power-mailinabox/management/wkd.py

#!/usr/local/lib/mailinabox/env/bin/python
# WDK (Web Key Directory) Manager: Facilitates discovery of keys by third-parties
# Current relevant documents: https://tools.ietf.org/id/draft-koch-openpgp-webkey-service-11.html

import pgp, utils, rtyaml, mailconfig, copy, shutil, os
from cryptography.hazmat.primitives import hashes

env = utils.load_environment()

wkdpath = f"{env['GNUPGHOME']}/.wkdlist.yml"

class WKDError(Exception):
	"""
	Errors specifically related to WKD.
	"""
	def __init__(self, msg):
		self.message = msg

	def __str__(self):
		return self.message

def sha1(message):
    h = hashes.Hash(hashes.SHA1())
    h.update(message)
    return h.finalize()

def zbase32(digest):
    # Crudely check if all quintets are complete
    if len(digest) % 5 != 0:
        raise ValueError("Digest cannot have incomplete chunks of 40 bits!")
    base = "ybndrfg8ejkmcpqxot1uwisza345h769"
    encoded = ""
    for i in range(0, len(digest), 5):
        chunk = int.from_bytes(digest[i:i+5], byteorder="big")
        for j in range(35, -5, -5):
            encoded += base[(chunk >> j) & 31]
    return encoded


# Strips and exports a key so that only the provided UID index(es) remain.
# This is to comply with the following requirement, set forth in section 5 of the draft:
#
# The mail provider MUST make sure to publish a key in a way
# that only the mail address belonging to the requested user
# is part of the User ID packets included in the returned key.
# Other User ID packets and their associated binding signatures
# MUST be removed before publication.
@pgp.fork_context
def strip_and_export(fpr, except_uid_indexes, context=None):
	context.armor = False # We need to disable armor output for this key
	k = pgp.get_key(fpr, context)
	if k is None:
		return None
	for i in except_uid_indexes:
		if i > len(k.uids):
			raise ValueError(f"UID index {i} out of bounds")

	switch = [(f"uid {i+1}" if i + 1 not in except_uid_indexes else "") for i in range(len(k.uids))] + ["deluid", "save"]
	stage = [-1] # Horrible hack: Because it's a reference (aka pointer), we can pass these around
	def interaction(request, prompt):
		print(f"{request}/{prompt}")
		if request in ["GOT_IT", "KEY_CONSIDERED", ""]:
			return 0
		elif request == "GET_BOOL":
			# No way to confirm interactively, so we just say yes
			return "y" # Yeah, I'd also rather just return True but that doesn't work
		elif request == "GET_LINE" and prompt == "keyedit.prompt":
			stage[0] += 1
			return switch[stage[0]]
		else:
			raise Exception("No idea of what to do!")

	context.interact(k, interaction)
	return pgp.export_key(fpr, context)

def email_compatible_with_key(email, fingerprint):
	# 1. Does the user exist?
	if not email in mailconfig.get_mail_users(env) + [a[0] for a in mailconfig.get_mail_aliases(env)]:
		raise ValueError(f"User or alias {email} not found!")

	if fingerprint is not None:
		key = pgp.get_key(fingerprint)
		# 2. Does the key exist?
		if key is None:
			raise ValueError(f"The key \"{fingerprint}\" does not exist!")

		# 3. Does the key have a user id with the email of the user?
		if email not in [u.email for u in key.uids]:
			raise WKDError(f"The key \"{fingerprint}\" has no such UID with the email \"{email}\"!")

		return key

	return None

# Gets a table with all the keys that can be served for each user and/or alias
def get_user_fpr_maps():
	uk_maps = {}
	for email in mailconfig.get_mail_users(env) + [a[0] for a in mailconfig.get_mail_aliases(env)]:
		uk_maps[email] = []
	for key in pgp.get_imported_keys() + [pgp.get_daemon_key()]:
		for userid in key.uids:
			try:
				uk_maps[userid.email].append(key.fpr)
			finally:
				# We don't host this email address, so ignore
				pass
	return uk_maps

# Gets the current WKD configuration
def get_wkd_config():
	with open(wkdpath, "r") as wkdfile:
		try:
			config = rtyaml.load(wkdfile)
			if (type(config) != dict):
				return {}
			return config
		except:
			return {}

# Sets the WKD key for an email address.
# email: An user or alias on this box. e.g. "administrator@example.com"
# fingerprint: The fingerprint of the key we want to bind it to. e.g "0123456789ABCDEF0123456789ABCDEF01234567"
def set_wkd_published(email, fingerprint=None):
	try:
		email_compatible_with_key(email, fingerprint)
	except Exception as err:
		raise err

	# All conditions met, do the necessary modifications
	with open(wkdpath, "a+") as wkdfile:
		wkdfile.seek(0)
		config = {}
		try:
			config = rtyaml.load(wkdfile)
			if (type(config) != dict):
				config = {}
		except:
			config = {}
		config[email] = fingerprint
		if fingerprint is None and email in config.keys():
			config.pop(email)
		wkdfile.truncate(0)
		wkdfile.write(rtyaml.dump(config))

# Looks for incompatible email/key pairs on the WKD configuration file
# and returns the uid indexes for compatible email/key pairs
def parse_wkd_list():
	removed = []
	uidlist = []
	with open(wkdpath, "a+") as wkdfile:
		wkdfile.seek(0)
		config = {}
		try:
			config = rtyaml.load(wkdfile)
			if (type(config) != dict):
				config = {}
		except:
			config = {}

		writeable = copy.deepcopy(config)
		for u, k in config.items():
			try:
				key = email_compatible_with_key(u, k)
				# Key is compatible

				index = []
				writeable[u] = key.fpr # Swap with the full-length fingerprint (if somehow this was changed by hand)
				for i in range(0, len(key.uids)):
					if key.uids[i].email == u:
						index.append(i)
				uidlist.append((u, key.fpr, index))
			except:
				writeable.pop(u)
				removed.append((u, k))
		# Shove the updated configuration back in the file
		wkdfile.truncate(0)
		wkdfile.write(rtyaml.dump(writeable))
	return (removed, uidlist)

WKD_LOCATION = "/var/lib/mailinabox/wkd"

def build_wkd():
	# Clean everything
	shutil.rmtree(WKD_LOCATION)
	os.mkdir(WKD_LOCATION, mode=0o666)

	# We serve WKD for all our emails and aliases (even if there are no keys)
	for domain in mailconfig.get_mail_domains(env, users_only=False):
		os.mkdir(f"{WKD_LOCATION}/{domain}/")

	for email, fpr, indexes in parse_wkd_list()[1]:
		local, domain = email.split("@", 1)
		localhash = zbase32(sha1(local.lower()))
		with open(f"{WKD_LOCATION}/{domain}/{localhash}", "wb") as k:
			k.write(strip_and_export(fpr, indexes))