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

[Gilad Ben-Yossef <gilad@benyossef.com>]

On Fri, Feb 7, 2020 at 9:56 AM Stephan Mueller <smueller@chronox.de> wrote:
>
> Am Freitag, 7. Februar 2020, 08:27:09 CET schrieb Eric Biggers:
>
> Hi Eric,
>
> > On Wed, Feb 05, 2020 at 04:48:16PM +0200, Gilad Ben-Yossef wrote:
> > > Probably another issue with my driver, but just in case -
> > >
> > > include/crypot/aead.h says:
> > >  * The scatter list pointing to the input data must contain:
> > >  *
> > >  * * for RFC4106 ciphers, the concatenation of
> > >  *   associated authentication data || IV || plaintext or ciphertext.
> > >  Note, the *   same IV (buffer) is also set with the
> > >  aead_request_set_crypt call. Note, *   the API call of
> > >  aead_request_set_ad must provide the length of the AAD and *   the IV.
> > >  The API call of aead_request_set_crypt only points to the size of *
> > >  the input plaintext or ciphertext.
> > >
> > > I seem to be missing the place where this is handled in
> > > generate_random_aead_testvec()
> > > and generate_aead_message()
> > >
> > > We seem to be generating a random IV for providing as the parameter to
> > > aead_request_set_crypt()
> > > but than have other random bytes set in aead_request_set_ad() - or am
> > > I'm missing something again?
> >
> > Yes, for rfc4106 the tests don't pass the same IV in both places.  This is
> > because I wrote the tests from the perspective of a generic AEAD that
> > doesn't have this weird IV quirk, and then I added the minimum quirks to
> > get the weird algorithms like rfc4106 passing.
> >
> > Since the actual behavior of the generic implementation of rfc4106 is that
> > the last 8 bytes of the AAD are ignored, that means that currently the
> > tests just avoid mutating these bytes when generating inauthentic input
> > tests.  They don't know that they're (apparently) meant to be another copy
> > of the IV.
> >
> > So it seems we need to clearly define the behavior when the two IV copies
> > don't match.  Should one or the other be used, should an error be returned,
> > or should the behavior be unspecified (in which case the tests would need
> > to be updated)?
> >
> > Unspecified behavior is bad, but it would be easiest for software to use
> > req->iv, while hardware might want to use the IV in the scatterlist...
> >
> > Herbert and Stephan, any idea what was intended here?
> >
> > - Eric
>
> The full structure of RFC4106 is the following:
>
> - the key to be set is always 4 bytes larger than required for the respective
> AES operation (i.e. the key is 20, 28 or 36 bytes respectively). The key value
> contains the following information: key || first 4 bytes of the IV (note, the
> first 4 bytes of the IV are the bytes derived from the KDF invoked by IKE -
> i.e. they come from user space and are fixed)
>
> - data block contains AAD || trailing 8 bytes of IV || plaintext or ciphertext
> - the trailing 8 bytes of the IV are the SPI which is updated for each new
> IPSec package
>
> aead_request_set_ad points to the AAD plus the 8 bytes of IV in the use case
> of rfc4106(gcm(aes)) as part of IPSec.
>
> Considering your question about the aead_request_set_ad vs
> aead_request_set_crypt I think the RFC4106 gives the answer: the IV is used in
> two locations considering that the IV is also the SPI in our case. If you see
> RFC 4106 chapter 3 you see the trailing 8 bytes of the IV as, well, the GCM IV
> (which is extended by the 4 byte salt as defined in chapter 4 that we provide
> with the trailing 4 bytes of the key). The kernel uses the SPI for this. In
> chapter 5 RFC4106 you see that the SP is however used as part of the AAD as
> well.
>
> Bottom line: if you do not set the same IV value for both, the AAD and the GCM
> IV, you deviate from the use case of rfc4106(gcm(aes)) in IPSec. Yet, from a
> pure mathematical point of view and also from a cipher implementation point of
> view, it does not matter whether the AAD and the IV point to the same value -
> the implementation must always process that data. The result however will not
> be identical to the IPSec use case.
>

It is correct, but is it smart?

Either we require the same IV to be passed twice as we do today, in which case
passing different IV should fail in a predictable manner OR we should define
the operation is taking two IV like structures - one as the IV and one as
bytes in the associated data and have the IPsec code use it in a specific way of
happen to pass the same IV in both places.

I don't care either way - but right now the tests basically relies on
undefined behaviour
which is always a bad thing, I think.

Gilad

-- 
Gilad Ben-Yossef
Chief Coffee Drinker

values of β will give rise to dom!