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Subject: Re: (Small) bias in generation of random passkeys for pairing
To:     Pavel Machek <pavel@ucw.cz>,
        kernel list <linux-kernel@vger.kernel.org>,
        security@kernel.org, linux-bluetooth@vger.kernel.org,
        johan.hedberg@gmail.com, marcel@holtmann.org
References: <20190619162456.GA9096@amd>
From:   Stefan Seyfried <stefan.seyfried@googlemail.com>
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Message-ID: <66098773-4d88-ebdb-02d5-8af0cc4ea99c@message-id.googlemail.com>
Date:   Thu, 20 Jun 2019 14:00:38 +0200
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Hi Pavel,

Am 19.06.19 um 18:24 schrieb Pavel Machek:
> Hi!
> 
> There's a (small) bias in passkey generation in bluetooth:
> 
>                 get_random_bytes(&passkey, sizeof(passkey));
>  		passkey %= 1000000;
> 		put_unaligned_le32(passkey, smp->tk);
> 
> (there are at least two places doing this).
> 
> All passkeys are not of same probability, passkey "000000" is more
> probable than "999999", but difference is small.

It is slightly different IMHO.

Unsigned 32bits passkey assumed (and all users I found were u32),
the passkeys "000000" to "967295" are slightly more probable than
"967296" to "999999".

If my math is right (which I doubt), the difference in probability
for both entities is 4294:4293.

> Do we care?

I, personally, don't (yet).
But then, I'm not a real security expert.

Have fun,
-- 
Stefan Seyfried

"For a successful technology, reality must take precedence over
 public relations, for nature cannot be fooled." -- Richard Feynman