[dave at farber.net: [IP] more on AP Story Justice Dept. Probing Domestic Spyin

[coderman]

On 1/1/06, J.A. Terranson <measl at mfn.org> wrote:
> ...
> Is there radioactive material which has has a known property that can be
> reliably and repetitively measured, that is useful as either a key or a
> seed, and that is guaranteed to change on a known schedule in a
> significant (i.e., keying data no longer relevant) way?
>
> The idea being something like msg xor radioseed "keys" = plaintext, but
> after 30 days, radioseed is different (and the original not knowable), and
> therefore message is dead.

it seems like this should be possible using a radioactive material
with a known short half-life and exposing it to a neutron source with
a mask (beryllium?) with the key space on it.

assume a grid of cells on a flat surface containing the radioactive
material; if a given cell emits over a threshold of radiation it is a
'1' bit, dead it is a 0 bit.  exposing the 0's to a neutron emitter
would fission the radioactive cells early leaving it's ionizing
radiation level below the threshold.

there would be some delay between when the key was usable with all
cells/bits readable (a few days, weeks, months?) and when it was still
holding a detectable / useful amount of key information that could be
used in a brute force attack against the unknown bits of key.

they let you put americium in smoke detectors but something tells me
it would be hard to get radioactive crypto keys commercially approved
for production. :)
(the neutron source would be another problem, although piezoelectric
fusion might work)