[Cryptography] Impossible trapdoor systems (was Re: Opening Discussion: Speculation on "BULLRUN")

Ray Dillinger bear at sonic.net
Wed Sep 11 23:37:03 EDT 2013

On 09/08/2013 11:49 AM, Perry E. Metzger wrote:

> That said, your hypothetical seems much like "imagine that you can
> float by the power of your mind alone". The construction of such a
> cipher with a single master key that operates just like any other key
> seems nearly impossible, and that should be obvious.

True.  A universal key that uses the same decryption operation as
a normal key is clearly stupid.

I guess the thing I was thinking of is that the "attacker" knows
a method that allows him to decrypt anything if he knows the IV,
but cannot recover the key used to encrypt it.

Which is of course a public-key system, where the decryption
method is the "private" key and the IV is the "public" key.
The thing I was thinking of as a "key" functions as a "nonce"
or subkey which allows people unrelated to the private key
holder to communicate semi-privately by shared secret, but
the private key is a backdoor on their communication.

Duh. Sorry, just wasn't thinking of the right "parallel mapping"
of what I described. For the cipher itself to function as a key
sort of escaped my attention.

Sorry to waste time.


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