Import release 0.1.4

[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. This means we have to handle retries, etc.

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 original plan]

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.

[The new plan]

It appears that people want to be able to use secnet on mobile
machines like laptops as well as to interconnect sites. In particular,
they want to be able to use their laptop in three situations:

1) connected to their internal LAN by a cable; no tunnel involved
2) connected via wireless, using a tunnel to protect traffic
3) connected to some other network, using a tunnel to access the
internal LAN.

They want the laptop to keep the same IP address all the time.

Case (1) is simple.

Case (2) requires that the laptop run a copy of secnet, and have a
tunnel configured between it and the main internal LAN default
gateway. secnet must support the concept of a 'soft' tunnel where it
adds a route and causes the gateway to do proxy-ARP when the tunnel is
up, and removes the route again when the tunnel is down.

The usual prohibition of packets coming in from one tunnel and going
out another must be relaxed in this case (in particular, the
destination address of packets from these 'mobile station' tunnels may
be another tunnel as well as the host).

(Quick sanity check: if chiark's secnet address was in
192.168.73.0/24, would this work properly? Yes, because there will be
an explicit route to it, and proxy ARP will be done for it. Do we want
packets from the chiark tunnel to be able to go out along other
routes? No. So, spotting a 'local' address in a remote site's list of
networks isn't sufficient to switch on routing for a site. We need an
explicit option. NB packets may be routed if the source OR the
destination is marked as allowing routing [otherwise packets couldn't
get back from eg. chiark to a laptop at greenend]).

** VPN-level configuration

At a high level we just want to be able to indicate which groups of
users can claim ownership of which ranges of IP addresses. Assuming
these users (or their representatives) all have accounts on a single
machine, we can automate the submission of keys and other information
to make up a 'sites' file for the entire VPN.

The distributed 'sites' file should be in a more restricted format
than the secnet configuration file, to prevent attackers who manage to
distribute bogus sites files from taking over their victim's machines.

The distributed 'sites' file is read one line at a time. Each line
consists of a keyword followed by other information. It defines a
number of VPNs; within each VPN it defines a number of locations;
within each location it defines a number of sites. These VPNs,
locations and sites are turned into a secnet.conf file fragment using
a script.

Some keywords are valid at any 'level' of the distributed 'sites'
file, indicating defaults.

The keywords are:

vpn n: we are now declaring information to do with VPN 'n'. Must come first.

location n: we are now declaring information for location 'n'.

site n: we are now declaring information for site 'n'.
endsite: we're finished declaring information for the current site

restrict-nets a b c ...: restrict the allowable 'networks' for the current
  level to those in this list.
end-definitions: prevent definition of further vpns and locations, and
  modification of defaults at VPN level

dh x y: the current VPN uses the specified group; x=modulus, y=generator

hash x: which hash function to use. Valid options are 'md5' and 'sha1'.

admin n: administrator email address for current level

key-lifetime n
setup-retries n
setup-timeout n
wait-time n
renegotiate-time n

address a b: a=dnsname, b=port
networks a b c ...
pubkey x y z: x=keylen, y=encryption key, z=modulus
mobile: declare this to be a 'mobile' site

** 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,protorange-A,nA

2) B->A: iA,iB,msg2,B,A,chosen-protocol,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,protorange-A,chosen-protocol,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.

If an attacker is trying to manipulate the chosen protocol, B can spot
this when it sees A's message 3.

4) B->A: {iA,iB,msg4,B,A,protorange-B,chosen-protocol,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.

A can abandon the exchange if the chosen protocol is not the one that
it would have chosen knowing the acceptable protocol ranges of A and
B.

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