Hiding cyphertext in cyphertext (FW)

Vin McLellan (vin@shore.net)
Sun, 18 Apr 1999 14:38:30 -0400

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From: "Jonathan Kennedy" <jkennedy@connecti.com>
To: <cypherpunks@cyberpass.net>
Subject: Hiding cyphertext in cyphertext
Date: Sun, 18 Apr 1999 12:47:27 -0000

I'm a beginner when it comes to cryptography but I after reading the Isreali
Spy example in Schneier's Book, Applied Cryptography, I began thinking about
a better way to hide cyphertext inside of cyphertext. I decided to post this
somewhere so I could recieve some peer-review. If this is the wrong place
for technical discussion, I apologize.

The problem with the Isreali Spy method is that the authorities would have
to actually believe that you used a simple XOR for encrypting your data. If
you've been under investigation they probably won't buy the excuse that
you're some amateur cryptographer just experimenting.

So after a few hours of thought this is what I came up with.

E(P) XOR D = K

send the recipient S(E(D), K) while hiding K in the signature.

The recipeint then does as follows.

D(E(D)) XOR K = E(P)
D(E(P)) = P

Where E() and D() is the encryption/decryption routines, P is the plaintext,
D is the dummy text and K is the secret key.

Upon request for the key by the authorities, the cryptographers can give
them k for E() and the intercepted message will decrypt to D, the dummy
message. The authorities would then have no reason to be suspicious because
E() can be any advanced encryption process you choose.

Better still, if the authorities try to crack the intercepted message, E(D),
they will just end up with D, the dummy message.

The whole process hinders upon the cryptographer's ability to send K
secretly. Since only a limited amount of data can be sent via digital
signature, would this prevent the sender from sending K in this way?

-JKennedy
--------
  "Cryptography is like literacy in the Dark Ages. Infinitely potent,
for good and ill... yet basically an intellectual construct, an idea,
which by its nature will resist efforts to restrict it to bureaucrats
and others who deem only themselves worthy of such Privilege."
  _A Thinking Man's Creed for Crypto _vbm

 * Vin McLellan + The Privacy Guild + <vin@shore.net> *
      53 Nichols St., Chelsea, MA 02150 USA <617> 884-5548

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