Re: Back Doors

From: Douglas A. Gwyn (DAGwyn_at_null.net)
Date: 12/20/03 

Date: Sat, 20 Dec 2003 02:15:00 -0600

"mike3" <mike4ty4@yahoo.com> wrote in message
news:1d54b7e4.0312192340.3f693332@posting.google.com...
> How can one implement "back doors" in an encryption algorithm that
> would allow someone to decode all messages encrypted with it provided
> they know some secret "magic" formula? Is this a very diffcult problem
> or an easy one?

It's neither, or both.
Back doors are simple enough in "toy" implementations, for example
    HELLO => QHSETLZLPO
More typical block cipher systems that don't expand the message size
would have to use more subtle means. Note that a sufficiently "weak"
(crackable) system can be considered to have a "back door", since all
one has to do is crack that easy-to-crack system. For example, if the
system hashes the key to, say, 8 bits and uses the result as the real key,
if it uses an inherently strong block encryptor the ciphertext will look
quite random, but it takes at most 2^8 = 256 trial decryptions to
recover the ciphertext!

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