Received-SPF: pass (google.com: domain of linux-crypto-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 29 May 2020 18:05:08 +1000
From:   Herbert Xu <herbert@gondor.apana.org.au>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Eric Biggers <ebiggers@kernel.org>,
        Stephan Mueller <smueller@chronox.de>
Subject: Re: [RFC/RFT PATCH 0/2] crypto: add CTS output IVs for arm64 and
 testmgr
Message-ID: <20200529080508.GA2880@gondor.apana.org.au>
References: <20200519190211.76855-1-ardb@kernel.org>
 <20200528073349.GA32566@gondor.apana.org.au>
 <CAMj1kXGkvLP1YnDimdLOM6xMXSQKXPKCEBGRCGBRsWKAQR5Stg@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAMj1kXGkvLP1YnDimdLOM6xMXSQKXPKCEBGRCGBRsWKAQR5Stg@mail.gmail.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-crypto-owner@vger.kernel.org
Precedence: bulk

On Thu, May 28, 2020 at 10:33:25AM +0200, Ard Biesheuvel wrote:
>
> The reason we return output IVs for CBC is because our generic
> implementation of CTS can wrap any CBC implementation, and relies on
> this output IV rather than grabbing it from the ciphertext directly
> (which may be tricky and is best left up to the CBC code)

No that's not the main reason.  The main user of chaining is
algif_skcipher.

> So if you are saying that we should never split up algif_skcipher
> requests into multiple calls into the underlying skcipher, then I
> agree with you. Even if the generic CTS seems to work more or less as
> expected by, e.g., the NIST validation tool, this is unspecified
> behavior, and definitely broken in various other places.

I was merely suggesting that requests to CTS/XTS shouldn't be
split up.  Doing it for others would be a serious regression.

However, having looked at this it would seem that the effort
in marking CTS/XTS is not that different to just adding support
to hold the last two blocks of data so that CTS/XTS can support
chaining.

I'll work on this.

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