Re: Why the auxiliary cipher in gss_krb5_crypto.c?

[Ard Biesheuvel]

On Mon, 7 Dec 2020 at 15:15, David Howells <dhowells@xxxxxxxxxx> wrote:
>
> Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> > > I wonder if it would help if the input buffer and output buffer didn't
> > > have to correspond exactly in usage - ie. the output buffer could be used
> > > at a slower rate than the input to allow for buffering inside the crypto
> > > algorithm.
> > >
> >
> > I don't follow - how could one be used at a slower rate?
>
> I mean that the crypto algorithm might need to buffer the last part of the
> input until it has a block's worth before it can write to the output.
>

This is what is typically handled transparently by the driver. When
you populate a scatterlist, it doesn't matter how misaligned the
individual elements are, the scatterlist walker will always present
the data in chunks that the crypto algorithm can manage. This is why
using a single scatterlist for the entire input is preferable in
general.

> > > The hashes corresponding to the kerberos enctypes I'm supporting are:
> > >
> > > HMAC-SHA1 for aes128-cts-hmac-sha1-96 and aes256-cts-hmac-sha1-96.
> > >
> > > HMAC-SHA256 for aes128-cts-hmac-sha256-128
> > >
> > > HMAC-SHA384 for aes256-cts-hmac-sha384-192
> > >
> > > CMAC-CAMELLIA for camellia128-cts-cmac and camellia256-cts-cmac
> > >
> > > I'm not sure you can support all of those with the instructions available.
> >
> > It depends on whether the caller can make use of the authenc()
> > pattern, which is a type of AEAD we support.
>
> Interesting.  I didn't realise AEAD was an API.
>
> > There are numerous implementations of authenc(hmac(shaXXX),cbc(aes)),
> > including h/w accelerated ones, but none that implement ciphertext
> > stealing. So that means that, even if you manage to use the AEAD layer to
> > perform both at the same time, the generic authenc() template will perform
> > the cts(cbc(aes)) and hmac(shaXXX) by calling into skciphers and ahashes,
> > respectively, which won't give you any benefit until accelerated
> > implementations turn up that perform the whole operation in one pass over
> > the input. And even then, I don't think the performance benefit will be
> > worth it.
>
> Also, the rfc8009 variants that use AES with SHA256/384 hash the ciphertext,
> not the plaintext.
>
> For the moment, it's probably not worth worrying about, then.  If I can manage
> to abstract the sunrpc bits out into a krb5 library, we can improve the
> library later.
>