RE: ghash

[Pascal Van Leeuwen <pvanleeuwen@verimatrix.com>]

> -----Original Message-----
> From: linux-crypto-owner@vger.kernel.org <linux-crypto-owner@vger.kernel.org> On Behalf Of Eric Biggers
> Sent: Friday, July 19, 2019 6:16 PM
> To: Pascal Van Leeuwen <pvanleeuwen@verimatrix.com>
> Cc: linux-crypto@vger.kernel.org; Herbert Xu <herbert@gondor.apana.org.au>; davem@davemloft.net
> Subject: Re: ghash
> 
> On Fri, Jul 19, 2019 at 02:05:01PM +0000, Pascal Van Leeuwen wrote:
> > Hi,
> >
> > While implementing GHASH support for the inside-secure driver and wondering why I couldn't get
> > the test vectors to pass I have come to the conclusion that ghash-generic.c actually does *not*
> > implement GHASH at all. It merely implements the underlying chained GF multiplication, which,
> > I understand, is convenient as a building block for e.g. aes-gcm but is is NOT the full GHASH.
> > Most importantly, it does NOT actually close the hash, so you can trivially add more data to the
> > authenticated block (i.e. the resulting output cannot be used directly without external closing)
> >
> > GHASH is defined as GHASH(H,A,C) whereby you do this chained GF multiply on a block of AAD
> > data padded to 16 byte alignment with zeroes, followed by a block of ciphertext padded to 16
> > byte alignment with zeroes, followed by a block that contains both AAD and cipher length.
> >
> > See also https://en.wikipedia.org/wiki/Galois/Counter_Mode
> >
> > Regards,
> > Pascal van Leeuwen
> > Silicon IP Architect, Multi-Protocol Engines @ Verimatrix
> > www.insidesecure.com
> >
> 
> Yes that's correct.  The hash APIs don't support multi-argument hashes, so
> there's no natural way for it to be "full GHASH".  So it relies on the caller to
> format the AAD and ciphertext into a single stream.  IMO it really should be
> called something like "ghash_core".
> 
> Do you have some question or suggestion, or was this just an observation?
> 
Well, considering it's pretending to be GHASH I was more less considering this a bug report ...

There's the inherent danger that someone not aware of the actual implementation tries to
use it as some efficient (e.g. due to instruction set support) secure authentication function.
Which, without proper external data formatting, it's surely not in its current form. This is 
not something you will actually notice when just using it locally for something (until
someone actually breaks it).

And then there was the issue of wanting the offload it to hardware, but that's kind of hard
if the software implementation does not follow the spec where the hardware does ...

I think more care should be taken with the algorithm naming - if it has a certain name, 
you expect it to follow the matching specification (fully). I have already identified 2 cases 
now (xts and ghash) where that is not actually the case.

Regards,
Pascal van Leeuwen
Silicon IP Architect, Multi-Protocol Engines @ Verimatrix
www.insidesecure.com