[146876] in cryptography@c2.net mail archive
Re: [Cryptography] Impossible trapdoor systems (was Re: Opening
daemon@ATHENA.MIT.EDU (James A. Donald)
Sun Sep 8 23:54:39 2013
X-Original-To: cryptography@metzdowd.com
Date: Mon, 09 Sep 2013 10:37:47 +1000
From: "James A. Donald" <jamesd@echeque.com>
To: cryptography@metzdowd.com
In-Reply-To: <20130908144953.63f7f313@jabberwock.cb.piermont.com>
Reply-To: jamesd@echeque.com
Errors-To: cryptography-bounces+crypto.discuss=bloom-picayune.mit.edu@metzdowd.com
On 2013-09-09 4:49 AM, Perry E. Metzger wrote:
> Your magic key must then take any block of N bits and magically
> produce the corresponding plaintext when any given ciphertext
> might correspond to many, many different plaintexts depending
> on the key. That's clearly not something you can do.
Suppose that the mappings from 2^N plaintexts to 2^N ciphertexts are not
random, but rather orderly, so that given one element of the map, one
can predict all the other elements of the map.
Suppose, for example the effect of encryption was to map a 128 bit block
to a group, map the key to the group, add the key to the block, and map
back. To someone who knows the group and the mapping, merely a heavily
obfuscated 128 bit Caesar cipher.
No magic key.
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