Re: [dm-devel] [PATCH] crypto/arc4: convert this stream cipher into a block cipher

[Herbert Xu <herbert gondor apana org au>]

On Mon, Feb 22, 2010 at 02:40:49PM -0500, Mikulas Patocka wrote:
>
> > So what we could do is simply add a new blkcipher arc4, alongside
> > the existing cipher arc4.  Then we can convert the existing users
> > across, and finally remove the old arc4.
> 
> arc4 can't be used as a block cipher --- see this paper 
> http://www.wisdom.weizmann.ac.il/~itsik/RC4/Papers/Rc4_ksa.ps , it says 
> that initialization vectors on RC4 are unreliable, if you use (unknown key 
> concatenated with known IV) or (known IV concatenated with unknown key) as 
> a RC4 key, the RC4 state can be exposed and the cipher is broken.

What we call a blkcipher is not really a block cipher.  In fact,
what we call "cipher" is really a block cipher.  So we're actually
changing arc4 so that it doesn't get used as a block cipher, i.e.,
you will no longer be able to say "cbc(arc4)" or some such.

I know it's confusing and perhaps one day we will rename blkcipher
to skcipher and cipher to blkcipher.

Cheers,
-- 
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Email: Herbert Xu ~{PmV>HI~} <herbert gondor apana org au>
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