Re:The strange story of “Extended Random”
by blattimwind on Sunday December 24, 2017 @07:56AM
(#15999330)
> For an extreme example: if all 128 bits of plaintext are known, and all 128 bits of ciphertext are also known, there should be a single 128 bit AES key that maps that input to that output, so the entropy left in the key would be zero.
Abstracting away a little bit. What is a block cipher? A bijective mapping between identical sets uniquely identified by the key. This makes it a permutation; and a good cipher says (among other things), that every key yields a unique permutation.
This doesn't mean that the mapping of one value identifies the key (i.e. the permutation itself). If the mapping for each key were truly random, then it would be essentially impossibly unlikely to end up not sharing a number of individual plain->cipher mappings between different keys.
You can do this as a little experiment using a small block size, 4 bits or so.
Abstracting away a little bit. What is a block cipher? A bijective mapping between identical sets uniquely identified by the key. This makes it a permutation; and a good cipher says (among other things), that every key yields a unique permutation.
This doesn't mean that the mapping of one value identifies the key (i.e. the permutation itself). If the mapping for each key were truly random, then it would be essentially impossibly unlikely to end up not sharing a number of individual plain->cipher mappings between different keys.
You can do this as a little experiment using a small block size, 4 bits or so.