[Cryptography] DSL modems - how would we detect wholesale subversion?

[John Gilmore]

> Many DSL modems contain a small switch, which if it's the only switch
> in a small home or office network, would make all packets among local
> nodes accessible to malware running in that DSL modem.

And most DSL modems are provided by your giant telco DSL provider --
such as AT&T -- which we already know has a long history of covertly
sucking up to NSA.  Besides their longstanding cooperation on domestic
and foreign fiber taps, they also produced the first-and-only Clipper
Chip subverted "telephone security device" for making voice calls that
"nobody but NSA" could listen to.  How hard would it be, really, for
them to subvert all their DSL modems to wiretap your LAN?

And how would you know if they had done so?  It's so convenient that
all AT&T DSL modems have a high bandwidth upstream connection to
AT&T's central office switches.  And even better that consumers have
no idea what packets are going up and down over that DSL signalling,
because they have no equipment for monitoring raw 2-wire DSL lines
(the way they could fairly easily detect inappropriate packets
traveling on an Ethernet, with a little free software and a little
replugging of Ethernet equipment).

Your DSL modem could be doing its main job (carrying your external
Internet traffic) using whatever fraction of the available bandwith
that requires in each millisecond, and using any spare capacity on the
DSL wire to mirror a select fraction, or all, of your local LAN
traffic up to the central office switch.  The switch would nominally
discard this 'filler traffic' -- but AT&T would be able to copy it to
NSA upon request, either by individual targeting of particular
customers, or wholesale.  In the better subverted DSL modems, the
filler/tap traffic would be fully encrypted between the modem and the
switch, so that even if you got professional equipment for monitoring
the DSL wire back to the central office, all you would see is 'random'
filler packets all the time.

Suppose AT&T and NSA really had no interest in doing this to you --
unlikely, I know -- but the Chinese manufacturers of DSL modems did
have such an interest?  The threat model is very similar, except the
Chinese would have to subvert the AT&T central office switches
covertly, without AT&T's willing cooperation, to extract your LAN
traffic from them.

You can guard against this threat by only plugging one Ethernet jack
into your DSL modem, and having that lead directly to a Linux or BSD
gateway box that is under your own control.  That way, the DSL modem
has no physical access to the rest of your LAN, and you can monitor
the upstream Ethernet to make sure that the only packets going to the
DSL modem are those that you intended to go upstream.

	John