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From:   "Van Leeuwen, Pascal" <pvanleeuwen@rambus.com>
To:     =?iso-8859-2?Q?Horia_Geant=E3?= <horia.geanta@nxp.com>,
        "Andrei Botila (OSS)" <andrei.botila@oss.nxp.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        "David S. Miller" <davem@davemloft.net>
CC:     "linux-crypto@vger.kernel.org" <linux-crypto@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Eric Biggers <ebiggers@kernel.org>
Subject: Re: [RFC] crypto: xts - limit accepted key length
Thread-Topic: [RFC] crypto: xts - limit accepted key length
Thread-Index: AQHV8Gr7c7FFE6Bn/kS7vrAYuVJAN6g09vgAgAHW44CAAAkGMIADTCMAgAAOtN4=
Date:   Thu, 5 Mar 2020 16:48:06 +0000
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>> What is wrong with software fallback for the 192 bit keys in your driver=
?
> More code to maintain.
>
That applies to many corner cases not relevant to and therefore not support=
ed by "my" HW as well ...
From personal experience, it's not generally accepted as an excuse though.

> AES-XTS-192 should be:
> -either rejected (since there's a standard in place) or
>
There is a standard for storage encryption _using_ AES in XTS mode, i.e. IE=
EE-P1619, which indeed does not mention 192 bit keys.
But there is no _standard_ for _generic_ XTS mode that prohibits the use of=
 keysizes of the underlying blockcipher.
There really is no good reason to disallow the use of 192 bit keys with AES=
 for XTS. As the software implementation as well as other hardware implemen=
tations can do it just fine.
Also, making an exception specifically for one particular blockcipher (bein=
g AES) inside the XTS wrapper is pretty ugly.

> -at most made optional (allowing for implementations to *optionally* supp=
ort
> more key sizes), meaning crypto fuzz testing shouldn't fail.
>
Agree that it is a major burden on hardware device drivers to support every=
 possible corner of a generic software implementation though software fallb=
ack mechanisms. Some mechanism allowing hardware drivers some freedom not t=
o support certain corner cases that are not relevant to the scenarios where=
 the driver is _known_ to be actually used would be terribly nice.

Regards,
Pascal van Leeuwen
Silicon IP Architect Multi-Protocol Engines, Rambus Security
Rambus ROTW Holding BV
+31-73 6581953

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