[crypto-philo]OTP or DES?

There is a thread going in coderpunks about some software company that
claims to have invented a software OTP that uses a PRNG.  The local experts
have ruled that this is impossible and I would have to agree, but this
thread got me thinking.  I would argue that the security of an OTP is
derived not from the fact that it really is secure, but from the fact that
it is claimed to be an OTP.  Imagine a plaintext, encrypted with triple-des.
It looks like a bunch of 1's and 0's to the casual observer, but to you it
is your secret plan to take over the world.  Or so you would have us believe
if we crack the cypher.
actually you plan to take over the world using a completely different plan.
It is quite easy to take the bits of the des-encrypted message, and
calculate the OTP key nessasary to decrypt the message into your real plan.
It would seem you could build a key to have your message say anything of
equal length.  Why then must a true OTP be based on a true RNG?  Because one
of the actual possible keys of a real OTP is indeed the encrypted des
message, you can claim that it is an otp and no-one can prove otherwise.
They can say "But we cracked your des key and can decypher this message!"
and you say "nope i used an OTP, that is a false message, here is what it
really says! The fact that by PURE coincidence that OTP could be decrypted
using that particular key means nothing, because a true OTP can generate
that bit sequence.
So in conclusion i would say that we can give pgp complete and perfect
security but the simple process of changing the header from ---Begin PGP
message--- to
----Begin OTP message----.  This makes you immune from decryption because no
one will ever know or can ever prove that the decryption they got is the
correct one.

Any comments?

Walrus

On Fri, 20 Oct 1995 02:45:28 -0500, walrus wrote:

   it is claimed to be an OTP.  Imagine a plaintext, encrypted with triple-des.
   It looks like a bunch of 1's and 0's to the casual observer, but to you it
   is your secret plan to take over the world.  Or so you would have us believe
   if we crack the cypher.
   actually you plan to take over the world using a completely different plan.
   It is quite easy to take the bits of the des-encrypted message, and
   calculate the OTP key nessasary to decrypt the message into your real plan.

Yes, but if you _really_ used a OTP to encrypt your real plans, the
probability that the ciphertext would decrypt via DES to anything
intelligible is so amazingly minute that no one would believe you.
It's like _one_ monkey typing out flawlessly the complete works of
Shakespeare.  On a Wednesday afternoon... :-)

   using that particular key means nothing, because a true OTP can generate
   that bit sequence.

And I can instantly break any encrypted message I see, by correctly
guessing the algorithm and the key.  I'd bet against it, though.

In fact, I don't even need to see it!!  I can guess the ciphertext, too
:-)

-- 
Paul Foley <mycroft@actrix.gen.nz>       ---         PGPmail preferred

	   PGP key ID 0x1CA3386D available from keyservers
    fingerprint = 4A 76 83 D8 99 BC ED 33  C5 02 81 C9 BF 7A 91 E8
----------------------------------------------------------------------
Fourth Law of Thermodynamics:  If the probability of success is not
almost one, it is damn near zero.
		-- David Ellis