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Subject: Re: Bluetooth ECDH selftest failed (endianness issue?)
From:   Marcel Holtmann <marcel@holtmann.org>
In-Reply-To: <CAO1O6sfFqcQvn2Sy_HOLAzuc9o-Wu0g_9oLH-OFOkCxfuA=Jtg@mail.gmail.com>
Date:   Mon, 21 Jan 2019 15:53:36 +0100
Cc:     Andrey Batyiev <batyiev@gmail.com>,
        Johan Hedberg <johan.hedberg@gmail.com>,
        Bluez mailing list <linux-bluetooth@vger.kernel.org>
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To:     Emil Lenngren <emil.lenngren@gmail.com>
Sender: linux-bluetooth-owner@vger.kernel.org
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Hi Emil,

>>>> On Sat, Dec 29, 2018 at 9:35 AM Marcel Holtmann <marcel@holtmann.org> wrote:
>>>> I think that our ECDH code was endian safe, but then it got changed at some point to use standard crypto and maybe something went wrong there. Can just provide the btmon -w trace.log for the SMP pairing so that I can have a look at the binary trace.
>>> 
>>> I found out that if I change "swap_digits" method in
>>> "net/bluetooth/ecdh_helper.c" to
>>> 
>>> static inline void swap_digits(u64 *in, u64 *out, unsigned int ndigits)
>>> {
>>> int i;
>>> 
>>> for (i = 0; i < ndigits; i++)
>>>       out[i] = in[ndigits - 1 - i];
>>> }
>>> 
>>> then BLE pairing on big-endian become operational. I'm not sure what
>>> proper fix should be: is it a problem with crypto API usage or a
>>> problem with crypto itself?
>> 
>> if the kernel ECC and ECDH crypto already swaps for us, then we don’t need to do it again. So all the swap_digits most likely can be removed from net/bluetooth/.
>> 
>> Regards
>> 
>> Marcel
>> 
> 
> The Bluetooth standard is a bit strange. It assumes the AES standard
> is big endian (although it is really just defined on a byte level),
> but since Bluetooth is little-endian everywhere, all AES 128-bit
> values must be reversed when a standard AES library is used. In
> particular, SMP reverses the AES values. So the swap_digits should be
> kept.

so you are saying just reversing is needed, but not swapping? But then this is no longer swap_digits, it is just a reverse. What do you want us to do in this case now?

Regards

Marcel