Re: [RFC PATCH] crypto: arc4: Implement a version optimized for memory usage

[Eric Biggers <ebiggers@kernel.org>]

On Tue, May 04, 2021 at 07:59:38PM +0200, Christophe JAILLET wrote:
> Le 04/05/2021 à 18:57, Eric Biggers a écrit :
> > On Sun, May 02, 2021 at 09:29:46PM +0200, Christophe JAILLET wrote:
> > > +#if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
> > > +#define S_type	u8
> > > +#else
> > > +#define S_type	u32
> > > +#endif
> > > +
> > >   struct arc4_ctx {
> > > -	u32 S[256];
> > > +	S_type S[256];
> > >   	u32 x, y;
> > >   };
> > 
> > Is it actually useful to keep both versions?  It seems we could just use the u8
> > version everywhere.  Note that there aren't actually any unaligned memory
> > accesses, so choosing the version conditionally on
> > CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS seems odd.  What are you trying to
> > determine by checking that?
> 
> Hi, this is a bad interpretation from me.
> 
> I thought that S[1] would likely use an odd address and would trigger an
> unaligned access. But as we would read only 1 byte, this is not the case.
> 
> Looking at [1], we have : "At this point, it should be clear that accessing
> a single byte (u8 or char) will never cause an unaligned access, because all
> memory addresses are evenly divisible by one."
> 
> 
> I wanted to avoid potential performance cost related to using char (i.e u8)
> instead of int (i.e. u32).
> On some architecture this could require some shift or masking or whatever to
> "unpack" the values of S.
> 
> 
> [1]:
> https://www.kernel.org/doc/html/latest/core-api/unaligned-memory-access.html
> 
> CJ
> 

arc4 is an insecure cipher which is only supported for use in legacy protocols.
So we don't really need to worry about optimizing performance on every
architecture.  If the byte-based version is *usually* faster as well as uses
less memory, we probably should just use it everywhere.

- Eric