linux-kernel - Re: constant half-byte from /proc/sys/kernel/random/uuid

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Message-ID: <1275397387.24218.20972.camel@zakaz.uk.xensource.com>
Date:	Tue, 01 Jun 2010 14:03:07 +0100
From:	Ian Campbell <ijc@...lion.org.uk>
To:	Petr Ovtchenkov <ptr@...d-ptr.info>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: constant half-byte from /proc/sys/kernel/random/uuid

On Tue, 2010-06-01 at 16:22 +0400, Petr Ovtchenkov wrote:
> Lack of randomness during generating uuid.
[...]
> See on 7th high half-byte: it '4' in
> all cases---4b, 42, 40, 43, 49, 41,
> 4b, 4b, 4e. Checked three comps, with
> similar results. All Linux-generated UUID (boot_id)
> that I see in internet has '4' in this
> half-byte.

This appears to be deliberate. From drivers/char/random.c:

        void generate_random_uuid(unsigned char uuid_out[16])
        {
                get_random_bytes(uuid_out, 16);
                /* Set UUID version to 4 --- truly random generation */
                uuid_out[6] = (uuid_out[6] & 0x0F) | 0x40;
                /* Set the UUID variant to DCE */
                uuid_out[8] = (uuid_out[8] & 0x3F) | 0x80;
        }
        EXPORT_SYMBOL(generate_random_uuid);

Version 4 UUIDs and the format used are described at
http://en.wikipedia.org/wiki/Universally_Unique_Identifier

Ian.

-- 
Ian Campbell
Current Noise: Jesu - Your Path To Divinity

Build a system that even a fool can use and only a fool will want to use it.

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