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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