add a mandatory-pgp-encryption submission port

setup/mail-postfix.sh

@@ -30,8 +30,10 @@ source setup/functions.sh # load our functions
 source /etc/mailinabox.conf # load global vars
 
 # Install packages.
+# python-libmilter is needed by our encryption milter.
 
 apt_install postfix postgrey postfix-pcre
+hide_output pip3 install git+https://github.com/mail-in-a-box/python-libmilter
 
 # Basic Settings
 
@@ -53,11 +55,20 @@ tools/editconf.py /etc/postfix/main.cf \
 # c) Add a new cleanup service specific to the submission service ('authclean')
 #    that filters out privacy-sensitive headers on mail being sent out by
 #    authenticated users.
+# d) Create an alternative one running on port 10587 that requires that all recipients have findable
+#    OpenPGP keys. Encrypts the message for the recipients using a milter on port 882. The milter
+#    precedes the DKIM milter on 8891 so that the message isn't touched after DKIM signing. If the
+#    encryption milter isn't running, reject the message so we dont send anything in the clear.
 tools/editconf.py /etc/postfix/master.cf -s -w \
 	"submission=inet n       -       -       -       -       smtpd
 	  -o syslog_name=postfix/submission
 	  -o smtpd_tls_ciphers=high -o smtpd_tls_protocols=!SSLv2,!SSLv3
 	  -o cleanup_service_name=authclean" \
+	"10587=inet n       -       -       -       -       smtpd
+	  -o syslog_name=postfix/submission-encrypted
+	  -o smtpd_tls_ciphers=high -o smtpd_tls_protocols=!SSLv2,!SSLv3
+	  -o cleanup_service_name=authclean
+	  -o smtpd_milters=inet:127.0.0.1:8892,inet:127.0.0.1:8891 -o milter_default_action=reject" \
 	"authclean=unix  n       -       -       -       0       cleanup
 	  -o header_checks=pcre:/etc/postfix/outgoing_mail_header_filters"
 
@@ -134,6 +145,7 @@ tools/editconf.py /etc/postfix/main.cf \
 
 ufw_allow smtp
 ufw_allow submission
+ufw_allow 10587
 
 # Restart services
 

tools/encryption-milter.py

@@ -0,0 +1,191 @@
+#!/usr/bin/env python3
+
+import sys
+import io
+import re
+import urllib.request, urllib.error
+import tempfile
+import shutil
+
+import libmilter
+import gnupg
+import dns.resolver
+from hashlib import sha224
+
+# Start logging to syslog.
+from syslog import syslog, openlog, LOG_MAIL
+openlog('encryption-milter', facility=LOG_MAIL)
+
+# Replace process title so it looks nicer in top.
+try:
+	import setproctitle
+	setproctitle.setproctitle("encryption-milter")
+except:
+	pass
+
+# Globals for DNS resolving. See:
+# http://tools.ietf.org/html/draft-ietf-dane-openpgpkey-00
+# http://tools.ietf.org/html/draft-ietf-dane-openpgpkey-usage-00
+# https://github.com/letoams/openpgpkey-milter/
+# I have not tested that this works at all.
+resolver = dns.resolver.get_default_resolver()
+#resolver.nameservers = [server]
+openpgp_rtype = 65280  # draft value - changes when RFC
+
+class EncryptionError(Exception):
+	pass
+
+class EncryptionMilter(libmilter.ForkMixin, libmilter.MilterProtocol):
+	def __init__(self, opts=0, protos=0):
+		libmilter.MilterProtocol.__init__(self, opts, protos)
+		libmilter.ForkMixin.__init__(self)
+
+		self.R = []  # list of recipient keys
+		self.fp = io.BytesIO() # storage for incoming body
+
+	def log(self, msg):
+		print(msg)
+		syslog('encryption-milter: ' + msg)
+
+	def rcpt(self, rcpt_to, cmdDict):
+		# Turn recipients into keys. If we don't have a key available,
+		# then reject the message.
+		try:
+			self.R.extend(self.get_pgp_keys(rcpt_to))
+			return libmilter.CONTINUE
+		except EncryptionError as e:
+			self.log(str(e))
+			self.setReply(b'554', b'5.7.1', str(e).encode("utf8"))
+			return libmilter.REJECT
+
+	@libmilter.noReply
+	def header(self, header, value, cmdDict):
+		self.fp.write(header)
+		self.fp.write(b': ')
+		self.fp.write(value)
+		self.fp.write(b'\n')
+		return libmilter.CONTINUE
+
+	@libmilter.noReply
+	def eoh(self, cmdDict):
+		self.fp.write(b'\n')
+		return libmilter.CONTINUE
+
+	@libmilter.noReply
+	def body(self, chunk, cmdDict):
+		self.fp.write(chunk)
+		return libmilter.CONTINUE
+
+	def eob(self, cmdDict):
+		msg = self.fp.getvalue()
+
+		gpgdir = tempfile.mkdtemp()
+		gpg = gnupg.GPG(gnupghome=gpgdir)
+		gpg.decode_errors = "ignore"
+		try:
+			# Add keys.
+			for key in self.R:
+				gpg.import_keys(key)
+
+			# Target message encryption to all imported keys.
+			fingerprints = ','.join(ikey['fingerprint'] for ikey in gpg.list_keys())
+
+			# Encrypt message.
+			enc_msg = gpg.encrypt(msg, fingerprints, always_trust=True)
+			if enc_msg.data == '':
+				# gpg binary and pythong wrapper is bad at giving us an error message
+				raise Exception('Encryption failed for an unknown reason. GPG failed.')
+
+			# Rewrite the message.
+
+			self.addHeader(b'X-OpenPGPKey', b'Encrypted to key(s): ' + fingerprints.encode("ascii"))
+			self.chgHeader(b'Subject', b'[pgp encrypted message]')
+			self.replBody(enc_msg.data)
+
+			return libmilter.CONTINUE
+
+		except ValueError: #Exception as e:
+			# Exceptions are thrown on things that would be temporary failures.
+			# But by now it's too late to tell the user there was a problem?
+			self.log(str(e))
+			self.setReply(b'554', b'5.7.1', str(e).encode("utf8"))
+			return libmilter.REJECT
+
+		finally:
+			shutil.rmtree(gpgdir)
+
+	def get_pgp_keys(self, email_addr):
+		keys = self.import_keys_from_keybase(email_addr)
+		if keys: return keys
+
+		keys = self.import_keys_from_dns(email_addr)
+		if keys: return keys
+
+		raise EncryptionError(email_addr.decode("utf8", "replace") + " does not have a known encryption key.")
+
+	def import_keys_from_keybase(self, email_addr):
+		# Extract the keybase username from the email address.
+		m = re.search(rb"\+keybase=(.*)@", email_addr)
+		if not m: return None
+		keybase_username = m.group(1)
+
+		# Query keybase.
+		try:
+			req = urllib.request.urlopen("https://keybase.io/%s/key.asc" % keybase_username.decode("ascii", "error"), timeout=20, cadefault=True)
+			openpgpkey = req.read()
+		except Exception as e:
+			if isinstance(e, urllib.error.HTTPError) and e.code == 404:
+				e = "User not found."
+			raise EncryptionError("Error getting public key for %s at Keybase.io: %s" % (keybase_username.decode("utf8", "replace"), str(e)))
+
+		# Return the key.
+		self.log("got keybase.io key for %s" % keybase_username.decode("utf8", "replace"))
+		return [openpgpkey]
+
+	def import_keys_from_dns(self, email_addr):
+		(username, domainname) = email_addr.split(b'@')
+		qname = '%s._openpgpkey.%s' % (sha224(username).hexdigest(), domainname)
+
+		try:
+			response = dns.resolver.query(qname, openpgp_rtype)
+		except dns.resolver.NoNameservers:
+			# could not connect to nameserver
+			raise EncryptionError("Could not connect to nameserver.")
+		except (dns.resolver.NXDOMAIN, dns.resolver.NoAnswer):
+			# host did not have an answer for this query; not sure what the
+			# difference is between the two exceptions
+			return None
+
+		if len(result) == 0:
+			# empty answer? probably not possible...
+			return None
+
+		# Return all keys found in DNS.
+		return [str(value) for value in result]
+
+
+def runMilter():
+	# Adapted from the python-libmilter example at
+	# https://github.com/crustymonkey/python-libmilter/blob/master/examples/testmilter.py
+
+	import signal, traceback
+
+	# Create the milter. Use the ForkFactor to handle each mail in a separate process.
+	f = libmilter.ForkFactory('inet:127.0.0.1:8892', EncryptionMilter,
+		libmilter.SMFIF_ADDHDRS | libmilter.SMFIF_CHGHDRS | libmilter.SMFIF_CHGBODY)
+
+	# Add a signal handler to cleanly exit.
+	def sigHandler(num, frame):
+		f.close()
+		sys.exit(0)
+	signal.signal(signal.SIGINT, sigHandler)
+
+	# Start the milter.
+	try:
+		f.run()
+	except Exception as e:
+		f.close()
+		raise
+
+if __name__ == '__main__':
+	runMilter()