linux-kernel - Re: [PATCH 08/21] KEYS: Add signature verification facility [ver #3]

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Date:	Wed, 18 Jan 2012 12:26:23 +0000
From:	David Howells <dhowells@...hat.com>
To:	"Kasatkin, Dmitry" <dmitry.kasatkin@...el.com>
Cc:	dhowells@...hat.com, keyrings@...ux-nfs.org,
	linux-crypto@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org, zohar@...ux.vnet.ibm.com,
	arjan.van.de.ven@...el.com, alan.cox@...el.com
Subject: Re: [PATCH 08/21] KEYS: Add signature verification facility [ver #3]

Kasatkin, Dmitry <dmitry.kasatkin@...el.com> wrote:

> It would also nice to have an API to supply pre-computed data hash.  For
> example IMA uses the same functionality to compute the hash of the file
> content, and then, based on security.ima type decided either verify it using
> just hash, or use digital signature.  We could pass a hash as data. But may
> be we do not want to have extra operation and compute hash over hash.

If I understand you correctly, you'd like to have the option to do the hashing
externally to this API?  Would you supply the completed hash or just a hash
with the data in it, and require this API to complete it (ie. chuck metadata
into it)?

I don't think it should be hard.  I could add an alternative to
verify_sig_add_data() perhaps.  Either that or one function that does the lot
and takes the precomputed hash as input.  There would be no need for the split
into four functions (begin, add_data, end, cancel) in such a case.  The reason
for the split is so that the caller can invoke add_data several times with
non-contiguous bits of data.

It might even make sense to expose the crypto hash object for direct access
rather than use add_data - but that then makes it hard to use crypto hardware
where you would just shovel the raw data into it and it does all the hashing
and cryptography in a black box.

David
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