Re: [PATCH] crypto: ccree - enable CTS support in AES-XTS

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

On Mon, Sep 9, 2019 at 5:38 PM Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
>
> On Mon, 9 Sep 2019 at 13:34, Gilad Ben-Yossef <gilad@benyossef.com> wrote:
> >
> > On Mon, Sep 9, 2019 at 3:20 PM Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> > >
> > > On Sun, 8 Sep 2019 at 09:04, Uri Shir <uri.shir@arm.com> wrote:
> > > >
> > > > In XTS encryption/decryption the plaintext byte size
> > > > can be >= AES_BLOCK_SIZE. This patch enable the AES-XTS ciphertext
> > > > stealing implementation in ccree driver.
> > > >
> > > > Signed-off-by: Uri Shir <uri.shir@arm.com>
> > > > ---
> > > >  drivers/crypto/ccree/cc_cipher.c | 16 ++++++----------
> > > >  1 file changed, 6 insertions(+), 10 deletions(-)
> > > >
> > > > diff --git a/drivers/crypto/ccree/cc_cipher.c b/drivers/crypto/ccree/cc_cipher.c
> > > > index 5b58226..a95d3bd 100644
> > > > --- a/drivers/crypto/ccree/cc_cipher.c
> > > > +++ b/drivers/crypto/ccree/cc_cipher.c
> > > > @@ -116,10 +116,6 @@ static int validate_data_size(struct cc_cipher_ctx *ctx_p,
> > > >         case S_DIN_to_AES:
> > > >                 switch (ctx_p->cipher_mode) {
> > > >                 case DRV_CIPHER_XTS:
> > > > -                       if (size >= AES_BLOCK_SIZE &&
> > > > -                           IS_ALIGNED(size, AES_BLOCK_SIZE))
> > > > -                               return 0;
> > > > -                       break;
> > >
> > > You should still check for size < block size.
> > Look again - he does via the fall through aspect of the case.
> >
>
> Ah right - I missed that.
>
> > >
> > > >                 case DRV_CIPHER_CBC_CTS:
> > > >                         if (size >= AES_BLOCK_SIZE)
> > > >                                 return 0;
> > > > @@ -945,7 +941,7 @@ static const struct cc_alg_template skcipher_algs[] = {
> > > >         {
> > > >                 .name = "xts(paes)",
> > > >                 .driver_name = "xts-paes-ccree",
> > > > -               .blocksize = AES_BLOCK_SIZE,
> > > > +               .blocksize = 1,
> > >
> > > No need for these blocksize changes - just keep them as they are.
> >
> > hm... I'm a little confused about this.
> > Why do we have, say CTR template, announce a block size of 1 (which
> > makes sense since it effectively turns a block cipher to a stream
> > cipher) but here stick to the underlying block size?
> > I mean, you can request processing for any granularity (subject to the
> > bigger than 1 block), just like CTR so I'm not sure what information
> > is supposed to be conveyed here.
> >
>
> The blocksize is primarily used by the walking code to ensure that the
> input is a round multiple. In the XTS case, we can't blindly use the
> skcipher walk interface to go over the data anyway, since the last
> full block needs special handling as well.
>
> So the answer is really that we had no reason to change it for the
> other drivers, and changing it here will trigger a failure in the
> testing code that compares against the generic implementations.

I see. That makes sense. Thanks for the explanation.

Gilad



-- 
Gilad Ben-Yossef
Chief Coffee Drinker

values of β will give rise to dom!