Re: AEAD question

[Eric Biggers <ebiggers@kernel.org>]

On Mon, Jul 22, 2019 at 12:55:39PM +0000, Pascal Van Leeuwen wrote:
> Eric & Herbert,
> 
> I noticed the testmgr fuzz tester generating (occasionally, see previous mail) tests cases with 
> authsize=0 for the AEAD ciphers. I'm wondering if that is intentional. Or actually, I'm wondering
> whether that should be considered a legal case.
> To me, it doesn't seem to make a whole lot of sense to do *authenticated* encryption and then
> effectively throw away the authentication result ... (it's just a waste of power and/or cycles)
> 
> The reason for this question is that supporting this requires some specific workaround in my 
> driver (yet again). And yes, I'm aware of the fact that I can advertise I don't support zero length
> authentication tags, but then probably/likely testmgr will punish me for that instead.
> 

As before you're actually talking about the "authenc" template for IPSec and not
about AEADs in general, right?  I'm not familiar with that algorithm, so you'll
have to research what the specification says, and what's actually using it.

Using an AEAD with authsize=0 is indeed silly, but perhaps someone using that in
some badly designed protocol where authentication is optional.  Also AFAICS from
the code, any authsize fits naturally into the algorithm; i.e., excluding 0
would be a special case.

But again, someone actually has to research this.  Maybe
crypto_aead_setauthsize() should simply reject authsize=0 for all AEADs.

What we should *not* do, IMO, is remove it from the tests and allow
implementations to do whatever they want.  If it's wrong we should fix it
everywhere, so that the behavior is consistent.

- Eric