From 910b473ea7eeb3e3413dd08a72f2d9dad18b3eaf Mon Sep 17 00:00:00 2001 From: Joshua Tauberer Date: Fri, 25 Jul 2014 13:21:13 +0000 Subject: [PATCH] add a mandatory-pgp-encryption submission port --- setup/mail-postfix.sh | 14 ++- tools/encryption-milter.py | 191 +++++++++++++++++++++++++++++++++++++ 2 files changed, 204 insertions(+), 1 deletion(-) create mode 100755 tools/encryption-milter.py diff --git a/setup/mail-postfix.sh b/setup/mail-postfix.sh index 23c6c649..c92091d9 100755 --- a/setup/mail-postfix.sh +++ b/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" @@ -134,7 +145,8 @@ tools/editconf.py /etc/postfix/main.cf \ ufw_allow smtp ufw_allow submission +ufw_allow 10587 # Restart services -restart_service postfix \ No newline at end of file +restart_service postfix diff --git a/tools/encryption-milter.py b/tools/encryption-milter.py new file mode 100755 index 00000000..ad426366 --- /dev/null +++ b/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()