Import release 0.06

[secnet] / NOTES

* Design of new, multi-subnet secnet protocol

Like the first (1995/6) version, we're tunnelling IP packets inside
UDP packets. To defeat various restrictions which may be imposed on us
by network providers (like the prohibition of incoming TCP
connections) we're sticking with UDP for everything this time,
including key setup.

Other new features include being able to deal with subnets hidden
behind changing 'real' IP addresses, and the ability to choose
algorithms and keys per pair of communicating sites.

** Configuration and structure

The network is made up from a number of 'sites'. These are collections
of machines with private IP addresses. The new secnet code runs on
machines which have interfaces on the private site network and some
way of accessing the 'real' internet.

Each end of a tunnel is identified by a name. Often it will be
convenient for every gateway machine to use the same name for each
tunnel endpoint, but this is not vital. Individual tunnels are
identified by their two endpoint names.

** Protocols

*** Protocol environment:

Each gateway machine serves a particular, well-known set of private IP
addresses (i.e. the agreement over which addresses it serves is
outside the scope of this discussion). Each gateway machine has an IP
address on the interconnecting network (usually the Internet), which
may be dynamically allocated and may change at any point.

Each gateway knows the RSA public keys of the other gateways with
which it wishes to communicate. The mechanism by which this happens is
outside the scope of this discussion. There exists a means by which
each gateway can look up the probable IP address of any other.

*** Protocol goals:

The ultimate goal of the protocol is for the originating gateway
machine to be able to forward packets from its section of the private
network to the appropriate gateway machine for the destination
machine, in such a way that it can be sure that the packets are being
sent to the correct destination machine, the destination machine can
be sure that the source of the packets is the originating gateway
machine, and the contents of the packets cannot be understood other
than by the two communicating gateways.

XXX not sure about the address-change stuff; leave it out of the first
version of the protocol. From experience, IP addresses seem to be
quite stable so the feature doesn't gain us much.

**** Protocol sub-goal 1: establish a shared key

Definitions:

A is the originating gateway machine
B is the destination gateway machine
PK_A is the public RSA key of A
PK_B is the public RSA key of B
PK_A^-1 is the private RSA key of A
PK_B^-1 is the private RSA key of B
x is the fresh private DH key of A
y is the fresh private DH key of B
k is g^xy mod m
g and m are generator and modulus for Diffie-Hellman
nA is a nonce generated by A
nB is a nonce generated by B
iA is an index generated by A, to be used in packets sent from B to A
iB is an index generated by B, to be used in packets sent from A to B
i? is appropriate index for receiver

Note that 'i' may be re-used from one session to the next, whereas 'n'
is always fresh.

Messages:

1) A->B: *,iA,msg1,A,B,nA

2) B->A: iA,iB,msg2,B,A,nB,nA

(The order of B and A reverses in alternate messages so that the same
code can be used to construct them...)

3) A->B: {iB,iA,msg3,A,B,nA,nB,g^x mod m}_PK_A^-1

If message 1 was a replay then A will not generate message 3, because
it doesn't recognise nA.

If message 2 was from an attacker then B will not generate message 4,
because it doesn't recognise nB.

4) B->A: {iA,iB,msg4,B,A,nB,nA,g^y mod m}_PK_B^-1

At this point, A and B share a key, k. B must keep retransmitting
message 4 until it receives a packet encrypted using key k.

5) A: iB,iA,msg5,(ping/msg5)_k

6) B: iA,iB,msg6,(pong/msg6)_k

(Note that these are encrypted using the same transform that's used
for normal traffic, so they include sequence number, MAC, etc.)

The ping and pong messages can be used by either end of the tunnel at
any time, but using msg0 as the unencrypted message type indicator.

**** Protocol sub-goal 2: end the use of a shared key

7) i?,i?,msg0,(end-session/msg7,A,B)_k

This message can be sent by either party. Once sent, k can be
forgotten. Once received and checked, k can be forgotten. No need to
retransmit or confirm reception. It is suggested that this message be
sent when a key times out, or the tunnel is forcibly terminated for
some reason.

XXX not yet implemented.

8) i?,i?,NAK/msg8

If the link-layer can't work out what to do with a packet (session has
gone away, etc.) it can transmit a NAK back to the sender.  The sender
can then try to verify whether the session is alive by sending ping
packets, and forget the key if it isn't. Potential denial-of-service
if the attacker can stop the ping/pong packets getting through (the
key will be forgotten and another key setup must take place), but if
they can delete packets then we've lost anyway...

The attacker can of course forge NAKs since they aren't protected. But
if they can only forge packets then they won't be able to stop the
ping/pong working. Trust in NAKs can be rate-limited...

Alternative idea (which is actually implemented): if you receive a
packet you can't decode, because there's no key established, then
initiate key setup...

Keepalives are probably a good idea.

**** Protocol sub-goal 3: send a packet

9) i?,i?,msg0,(send-packet/msg9,packet)_k