New UK crypto law and an idea on how to defeat it

Adam Lock
Wed, 01 Dec 1999 16:19:48 +0000

I understand that it will (or might) soon be necessary in the UK to hand
over crypto keys to the police if they so demand them. The penalty for
not doing so is a term in prison.

So here's an idea on how to defeat it.

Imagine Bob is having an affair with the police chief's wife and has all
their encrypted love letters in his possession. The police chief
suspects the affair and is prepared to abuse his powers to obtain the
letters for a divorce proceeding in his favour. He knows he can force
Bob to give him the decryption key by threatening him with prison term
for not doing so. Bob doesn't want to go to prison so he "reluctantly"
hands over the key. The chief gleefully decrypts the message hoping at
last for evidence of his wife's infidelity but all he sees is a cooking
recipe! Curse that Bob!

How is this done?

Simple. Write a tool that encrypts two or more plaintexts each with a
separate key and concatenates them into a single ciphertext.

But there's a problem. The police chief finds out that there are
multiple plaintexts in the ciphertext. He goes back to Bob and demands
the proper key or he'll definitely go to prison. How can Bob claim that
he has "truthfully" given up the correct key the first time and has no
idea what the other key is?

Again simple. Make the tool capable of encrypting one or more plaintexts
and zero or more *random* plaintexts (with random keys) into a single

Bob can't be sent to jail because he can validly claim that the other
plaintexts in the cipher were randomly generated and so he couldn't
possibly know what the other key was let alone hand it over. The police
chief might *suspect* Bob was lying, but there's no way he could prove
it short of Bob's admission. Effectively, Bob has defeated the
requirement to hand over his key, but has still kept his secrets secret.

Does this sound like a feasible idea?

Adam Lock