🚀 go-pugleaf

>>In article <BvpEx7.7Iq@fulcrum.bt.co.uk> igb@fulcrum.bt.co.uk (Ian G Batten)  
>>writes:
>> "Suppose I strip off the bottom bit
>> "of each sample in a piece of music on CD and replace it with my ideal
>> "random bits, and press the CDs.
>>
I know that during the war two forms of "personal codes" for agents were used.
One was the book code where the agent and his control used a common book as the
source of a "random pad" for encoding messages.

For example if agent X used Hamlet for his book code he may have used

"Alas poor Yoric, I knew him..........."
as the random sequence to encode the plain text. Both agent and control
know that that paragraph is the "random pad" of the day but a potential 
enemy would have to know at least the book, if not the passage to decode
the cyphertext.

SOE agents used poems because they can be easily memorised so the agent need
not carry a book (and thus the basis of their personal code ) with them.
This has the secondary advantage that the code dies with the agent. A book
found with an agent could be used for spoofing even if the agent where
uncooperative or dead. When the Germans became aware of the poem codes and
checked all published poems against received cyphertext the British just wrote
new unpublished poems, some of which are the most beautiful coding systems
you have ever heared :-)

My point is this. Why with the CD system is it nescessary to produce and
 distribute special CD's.
Instead the agent has a PC and a CDROM player and a very large collection of
 standard off the shelf CD's.

The agent knows that in December the "disk of the month" is for instance
Queen's "A Kind of Magic". This is a standard CD bought at a local music shop.
The software on the PC takes a beginning block postion or time on the CD and
uses that as a start point for encryption. As the disk contains a lot of
effectively random data and the disk is changed regularly the method of
finding the start position can be real simple without compromising the code.
Suppose the spy takes the date and mutiplys it by 2 for example.
The start position for today would be 921007 * 2 = 1842014, which is either
a block number or a time in ms.

This has to be a perfect cypher. The musical data is effectively an agreed
random number sequence, both the disk used and the start point are known
only to the agent and his control. Even if the agent was discovered the
huge amount of possible sequences even with a modest CD collection
would be astronomical and make the code resistant to spoofing without the
help of the agent (and all codes would fail in that case!!!!).

If the agent only bought his CD's as he needed them the code would be even
better. IF the FBI raids agent X's house on August 20th they may discover
his complete setup (v. unlikely) but if agent x hasn't actualy
bought the disk which is the basis of Septembers code sequence, they stand
no chance of discovering what it is without his assistance.

Well what do you think???

Fletch