linux-kernel - Re: [PATCH v3 5/7] crypto: AF_ALG: add random number generator support

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Message-ID: <20141124143150.GC31469@gondor.apana.org.au>
Date:	Mon, 24 Nov 2014 22:31:50 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Stephan Mueller <smueller@...onox.de>
Cc:	Daniel Borkmann <dborkman@...hat.com>,
	'Quentin Gouchet' <quentin.gouchet@...il.com>,
	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-crypto@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH v3 5/7] crypto: AF_ALG: add random number generator
 support

On Fri, Nov 21, 2014 at 06:32:52AM +0100, Stephan Mueller wrote:
> This patch adds the random number generator support for AF_ALG.
> 
> A random number generator's purpose is to generate data without
> requiring the caller to provide any data. Therefore, the AF_ALG
> interface handler for RNGs only implements a callback handler for
> recvmsg.
> 
> The following parameters provided with a recvmsg are processed by the
> RNG callback handler:
> 
>         * sock - to resolve the RNG context data structure accessing the
>           RNG instance private to the socket
> 
>         * len - this parameter allows userspace callers to specify how
>           many random bytes the RNG shall produce and return. As the
>           kernel context for the RNG allocates a buffer of 128 bytes to
>           store random numbers before copying them to userspace, the len
>           parameter is checked that it is not larger than 128. If a
>           caller wants more random numbers, a new request for recvmsg
>           shall be made.
> 
> The size of 128 bytes is chose because of the following considerations:
> 
>         * to increase the memory footprint of the kernel too much (note,
>           that would be 128 bytes per open socket)
> 
>         * 128 is divisible by any typical cryptographic block size an
>           RNG may have
> 
>         * A request for random numbers typically only shall supply small
>           amount of data like for keys or IVs that should only require
>           one invocation of the recvmsg function.
> 
> Note, during instantiation of the RNG, the code checks whether the RNG
> implementation requires seeding. If so, the RNG is seeded with output
> from get_random_bytes.
> 
> A fully working example using all aspects of the RNG interface is
> provided at http://www.chronox.de/libkcapi.html
> 
> Signed-off-by: Stephan Mueller <smueller@...onox.de>

Sorry but who is going to use this and for what purpose?

Every other algif interface exports real hardware features that
cannot otherwise be accessed from user-space.  All crypto RNGs
are by definition software-only, so what is the point of this?

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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