Maynard Handley (name99.delete@this.name99.org) on December 12, 2018 10:32 am wrote:
> Anon (No.delete@this.email.com) on December 11, 2018 11:29 pm wrote:
> > Maynard Handley (name99.delete@this.name99.org) on December 11, 2018 4:32 pm wrote:
> > > Anon (no.delete@this.email.com) on December 11, 2018 4:21 pm wrote:
> > > > Maynard Handley (name99.delete@this.name99.org) on December 11, 2018 7:20 am wrote:
> > > > >......
> > > > > The crypto has to be appropriate to the problem. This is NOT the equivalent of network crypto.
> > > > > For example, suppose the equivalent of a random key burned into the device is XOR'd with everything
> > > > > going into and out of an MRAM cache (maybe also XOR-in the relevant addresses).
> > > > > Is that good enough? On the off-chance that someone could extract the contents
> > > > > of the MRAM cache (which may or may not be true?) the assumption is always
> > > > > that they CANNOT extract the contents of these sorts of hardware keys.
> > > >
> > > > Such a scheme would be about as secure as writing a key on a yellow postit on your monitor.
> > >
> > > Why do you say that?
> > > This is not a situation where one can watch a constant stream of data and extract statistics.
> > > My threat model is that someone steals a laptop, delids the chip, and has access to the
> > > frozen contents of the MRAM data. Not even the cache tags, just a collection of lines
> > > of data; and not a constant, changing, stream of data as the CPU executes.
> > >
> > > Is that the threat model you believe you have in mind? If not, well, we are not discussing the same thing.
> >
> > Unfortunately it makes no difference, a stream versus a static
> > corpus does not make a difference to many forms of attack.
> >
> > >
> > >
> > > > Unfortunately the fact is that good encryption is hard, and somewhat computationally involved.
> > > >
> > > > Not to mention the fact that with a static key, both ends
> > > > need that key, as as soon as it leaks, its game over.
> > >
> > > Slogans are great. But the relevance of this one is not especially clear to me.
> >
> > What do you think is a slogan? That is a simple fact. What part of it doesn't make sense?
> > The key is the key.. Abs it must exist at both ends.. If I've of them is insecure
> > then the game is over. Too much like a slogan? Sorry but it is simple fact.
>
> I don't know what "both ends need that key" provides in this context.
> I'm assuming a fused key that cannot be read by external means and that this
> key is held in NON-persistent storage (ie a standard register) by the cache
> controller. I'm not sure how these two facts make the system insecure.
>
> If you're assuming the fused key end can be read, well, then you're assuming
> away the security in pretty much EVERY piece of hardware on earth.
> If you're assuming that the laptop can be snatched away and data read from non-persistent registers,
> well, then why is MRAM adding any new insecurity --- your magic microscope that can read registers
> can just read whatever it wants from any part of the laptop you have snatched.

That might work, but it defeats nearly all the points of having non-volitile cache (except the ability to write-back). I'd expect to simply use SRAM if I needed a duplicate SRAM cache with my MRAM cache, but perhaps those that want persistent storage care enough for such an encryption model.