Re: (Small) bias in generation of random passkeys for pairing

From: Stefan Seyfried
Date: Thu Jun 20 2019 - 08:05:59 EST

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