🚀 go-pugleaf

>>Iain McCord raises the possibiity that the police will "plant" an
>>encrypted file, or worse, plant an incriminating encrypted file of 
>>their own design.
>>
>>This is not a cryptographic issue. They could do the same with any
>>other kind of evidence. One's protections against this are also
>>the same, including the ability to challenge the 'chain of evidence'
>>in court, the ability to introduce contrary evidence, etc.
>
>Oh, but it IS a cryptographic issue.
>
>With a public key system you can be ABSOLUTELY certain that a particular
>ciphertext was created with one key if you can decode it with the other.
>Therefore, if the keys are not compromised you can, with certainty,
>prove whether or not it was written by the individual in question.

I am going to presume you are talking about "signing" a message.  If you
are talking about using a public key to encrypt a message, then presumably
anyone could have done this.

While I am not going to try to refute the essance of what you say, I think
you make a giant leap of faith with your supposition "if the keys are not
compromised".  It is darn neigh impossible to conclude this once you no
longer have control of the computer.  If the police are of a mind to, they
could generate a public/private key pair, sign a message with it, and
claim it was signed by you.  "How can you prove it was signed by me?"
"Because it can be decrypted with your public key"  "How do you know that
is my public key?"  "It was on your own public keyring with you as the
indicated owner."

Once the police have sole control over the computer, if the police are both
technically competant and corrupt, all bets are off:  Your defense is to
convince a jury that the police have not been impartial.  About the only
thing cryptography adds in this regard is a degree of complexity that makes
it beyond the scope of the average judge and jurist to understand.
-- 
Rob Stampfli  rob@colnet.cmhnet.org      The neat thing about standards:
614-864-9377  HAM RADIO: kd8wk@n8jyv.oh  There are so many to choose from.