       n       -       -       pipe
  flags=DRhu user=webkey argv=/path/to/easywks process

Then tell postfix to forward mails to the submission addresses to said transport, e.g. in /etc/postfix/transport:

# The colon at the end is important, it lets postfix know
# that "webkey" is a transport, not a name
gpgwks@example.org   webkey:
webkey@example.com   webkey:

Configure EasyWKS to send outgoing mails via SMTP.

LMTP Server

Configure EasyWKS to run the LMTP server, e.g. using the following systemd unit:

[Unit]
Description=OpenPGP WKS for Human Beings - LMTP Server

[Service]
Type=simple
ExecStart=/path/to/easywks lmtpd
Restart=on-failure
User=webkey
Group=webkey
WorkingDirectory=/var/lib/easywks

[Install]
WantedBy=multi-user.target

Also configure EasyWKS to send outgoing mails via SMTP.