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Date:   Mon, 15 Jun 2020 17:30:24 +1000
From:   Herbert Xu <herbert@gondor.apana.org.au>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Stephan Mueller <smueller@chronox.de>,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        Eric Biggers <ebiggers@kernel.org>
Subject: Re: [v2 PATCH 0/3] crypto: skcipher - Add support for no chaining
 and partial chaining
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On Fri, Jun 12, 2020 at 06:10:57PM +0200, Ard Biesheuvel wrote:
>
> First of all, the default fcsize for all existing XTS implementations
> should be -1 as well, given that chaining is currently not supported
> at all at the sckipher interface layer for any of them (due to the
> fact that the IV gets encrypted with a different key at the start of

Sure.  I was just too lazy to actually set the -1 everywhere.  I'll
try to do that before I repost again.

> the operation). This also means it is going to be rather tricky to
> implement for h/w accelerated XTS implementations, and it seems to me
> that the only way to deal with this is to decrypt the IV in software
> before chaining the next operation, which is rather horrid and needs
> to be implemented by all of them.

I don't think we should support chaining for XTS at all so I don't
see why we need to worry about the hardware accelerated XTS code.

> Given that
> 
> a) this is wholly an AF_ALG issue, as there are no in-kernel users
> currently suffering from this afaik,
> b) using AF_ALG to get access to software implementations is rather
> pointless in general, given that userspace can simply issue the same
> instructions directly
> c) fixing all XTS and CTS implementation on all arches and all
> accelerators is not a small task
> 
> wouldn't it be better to special case XTS and CBC-CTS in
> algif_skcipher instead, rather than polluting the skipcher API this
> way?

As I said we need to be able to differentiate between the ones
that can chain vs. the ones that can't.  Putting this knowledge
directly into algif_skcipher is just too horrid.

The alternative is to add this marker into the algorithms.  My
point was that if you're going to do that you might as well go
a step further and allow cts to chain as it is so straightforward.

Cheers,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt