🚀 go-pugleaf

In article <1992Oct28.114927.28223@news.cs.indiana.edu>, Marc VanHeyningen <mvanheyn@whale.cs.indiana.edu> writes:
> A reasonable idea, but unfortunately it is subject to the same problems
> as a single trustee:  There is no way to "prove" that someone is using
> an unregistered key until you get probable cause and get a court order
> for the keys and discover that you can't decrypt the traffic.

Let's turn our attention to technical matters for a while...
Basically, I'm not convinced you're right.  Consider, say, the Enigma
and the Index of Coincidence.  Using the latter, it was possible to
tell when two ciphertexts were encrypted using the same key.  Thus, one
could supply, with protected key registration, some innocuous plaintext
encrypted with the same key.  The trusted registrar could verify the
encryption, and anyone possessing just the sample of ciphertext could
tell if another message really did use that same key.

Now -- the Index of Coincidence won't work for DES because there are
too few repeats of 8-graphs, and that's without even bringing block
chaining modes into the discussion.  Besides, most modern cryptosystems
use random session keys, and use master keys only to exchange the
session keys.  Disregarding the last point, it's an interesting
question whether or not there's an analogous test that could be used.
Suggestions?  (Btw, in the context of the discussion it's perfectly
proper to decline to work on the problem, given the possibility of a
positive answer being abused.)