[Cryptography] Keyed affine transform

[Pierre Abbat]

I'm thinking of inventing a keyed affine transform, which is like a cipher, 
except that it's affine, so it's easy to break. It will have the following 
properties:

* The plaintext and ciphertext can have any number of bytes (the ciphertext is 
as long as the plaintext).

* The key could be fixed-length (probably 32 bytes) or variable-length. I 
haven't decided.

* The transform is invertible: given the ciphertext and key, you can easily 
compute the plaintext.

* If you hold the key fixed, and flip a bit of the plaintext, about half the 
ciphertext bits flip, and if you flip a different plaintext bit, about a fourth 
of the ciphertext bits flipped both times. However, if you take a different 
plaintext and flip the same bit, the same ciphertext bits flip, unlike a cipher 
with the SAC. Similarly if you flip one ciphertext bit, about half the 
plaintext bits flip.

* If you hold the plaintext bit you flip fixed, and vary the key, the sets of 
ciphertext bits that flip look random, even if you compare keys that differ in 
the same key bit.

What could such a transform be used for?

Pierre
-- 
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