RE: Possible issue with new inauthentic AEAD in extended crypto tests

["Van Leeuwen, Pascal" <pvanleeuwen@rambus.com>]

Hi Stephan,

> -----Original Message-----
> From: linux-crypto-owner@vger.kernel.org <linux-crypto-owner@vger.kernel.org> On Behalf Of Stephan Mueller
> Sent: Wednesday, January 29, 2020 2:27 AM
> To: Van Leeuwen, Pascal <pvanleeuwen@rambus.com>
> Cc: Eric Biggers <ebiggers@kernel.org>; Gilad Ben-Yossef <gilad@benyossef.com>; Herbert Xu <herbert@gondor.apana.org.au>;
> Linux Crypto Mailing List <linux-crypto@vger.kernel.org>; Geert Uytterhoeven <geert@linux-m68k.org>; David Miller
> <davem@davemloft.net>; Ofir Drang <Ofir.Drang@arm.com>
> Subject: Re: Possible issue with new inauthentic AEAD in extended crypto tests
>
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> Am Mittwoch, 29. Januar 2020, 01:18:29 CET schrieb Van Leeuwen, Pascal:
>
> Hi Pascal,
>
> > > -----Original Message-----
> > > From: linux-crypto-owner@vger.kernel.org
> > > <linux-crypto-owner@vger.kernel.org> On Behalf Of Eric Biggers
>  Sent:
> > > Tuesday, January 28, 2020 10:13 PM
> > > To: Gilad Ben-Yossef <gilad@benyossef.com>
> > > Cc: Herbert Xu <herbert@gondor.apana.org.au>; Stephan Mueller
> > > <smueller@chronox.de>; Linux Crypto Mailing List <linux-
> > > crypto@vger.kernel.org>; Geert Uytterhoeven <geert@linux-m68k.org>; David
> > > Miller <davem@davemloft.net>; Ofir Drang <Ofir.Drang@arm.com>
> > > Subject: Re: Possible issue with new inauthentic AEAD in extended crypto
> > > tests
> >
> > >
> > >
> > > <<< External Email >>>
> > > CAUTION: This email originated from outside of the organization. Do not
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> > >
> > >
> > >
> > > On Tue, Jan 28, 2020 at 09:24:25AM +0200, Gilad Ben-Yossef wrote:
> > >
> > > > - The source is presumed to have enough room for both the associated
> > > > data and the plaintext.
> > > > - Unless it's in-place encryption, in which case, you also presume to
> > > > have room for the authentication tag
> > >
> > >
> > >
> > > The authentication tag is part of the ciphertext, not the plaintext.  So
> > > the
>  rule is just that the ciphertext buffer needs to have room for it,
> > > not the plaintext.
> > >
> > >
> > >
> > > Of course, when doing in-place encryption/decryption, the two buffers are
> > > the
>  same, so both will have room for it, even though the tag is only
> > > meaningful on the ciphertext side.  That's just the logical consequence
> > > of "in-place".>
> > >
> > >
> > > > - The only way to tell if this is in-place encryption or not is to
> > > > compare the pointers to the source and destination - there is no flag.
> > >
> > >
> > >
> > > Requiring users to remember to provide a flag to indicate in-place
> > > encryption/decryption, in addition to passing the same scatterlist, would
> > > make
>  the API more complex.
> > >
> > >
> >
> > Also, what would the benefit? You'd still have to compare the flag. The
> > performance
>  difference of comparing the flag vs comparing 2 pointers (that
> > you need to read anyway) is likely completely negligible on most modern CPU
> > architectures ...
> >
> > > > - You can count on the scattergather list not having  a first NULL
> > > > buffer, *unless* the plaintext and associated data length are both
> > > > zero AND it's not in place encryption.
> > > > - You can count on not getting NULL as a scatterlist point, *unless*
> > > > the plaintext and associated data length are both zero AND it's not in
> > > > place encryption. (I'm actually unsure of this one?)
> > >
> > >
> > >
> > > If we consider that the input is not just a scatterlist, but rather a
> > > scatterlist and a length, then these observations are really just "you
> > > can
> > > access the first byte, unless the length is 0" -- which is sort of
> > > obvious.  And requiring a dereferencable pointer for length = 0 is
> > > generally considered to be bad API design; see the memcpy() fiasco
> > > (https://www.imperialviolet.org/2016/06/26/nonnull.html).
> > >
> > >
> > >
> > > The API could be simplified by only supporting full scatterlists, but it
> > > seems that users are currently relying on being able to encrypt/decrypt
> > > just a prefix.>
> > >
> > >
> > > IMO, the biggest problems with the AEAD API are actually things you
> > > didn't
> > > mention, such as the fact that the AAD isn't given in a separate
> > > scatterlist,
> >
> > >
> >
> > While I can understand this may be beneficial in some cases, I believe they
> > do not
> > outweigh the downsides:
> > - In many use cases, AAD+cipher text are stored as one contiguous string.
>
> Then refer to that one linear buffer with one SGL entry.
>
Hmm ... I believe having a seperate scatter list for AAD would imply that you have
seperate scatter entries for AAD (in that list) and Crypto[+TAG] (in the other list).
So you still have the burden of constructing 2 scatterlists instead of one, figuring
out where the second one starts. Plus the burden of any hardware accelerator
having to handle 2 particles instead of one.

Note that even with one scatterlist you can still have the AAD data coming from
some specific AAD-only buffer(s). Just put it it its own (set of) particle(s), seperate
from the crypto data particles. So that is not a reason to have seperate *lists*.

The only advantage of having AAD seperate I can think of is for software
crypto implementations, not having to skip over the AAD for the scatterlist they
send to the parallel encryption part. Which IMHO is only a minor inconvenience
that you shouldn't push to all the users of the API.

> > Requiring this
> > string to be spit into seperate particles for AAD and
> > ciphertext would be a burden.
>
> There is no need to split a string. All that is said is that the SGL needs to
> point to memory that is AAD||PT or AAD||CT||TAG. There is no statement about
> the number of SGL entries to point to these buffer(s). So you could have one
> linear buffer for these components pointing to it with an SGL holding one
> entry.
>
The remark I responded to was about having a seperate scatterlist for AAD data.
Which, in my world, implies that the *other* scatterlist does NOT include the AAD
data. So that one would then need to be only PT or CT||TAG. Which does require
"splitting the string" (virtually, anyway) between AAD and PT/CT.

It's not about splitting the data physically (i.e. moving it). It's about splitting the
particles, creating 2 particles (in 2 lists) where you would now only need 1.

> > - For hardware accelerators, there is a cost
> > associated with each additional particle, in terms of either bandwidth or
> > performance or both. So less particles = better, generally.
> > The only thing that I find odd is that if you do a non-inplace operation you
> > have this
> > undefined(?) gap in the output data where the AAD would be for
> > inplace. That makes little sense to me and requires extra effort to skip
> > over in the driver.
> >
> > > and that the API only supports scatterlists and not virtual addresses
> > > (which makes it difficult to use in some cases).
> > >
> > >
> >
> > While I can understand that this is difficult if the API user just got this
> > virtual address
>  provided from somewhere else and needs to do the
> > translation, the other side of the medal is that any hardware driver would
> > otherwise have to do address translation and scatterlist building on the
> > fly (as hardware needs to access contiguous physical memory), which would
> > be real burden there. While many API users_are_ able to provide a nice
> > scatterlist at negligible extra cost. So why burden those?
> >
> >
> > > In any case we do need much better documentation.  I'm planning to improve
> > > some
>  of the crypto API documentation, but I'll probably do the hash and
> > > skcipher algorithm types first before getting to AEAD.  So if you want to
> > > improve the AEAD documentation in the mean time, please go ahead.
> > >
> > >
> > >
> > > - Eric
> >
> >
> > Regards,
> > Pascal van Leeuwen
> > Silicon IP Architect Multi-Protocol Engines, Rambus Security
> > Rambus ROTW Holding BV
> > +31-73 6581953
> >
> > Note: The Inside Secure/Verimatrix Silicon IP team was recently acquired by
> > Rambus.
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>
>
> Ciao
> Stephan

Regards,
Pascal van Leeuwen
Silicon IP Architect Multi-Protocol Engines, Rambus Security
Rambus ROTW Holding BV
+31-73 6581953

Note: The Inside Secure/Verimatrix Silicon IP team was recently acquired by Rambus.
Please be so kind to update your e-mail address book with my new e-mail address.


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