Re: [dm-crypt] cryptsetup with arc4 cipher

[Milan Broz <mbroz@redhat.com>]

Sebastian Andrzej Siewior wrote:
> Don't use this as a block cipher in dm-crypt, it is a bad idea.
> 
> The long story:
> ARC4 is a stream cipher and not a block cipher. Its internal state is
> reseted in setkey() and every crypto request (encrypt/decrypt don't
> matter) update the internal state of the stream cipher. That's why you
> get a different result every time you read the same block.
> 
> If you want to use this stream cipher in dm-crypt you would have to
> setup it up in ECB mode and use a key like "passphrase-IV". You have to
> set this key before a requests and wait until its done until you
> issue another crypto request (which includes setkey).

yes, I understand why this happens. I do not want to use stream cipher,
but apparently users will do that:-)

My question was why crypto allows this setting?
Or maybe what to do in dm-crypt to not allow user set such cipher
mapping (because it cannot produce anything useful).

IMHO it will not work even in ECB mode here for dm-crypt.
(also dm-crypt uses own IV function generators, but it is another story)

> CBC will not work because one of its requirements is that the cipher
> provides an inverse function which ARC4 simply does not have.

Again, why crypto API allows to use it and do not produce error then?

Milan
--
mbroz@redhat.com