Re: [RFC PATCH v2] md/dm-crypt - reuse eboiv skcipher for IV generation

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Aug 09, 2019 at 08:29:59PM +0000, Pascal Van Leeuwen wrote:
> > 
> > There's no proof that other attacks don't exist.
> >
> As you can't prove something doesn't exist ...

Of course you can, that's what the security proofs for crypto constructions
always do.  They prove that no efficient attack exists (in some attack model)
unless the underlying crypto primitives are weak.

> 
> > If you're going to advocate
> > for using it regardless, then you need to choose a different (weaker) attack
> > model, then formally prove that the construction is secure under that model.
> > Or show where someone else has done so.
> > 
> I'm certainly NOT advocating the use of this. I was merely pointing out a
> legacy use case that happens to be very relevant to people stuck with it,
> which therefore should not be dismissed so easily.
> And how this legacy use case may have further security implications (like
> the tweak encryption being more sensitive than was being assumed, so you
> don't want to run that through an insecure implementation).

Obviously there are people already using bad crypto, whether this or something
else, and they often need to continue to be supported.  I'm not disputing that.

What I'm disputing is your willingness to argue that it's not really that bad,
without a corresponding formal proof which crypto constructions always have.

- Eric