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Date:   Thu, 20 Jun 2019 09:13:25 +0800
From:   Herbert Xu <herbert@gondor.apana.org.au>
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        linux-crypto@vger.kernel.org, dm-devel@redhat.com,
        linux-fscrypt@vger.kernel.org,
        Gilad Ben-Yossef <gilad@benyossef.com>,
        Milan Broz <gmazyland@gmail.com>
Subject: Re: [PATCH v3 1/6] crypto: essiv - create wrapper template for ESSIV
 generation
Message-ID: <20190620011325.phmxmeqnv2o3wqtr@gondor.apana.org.au>
References: <20190619162921.12509-1-ard.biesheuvel@linaro.org>
 <20190619162921.12509-2-ard.biesheuvel@linaro.org>
 <20190620010417.GA722@sol.localdomain>
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On Wed, Jun 19, 2019 at 06:04:17PM -0700, Eric Biggers wrote:
>
> > +#define ESSIV_IV_SIZE		sizeof(u64)	// IV size of the outer algo
> > +#define MAX_INNER_IV_SIZE	16		// max IV size of inner algo
> 
> Why does the outer algorithm declare a smaller IV size?  Shouldn't it just be
> the same as the inner algorithm's?

In general we allow outer algorithms to have distinct IV sizes
compared to the inner algorithm.  For example, rfc4106 has a
different IV size compared to gcm.

In this case, the outer IV size is the block number so that's
presumably why 64 bits is sufficient.  Do you forsee a case where
we need 128-bit block numbers?

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