How Clipper's protection works

[Ross Anderson <rja14 () cl cam ac uk>]

Our spies have managed to find out how the Clipper chip's protection works.

The encryption circuit is broken up into a number of pieces. These, together
with a lot of chunks of unrelated circuitry, are in the chip mask, which is
unclassified; I understand that `there are even foreign nationals working
on the 
production line'.

The circuit is only connected together during programming, which is done in a 
secure room by applying high voltage pulses to pairs of pins. These pulses blow 
conducting paths in amorphous silicon links between the circuit fragments.
As the 
links are buried between two metal layers in the chip, and blown links refract 
X-rays to the same extent as unblown ones, the paths cannot be discovered by 
passive inspection.

This leaves open the possibility that the attacker might power the chip up and
try to track the current flows using a scanning electron microscope. In order
to prevent this, the chip surface is salted with a large number of oscillators.

The links are not just used to implement the algorithm, but also to embed the 
various secret keys in each chip. The NSA's assessment of the technology is
that 
defeating it would take new technical advances; these are costed by the
agency at 
a minimum of 3-5 years effort and a budget of \$50 million or more.

So far so good. What I would very much like to get hold now of is a copy of US
patent 4,766,516 by Hughes Aircraft Company called `Method and Apparatus for
Securing Integrated Circuits from Unauthorised Copying and Use'.

This apparently describes a competing system which is used in some military
products. Does anybody have access to a US patent library? Here in Europe it's
rather inconvenient to get hold of such documents.

These technologies could have a lot of interesting civilian uses, and it
would be a shame if the military get to hog them,

Ross