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Subject: Re: [RFC PATCH v2] md/dm-crypt - reuse eboiv skcipher for IV
 generation
To:     Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        linux-crypto@vger.kernel.org
Cc:     herbert@gondor.apana.org.au, ebiggers@kernel.org, agk@redhat.com,
        snitzer@redhat.com, dm-devel@redhat.com
References: <20190807055022.15551-1-ard.biesheuvel@linaro.org>
From:   Milan Broz <gmazyland@gmail.com>
Openpgp: preference=signencrypt
Message-ID: <c83ec561-4359-3f38-3da3-65f9f18c1822@gmail.com>
Date:   Wed, 7 Aug 2019 10:08:58 +0200
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On 07/08/2019 07:50, Ard Biesheuvel wrote:
> Instead of instantiating a separate cipher to perform the encryption
> needed to produce the IV, reuse the skcipher used for the block data
> and invoke it one additional time for each block to encrypt a zero
> vector and use the output as the IV.
> 
> For CBC mode, this is equivalent to using the bare block cipher, but
> without the risk of ending up with a non-time invariant implementation
> of AES when the skcipher itself is time variant (e.g., arm64 without
> Crypto Extensions has a NEON based time invariant implementation of
> cbc(aes) but no time invariant implementation of the core cipher other
> than aes-ti, which is not enabled by default)
> 
> This approach is a compromise between dm-crypt API flexibility and
> reducing dependence on parts of the crypto API that should not usually
> be exposed to other subsystems, such as the bare cipher API.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

Yes, this is a good idea, I'll test it. Thanks!

Milan