Merge remote-tracking branch 'drebs/feature/4324_prevent-double-encryption-when-relaying-2' into develop

1 files changed, 585 insertions, 0 deletions

diff --git a/src/leap/mail/smtp/gateway.py b/src/leap/mail/smtp/gateway.py
new file mode 100644
index 0000000..06405b4
--- /dev/null
+++ b/src/leap/mail/smtp/gateway.py
@@ -0,0 +1,585 @@
+# -*- coding: utf-8 -*-
+# gateway.py
+# Copyright (C) 2013 LEAP
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program. If not, see <http://www.gnu.org/licenses/>.
+
+"""
+LEAP SMTP encrypted gateway.
+
+The following classes comprise the SMTP gateway service:
+
+    * SMTPFactory - A twisted.internet.protocol.ServerFactory that provides
+      the SMTPDelivery protocol.
+    * SMTPDelivery - A twisted.mail.smtp.IMessageDelivery implementation. It
+      knows how to validate sender and receiver of messages and it generates
+      an EncryptedMessage for each recipient.
+    * SSLContextFactory - Contains the relevant ssl information for the
+      connection.
+    * EncryptedMessage - An implementation of twisted.mail.smtp.IMessage that
+      knows how to encrypt/sign itself before sending.
+
+
+"""
+
+import re
+from StringIO import StringIO
+from email.Header import Header
+from email.utils import parseaddr
+from email.parser import Parser
+from email.mime.application import MIMEApplication
+
+from zope.interface import implements
+from OpenSSL import SSL
+from twisted.mail import smtp
+from twisted.internet.protocol import ServerFactory
+from twisted.internet import reactor, ssl
+from twisted.internet import defer
+from twisted.python import log
+
+from leap.common.check import leap_assert, leap_assert_type
+from leap.common.events import proto, signal
+from leap.keymanager import KeyManager
+from leap.keymanager.openpgp import OpenPGPKey
+from leap.keymanager.errors import KeyNotFound
+from leap.mail.smtp.rfc3156 import (
+    MultipartSigned,
+    MultipartEncrypted,
+    PGPEncrypted,
+    PGPSignature,
+    RFC3156CompliantGenerator,
+    encode_base64_rec,
+)
+
+# replace email generator with a RFC 3156 compliant one.
+from email import generator
+generator.Generator = RFC3156CompliantGenerator
+
+
+#
+# Helper utilities
+#
+
+def validate_address(address):
+    """
+    Validate C{address} as defined in RFC 2822.
+
+    :param address: The address to be validated.
+    :type address: str
+
+    @return: A valid address.
+    @rtype: str
+
+    @raise smtp.SMTPBadRcpt: Raised if C{address} is invalid.
+    """
+    leap_assert_type(address, str)
+    # in the following, the address is parsed as described in RFC 2822 and
+    # ('', '') is returned if the parse fails.
+    _, address = parseaddr(address)
+    if address == '':
+        raise smtp.SMTPBadRcpt(address)
+    return address
+
+
+#
+# SMTPFactory
+#
+
+class SMTPFactory(ServerFactory):
+    """
+    Factory for an SMTP server with encrypted gatewaying capabilities.
+    """
+
+    def __init__(self, userid, keymanager, host, port, cert, key,
+                 encrypted_only):
+        """
+        Initialize the SMTP factory.
+
+        :param userid: The user currently logged in
+        :type userid: unicode
+        :param keymanager: A KeyManager for retrieving recipient's keys.
+        :type keymanager: leap.common.keymanager.KeyManager
+        :param host: The hostname of the remote SMTP server.
+        :type host: str
+        :param port: The port of the remote SMTP server.
+        :type port: int
+        :param cert: The client certificate for authentication.
+        :type cert: str
+        :param key: The client key for authentication.
+        :type key: str
+        :param encrypted_only: Whether the SMTP gateway should send unencrypted
+                               mail or not.
+        :type encrypted_only: bool
+        """
+        # assert params
+        leap_assert_type(keymanager, KeyManager)
+        leap_assert_type(host, str)
+        leap_assert(host != '')
+        leap_assert_type(port, int)
+        leap_assert(port is not 0)
+        leap_assert_type(cert, unicode)
+        leap_assert(cert != '')
+        leap_assert_type(key, unicode)
+        leap_assert(key != '')
+        leap_assert_type(encrypted_only, bool)
+        # and store them
+        self._userid = userid
+        self._km = keymanager
+        self._host = host
+        self._port = port
+        self._cert = cert
+        self._key = key
+        self._encrypted_only = encrypted_only
+
+    def buildProtocol(self, addr):
+        """
+        Return a protocol suitable for the job.
+
+        :param addr: An address, e.g. a TCP (host, port).
+        :type addr:  twisted.internet.interfaces.IAddress
+
+        @return: The protocol.
+        @rtype: SMTPDelivery
+        """
+        smtpProtocol = smtp.SMTP(SMTPDelivery(
+            self._userid, self._km, self._host, self._port, self._cert,
+            self._key, self._encrypted_only))
+        smtpProtocol.factory = self
+        return smtpProtocol
+
+
+#
+# SMTPDelivery
+#
+
+class SMTPDelivery(object):
+    """
+    Validate email addresses and handle message delivery.
+    """
+
+    implements(smtp.IMessageDelivery)
+
+    def __init__(self, userid, keymanager, host, port, cert, key,
+                 encrypted_only):
+        """
+        Initialize the SMTP delivery object.
+
+        :param userid: The user currently logged in
+        :type userid: unicode
+        :param keymanager: A KeyManager for retrieving recipient's keys.
+        :type keymanager: leap.common.keymanager.KeyManager
+        :param host: The hostname of the remote SMTP server.
+        :type host: str
+        :param port: The port of the remote SMTP server.
+        :type port: int
+        :param cert: The client certificate for authentication.
+        :type cert: str
+        :param key: The client key for authentication.
+        :type key: str
+        :param encrypted_only: Whether the SMTP gateway should send unencrypted
+                               mail or not.
+        :type encrypted_only: bool
+        """
+        self._userid = userid
+        self._km = keymanager
+        self._userid = userid
+        self._km = keymanager
+        self._host = host
+        self._port = port
+        self._cert = cert
+        self._key = key
+        self._encrypted_only = encrypted_only
+        self._origin = None
+
+    def receivedHeader(self, helo, origin, recipients):
+        """
+        Generate the 'Received:' header for a message.
+
+        :param helo: The argument to the HELO command and the client's IP
+            address.
+        :type helo: (str, str)
+        :param origin: The address the message is from.
+        :type origin: twisted.mail.smtp.Address
+        :param recipients: A list of the addresses for which this message is
+            bound.
+        :type: list of twisted.mail.smtp.User
+
+        @return: The full "Received" header string.
+        :type: str
+        """
+        myHostname, clientIP = helo
+        headerValue = "by %s from %s with ESMTP ; %s" % (
+            myHostname, clientIP, smtp.rfc822date())
+        # email.Header.Header used for automatic wrapping of long lines
+        return "Received: %s" % Header(headerValue)
+
+    def validateTo(self, user):
+        """
+        Validate the address of C{user}, a recipient of the message.
+
+        This method is called once for each recipient and validates the
+        C{user}'s address against the RFC 2822 definition. If the
+        configuration option ENCRYPTED_ONLY_KEY is True, it also asserts the
+        existence of the user's key.
+
+        In the end, it returns an encrypted message object that is able to
+        send itself to the C{user}'s address.
+
+        :param user: The user whose address we wish to validate.
+        :type: twisted.mail.smtp.User
+
+        @return: A Deferred which becomes, or a callable which takes no
+            arguments and returns an object implementing IMessage. This will
+            be called and the returned object used to deliver the message when
+            it arrives.
+        @rtype: no-argument callable
+
+        @raise SMTPBadRcpt: Raised if messages to the address are not to be
+            accepted.
+        """
+        # try to find recipient's public key
+        try:
+            address = validate_address(user.dest.addrstr)
+            # verify if recipient key is available in keyring
+            self._km.get_key(address, OpenPGPKey)  # might raise KeyNotFound
+            log.msg("Accepting mail for %s..." % user.dest.addrstr)
+            signal(proto.SMTP_RECIPIENT_ACCEPTED_ENCRYPTED, user.dest.addrstr)
+        except KeyNotFound:
+            # if key was not found, check config to see if will send anyway.
+            if self._encrypted_only:
+                signal(proto.SMTP_RECIPIENT_REJECTED, user.dest.addrstr)
+                raise smtp.SMTPBadRcpt(user.dest.addrstr)
+            log.msg("Warning: will send an unencrypted message (because "
+                    "encrypted_only' is set to False).")
+            signal(
+                proto.SMTP_RECIPIENT_ACCEPTED_UNENCRYPTED, user.dest.addrstr)
+        return lambda: EncryptedMessage(
+            self._origin, user, self._km, self._host, self._port, self._cert,
+            self._key)
+
+    def validateFrom(self, helo, origin):
+        """
+        Validate the address from which the message originates.
+
+        :param helo: The argument to the HELO command and the client's IP
+            address.
+        :type: (str, str)
+        :param origin: The address the message is from.
+        :type origin: twisted.mail.smtp.Address
+
+        @return: origin or a Deferred whose callback will be passed origin.
+        @rtype: Deferred or Address
+
+        @raise twisted.mail.smtp.SMTPBadSender: Raised if messages from this
+            address are not to be accepted.
+        """
+        # accept mail from anywhere. To reject an address, raise
+        # smtp.SMTPBadSender here.
+        if str(origin) != str(self._userid):
+            log.msg("Rejecting sender {0}, expected {1}".format(origin,
+                                                                self._userid))
+            raise smtp.SMTPBadSender(origin)
+        self._origin = origin
+        return origin
+
+
+#
+# EncryptedMessage
+#
+
+class SSLContextFactory(ssl.ClientContextFactory):
+    def __init__(self, cert, key):
+        self.cert = cert
+        self.key = key
+
+    def getContext(self):
+        self.method = SSL.TLSv1_METHOD  # SSLv23_METHOD
+        ctx = ssl.ClientContextFactory.getContext(self)
+        ctx.use_certificate_file(self.cert)
+        ctx.use_privatekey_file(self.key)
+        return ctx
+
+
+def move_headers(origmsg, newmsg):
+    headers = origmsg.items()
+    unwanted_headers = ['content-type', 'mime-version', 'content-disposition',
+                        'content-transfer-encoding']
+    headers = filter(lambda x: x[0].lower() not in unwanted_headers, headers)
+    for hkey, hval in headers:
+        newmsg.add_header(hkey, hval)
+        del(origmsg[hkey])
+
+
+class EncryptedMessage(object):
+    """
+    Receive plaintext from client, encrypt it and send message to a
+    recipient.
+    """
+    implements(smtp.IMessage)
+
+    def __init__(self, fromAddress, user, keymanager, host, port, cert, key):
+        """
+        Initialize the encrypted message.
+
+        :param fromAddress: The address of the sender.
+        :type fromAddress: twisted.mail.smtp.Address
+        :param user: The recipient of this message.
+        :type user: twisted.mail.smtp.User
+        :param keymanager: A KeyManager for retrieving recipient's keys.
+        :type keymanager: leap.common.keymanager.KeyManager
+        :param host: The hostname of the remote SMTP server.
+        :type host: str
+        :param port: The port of the remote SMTP server.
+        :type port: int
+        :param cert: The client certificate for authentication.
+        :type cert: str
+        :param key: The client key for authentication.
+        :type key: str
+        """
+        # assert params
+        leap_assert_type(user, smtp.User)
+        leap_assert_type(keymanager, KeyManager)
+        # and store them
+        self._fromAddress = fromAddress
+        self._user = user
+        self._km = keymanager
+        self._host = host
+        self._port = port
+        self._cert = cert
+        self._key = key
+        # initialize list for message's lines
+        self.lines = []
+
+    #
+    # methods from smtp.IMessage
+    #
+
+    def lineReceived(self, line):
+        """
+        Handle another line.
+
+        :param line: The received line.
+        :type line: str
+        """
+        self.lines.append(line)
+
+    def eomReceived(self):
+        """
+        Handle end of message.
+
+        This method will encrypt and send the message.
+        """
+        log.msg("Message data complete.")
+        self.lines.append('')  # add a trailing newline
+        try:
+            self._maybe_encrypt_and_sign()
+            return self.sendMessage()
+        except KeyNotFound:
+            return None
+
+    def parseMessage(self):
+        """
+        Separate message headers from body.
+        """
+        parser = Parser()
+        return parser.parsestr('\r\n'.join(self.lines))
+
+    def connectionLost(self):
+        """
+        Log an error when the connection is lost.
+        """
+        log.msg("Connection lost unexpectedly!")
+        log.err()
+        signal(proto.SMTP_CONNECTION_LOST, self._user.dest.addrstr)
+        # unexpected loss of connection; don't save
+        self.lines = []
+
+    def sendSuccess(self, r):
+        """
+        Callback for a successful send.
+
+        :param r: The result from the last previous callback in the chain.
+        :type r: anything
+        """
+        log.msg(r)
+        signal(proto.SMTP_SEND_MESSAGE_SUCCESS, self._user.dest.addrstr)
+
+    def sendError(self, e):
+        """
+        Callback for an unsuccessfull send.
+
+        :param e: The result from the last errback.
+        :type e: anything
+        """
+        log.msg(e)
+        log.err()
+        signal(proto.SMTP_SEND_MESSAGE_ERROR, self._user.dest.addrstr)
+
+    def sendMessage(self):
+        """
+        Send the message.
+
+        This method will prepare the message (headers and possibly encrypted
+        body) and send it using the ESMTPSenderFactory.
+
+        @return: A deferred with callbacks for error and success of this
+            message send.
+        @rtype: twisted.internet.defer.Deferred
+        """
+        msg = self._msg.as_string(False)
+
+        log.msg("Connecting to SMTP server %s:%s" % (self._host, self._port))
+
+        d = defer.Deferred()
+        # we don't pass an ssl context factory to the ESMTPSenderFactory
+        # because ssl will be handled by reactor.connectSSL() below.
+        factory = smtp.ESMTPSenderFactory(
+            "",  # username is blank because server does not use auth.
+            "",  # password is blank because server does not use auth.
+            self._fromAddress.addrstr,
+            self._user.dest.addrstr,
+            StringIO(msg),
+            d,
+            requireAuthentication=False,
+            requireTransportSecurity=True)
+        signal(proto.SMTP_SEND_MESSAGE_START, self._user.dest.addrstr)
+        reactor.connectSSL(
+            self._host, self._port, factory,
+            contextFactory=SSLContextFactory(self._cert, self._key))
+        d.addCallback(self.sendSuccess)
+        d.addErrback(self.sendError)
+        return d
+
+    #
+    # encryption methods
+    #
+
+    def _encrypt_and_sign(self, pubkey, signkey):
+        """
+        Create an RFC 3156 compliang PGP encrypted and signed message using
+        C{pubkey} to encrypt and C{signkey} to sign.
+
+        :param pubkey: The public key used to encrypt the message.
+        :type pubkey: leap.common.keymanager.openpgp.OpenPGPKey
+        :param signkey: The private key used to sign the message.
+        :type signkey: leap.common.keymanager.openpgp.OpenPGPKey
+        """
+        # create new multipart/encrypted message with 'pgp-encrypted' protocol
+        newmsg = MultipartEncrypted('application/pgp-encrypted')
+        # move (almost) all headers from original message to the new message
+        move_headers(self._origmsg, newmsg)
+        # create 'application/octet-stream' encrypted message
+        encmsg = MIMEApplication(
+            self._km.encrypt(self._origmsg.as_string(unixfrom=False), pubkey,
+                             sign=signkey),
+            _subtype='octet-stream', _encoder=lambda x: x)
+        encmsg.add_header('content-disposition', 'attachment',
+                          filename='msg.asc')
+        # create meta message
+        metamsg = PGPEncrypted()
+        metamsg.add_header('Content-Disposition', 'attachment')
+        # attach pgp message parts to new message
+        newmsg.attach(metamsg)
+        newmsg.attach(encmsg)
+        self._msg = newmsg
+
+    def _sign(self, signkey):
+        """
+        Create an RFC 3156 compliant PGP signed MIME message using C{signkey}.
+
+        :param signkey: The private key used to sign the message.
+        :type signkey: leap.common.keymanager.openpgp.OpenPGPKey
+        """
+        # create new multipart/signed message
+        newmsg = MultipartSigned('application/pgp-signature', 'pgp-sha512')
+        # move (almost) all headers from original message to the new message
+        move_headers(self._origmsg, newmsg)
+        # apply base64 content-transfer-encoding
+        encode_base64_rec(self._origmsg)
+        # get message text with headers and replace \n for \r\n
+        fp = StringIO()
+        g = RFC3156CompliantGenerator(
+            fp, mangle_from_=False, maxheaderlen=76)
+        g.flatten(self._origmsg)
+        msgtext = re.sub('\r?\n', '\r\n', fp.getvalue())
+        # make sure signed message ends with \r\n as per OpenPGP stantard.
+        if self._origmsg.is_multipart():
+            if not msgtext.endswith("\r\n"):
+                msgtext += "\r\n"
+        # calculate signature
+        signature = self._km.sign(msgtext, signkey, digest_algo='SHA512',
+                                  clearsign=False, detach=True, binary=False)
+        sigmsg = PGPSignature(signature)
+        # attach original message and signature to new message
+        newmsg.attach(self._origmsg)
+        newmsg.attach(sigmsg)
+        self._msg = newmsg
+
+    def _maybe_encrypt_and_sign(self):
+        """
+        Attempt to encrypt and sign the outgoing message.
+
+        The behaviour of this method depends on:
+
+            1. the original message's content-type, and
+            2. the availability of the recipient's public key.
+
+        If the original message's content-type is "multipart/encrypted", then
+        the original message is not altered. For any other content-type, the
+        method attempts to fetch the recipient's public key. If the
+        recipient's public key is available, the message is encrypted and
+        signed; otherwise it is only signed.
+
+        Note that, if the C{encrypted_only} configuration is set to True and
+        the recipient's public key is not available, then the recipient
+        address would have been rejected in SMTPDelivery.validateTo().
+
+        The following table summarizes the overall behaviour of the gateway:
+
+        +---------------------------------------------------+----------------+
+        | content-type        | rcpt pubkey | enforce encr. | action         |
+        +---------------------+-------------+---------------+----------------+
+        | multipart/encrypted | any         | any           | pass           |
+        | other               | available   | any           | encrypt + sign |
+        | other               | unavailable | yes           | reject         |
+        | other               | unavailable | no            | sign           |
+        +---------------------+-------------+---------------+----------------+
+        """
+        # pass if the original message's content-type is "multipart/encrypted"
+        self._origmsg = self.parseMessage()
+        if self._origmsg.get_content_type() == 'multipart/encrypted':
+            self._msg = self._origmsg
+            return
+
+        from_address = validate_address(self._fromAddress.addrstr)
+        signkey = self._km.get_key(from_address, OpenPGPKey, private=True)
+        log.msg("Will sign the message with %s." % signkey.fingerprint)
+        to_address = validate_address(self._user.dest.addrstr)
+        try:
+            # try to get the recipient pubkey
+            pubkey = self._km.get_key(to_address, OpenPGPKey)
+            log.msg("Will encrypt the message to %s." % pubkey.fingerprint)
+            signal(proto.SMTP_START_ENCRYPT_AND_SIGN,
+                   "%s,%s" % (self._fromAddress.addrstr, to_address))
+            self._encrypt_and_sign(pubkey, signkey)
+            signal(proto.SMTP_END_ENCRYPT_AND_SIGN,
+                   "%s,%s" % (self._fromAddress.addrstr, to_address))
+        except KeyNotFound:
+            # at this point we _can_ send unencrypted mail, because if the
+            # configuration said the opposite the address would have been
+            # rejected in SMTPDelivery.validateTo().
+            log.msg('Will send unencrypted message to %s.' % to_address)
+            signal(proto.SMTP_START_SIGN, self._fromAddress.addrstr)
+            self._sign(signkey)
+            signal(proto.SMTP_END_SIGN, self._fromAddress.addrstr)