Re: Crypto Update for 2.6.38

Dag Arne Osvik
2011-01-10T19:12:19Z

> On Sat, Jan 08, 2011 at 03:23:04PM +0200, Nikos Mavrogiannopoulos wrote:
> > On Fri, Jan 7, 2011 at 2:04 PM, Neil Horman <nhorman@tuxdriver.com>
> > wrote:
> >
> > > > Btw, it doesn't have to be about performance per se. Does this
> > > > allow people to use keys without actually _seeing_ those keys?
> > > > Your example implies that that is not the case, but that's
> > > > actually one of the few reasons to actually support a kernel
> > > > crypto interface - the ability to have private personal keys
> > > > around, but not having to actually let possibly untrusted programs
> > > > see them.
> > > This actually is an indirect feature of this interface.  Using it,
> > > you can open a algorithm socket, select a specific alg, assign a
> > > key, and then pass that socket descriptor over a unix socket to an
> > > another process using an SCM_RIGHTS ancilliary message.  The
> > > receiving process can then use children acceppted from that passed
> > > socket to preform the configured crypto operation without any
> > > knoweldge of the keys used in it.  I can write a demo app if you
> > > like.
> >
> > Several things have to be considered when extending an interface like
> > that. For example, do the algorithm implementations protect against
> > timing attacks, or keys can be recovered, using them? What is the
> No, the kernel does not implement any protection against timing attacks
> in the algorithms per-se, but preforming a timing attack against a
> kernel crypto operation is going to be near impossible anyway, as
> precise timing measurements are going to get obscured by interupts,
> scheduling jitter, lock contention, and various other factors that will
> make measuring syscall time fairly useless.

Let me just point out that this is not near impossible at all; instead it has already been done more than 6 years ago. And it's not only syscall time that leaks information. One practical example is recovery of a full AES key in a couple of seconds, using cache attacks against an encrypted file system. AES-NI is immune to this kind of attack, but other algorithms typically implemented using lookup tables are at risk.

Dag Arne