tinc/doc/SECURITY2

This is the security documentation for tinc, a Virtual Private Network daemon.

   Copyright 2001-2002 Guus Sliepen <guus@sliepen.eu.org>,
             2001-2002 Wessel Dankers <wsl@nl.linux.org>

   Permission is granted to make and distribute verbatim copies of
   this documentation provided the copyright notice and this
   permission notice are preserved on all copies.

   Permission is granted to copy and distribute modified versions of
   this documentation under the conditions for verbatim copying,
   provided that the entire resulting derived work is distributed
   under the terms of a permission notice identical to this one.

   $Id: SECURITY2,v 1.1.2.4 2002/09/15 22:19:37 guus Exp $

Proposed new authentication scheme
----------------------------------

A new scheme for authentication in tinc has been devised, which offers some
improvements over the protocol used in 1.0pre2 and 1.0pre3. Explanation is
below.

daemon  message
--------------------------------------------------------------------------
client  <attempts connection>

server  <accepts connection>

client  ID client 12
              |   +---> version
              +-------> name of tinc daemon

server  ID server 12
              |   +---> version
              +-------> name of tinc daemon

client  META_KEY 5f0823a93e35b69e...7086ec7866ce582b
                 \_________________________________/
                                 +-> RSAKEYLEN bits totally random string S1,
                                     encrypted with server's public RSA key

server  META_KEY 6ab9c1640388f8f0...45d1a07f8a672630
                 \_________________________________/
                                 +-> RSAKEYLEN bits totally random string S2,
                                     encrypted with client's public RSA key

From now on:
 - the client will symmetrically encrypt outgoing traffic using S1
 - the server will symmetrically encrypt outgoing traffic using S2

client  CHALLENGE da02add1817c1920989ba6ae2a49cecbda0
                  \_________________________________/
                                 +-> CHALLEN bits totally random string H1

server  CHALLENGE 57fb4b2ccd70d6bb35a64c142f47e61d57f
                  \_________________________________/
                                 +-> CHALLEN bits totally random string H2

client  CHAL_REPLY 816a86
                      +-> 160 bits SHA1 of H2

server  CHAL_REPLY 928ffe
                      +-> 160 bits SHA1 of H1

After the correct challenge replies are recieved, both ends have proved
their identity. Further information is exchanged.

client  ACK 655 123 0
             |   |  +-> options
	         |   +----> estimated weight
	         +--------> listening port of client

server  ACK 655 321 0
             |   |  +-> options
	         |   +----> estimated weight
	         +--------> listening port of server
--------------------------------------------------------------------------

This new scheme has several improvements, both in efficiency and security.

First of all, the server sends exactly the same kind of messages over the wire
as the client. The previous versions of tinc first authenticated the client,
and then the server. This scheme even allows both sides to send their messages
simultaneously, there is no need to wait for the other to send something first.
This means that any calculations that need to be done upon sending or receiving
a message can also be done in parallel. This is especially important when doing
RSA encryption/decryption. Given that these calculations are the main part of
the CPU time spent for the authentication, speed is improved by a factor 2.

Second, only one RSA encrypted message is sent instead of two. This reduces the
amount of information attackers can see (and thus use for a crypto attack). It
also improves speed by a factor two, making the total speedup a factor 4.

Third, and most important:

The symmetric cipher keys are exchanged first, the challenge is done
afterwards. In the previous authentication scheme, because a man-in-the-middle
could pass the challenge/chal_reply phase (by just copying the messages between
the two real tinc daemons), but no information was exchanged that was really
needed to read the rest of the messages, the challenge/chal_reply phase was of
no real use. The man-in-the-middle was only stopped by the fact that only after
the ACK messages were encrypted with the symmetric cipher. Potentially, it
could even send it's own symmetric key to the server (if it knew the server's
public key) and read some of the metadata the server would send it (it was
impossible for the mitm to read actual network packets though). The new scheme
however prevents this.

This new scheme makes sure that first of all, symmetric keys are exchanged. The
rest of the messages are then encrypted with the symmetric cipher. Then, each
side can only read received messages if they have their private key. The
challenge is there to let the other side know that the private key is really
known, because a challenge reply can only be sent back if the challenge is
decrypted correctly, and that can only be done with knowledge of the private
key.

Fourth: the first thing that is send via the symmetric cipher encrypted
connection is a totally random string, so that there is no known plaintext (for
an attacker) in the beginning of the encrypted stream.

Some things to be discussed:

 - What should CHALLEN be? Same as RSAKEYLEN? 256 bits? More/less?