[Cryptography] Shortening block cipher length...

[Peter Gutmann]

Jon Callas <jon at callas.org> writes:

>Colin's trick is essentially isomorphic to XEX, and not as fast. He generates
>a CFB IV via a fast hash. Then it's just CFB. 

We're talking about different things, just use of CFB isn't really much of a
trick.  This involves running two passes of CBC over a data block without
resetting the IV on the second pass.  To decrypt the data, decrypt the n-th
block using the n-1th block as the IV, which gives you the IV used for the
outer encryption.  Then just run two passes of decryption.  The result is an
IV-less encryption for which a single bit change affects the entire block that
can be constructed with any off-the-shelf block cipher with no special modes
or operations required.

Peter.