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Subject: Re: another testmgr question
To:     Pascal Van Leeuwen <pvanleeuwen@insidesecure.com>,
        Eric Biggers <ebiggers@kernel.org>
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From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <907eb6a5-dc76-d5ee-eccf-e7bd426a0868@c-s.fr>
Date:   Fri, 24 May 2019 10:46:37 +0200
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Le 24/05/2019 à 10:44, Pascal Van Leeuwen a écrit :
>>> Valid? A totally fabricated case, if you ask me. Yes, you could do that,
>>> but is it *useful* at all? Really?
>>> No, it's not because a file of length 0 is a file of length 0, the length
>>> in itself is sufficient guarantee of its contents. The hash does not add
>>> *anything* in this case. It's a constant anyway, the same value for *any*
>>> zero-length file. It doesn't tell you anything you didn't already know.
>>> IMHO the tool should just return a message stating "hashing an empty file
>>> does not make any sense at all ...".
>>>
>>
>> Of course it's useful.  It means that *every* possible file has a SHA-256
>> digest.  So if you're validating a file, you just check the SHA-256 digest;
>> or
>> if you're creating a manifest, you just hash the file and list the SHA-256
>> digest.  Making everyone handle empty files specially would be insane.
>>
> As I already mentioned in another thread somewhere, this morning in the
> shower I realised that this may be useful if you have no expectation of
> the length itself. But it's still a pretty specific use case which was
> never considered for our hardware. And our HW doesn't seem to be alone in
> this.
> Does shaXXXsum or md5sum use the kernel crypto API though?

The ones from libkcapi do (http://www.chronox.de/libkcapi.html)

Christophe