Subject: Re: Encrypted compressed vnds
To: Garrett D'Amore <>
From: Thor Lancelot Simon <>
List: tech-kern
Date: 08/07/2006 11:49:35
On Mon, Aug 07, 2006 at 08:21:12AM -0700, Garrett D'Amore wrote:
> Joerg Sonnenberger wrote:
> >
> > If you want something simple, at least use a secure cipher. AES and
> > Blowfish are in the kernel already, so use them. RC4 is as well, but
> > considered weak now. DES is just not worth the effort.
> >
> > Joerg
> I'm not aware of any cryptographers that consider RC4 weak.  RC4 is fast
> and simple, and it is a stream cipher.  I think RC4 has gotten a bad
> name because it was misused in WEP.  The usage created the weakness --
> properly used RC4 should be pretty strong.

I disagree.  There is a long history of work, going back about a decade
now, that, if nothing else, demonstrates that RC4 fails to meet what are
thought of as good design requirements for a stream cipher -- its output
can be all too easily distinguished from a random bitstream, to begin
with.  This has come up a couple of times on the cryptography list in the
past few months and certainly my sense of it is that very few people would
endorse the use of RC4 for new designs now.

There's also a considerable problem with using RC4 for encryption of data
like disk blocks: because the output of RC4 is so non-random when it first
starts up after rekeying, you do a significant amount of wasted work just
to get the cipher to a "safe" state, then use it to encrypt only 512 bytes.

Finally, there is the highly questionable notion of using a stream cipher,
with its easy susceptibility to bit-flipping, to encrypt data in persistent
storage on disk: yes, we do know attacks that allow modification of the
decrypted plaintext under block cipher modes like CBC, but stream ciphers
make it trivial, and surely that is not a good thing.  To use RC4 for this
purpose would require that it be used with a keyed MAC, and at that point
any speed advantage disappears.

In general I am opposed to adding encryption code to vnd and question
whether adding compression code was a good idea, either: it seems to
me the _only reason_ we would find it convenient to add encryption to
vnd now is that compression was added to it instead of being implemented
as a separate compressing device layer, which would have allowed the
stacking of the encryption and compression transforms in the right order.