Re: Is there a way to force legacy LE pairing for a device

[Marcel Holtmann <marcel@holtmann.org>]

Hi Jacek,

> I have a problem connecting to a BT device from a Linux desktop –
> pairing fails, while an Android phone pairs properly.
> 
> The obvious difference in the btsnoop logs is in the pairing request.
> 
> Android:
> < ACL Data TX: Handle 65 flags 0x00 dlen 11
> 
> 
> #995 150.491086
>      SMP: Pairing Request (0x01) len 6
>        IO capability: KeyboardDisplay (0x04)
>        OOB data: Authentication data not present (0x00)
>        Authentication requirement: Bonding, MITM, Legacy, No Keypresses
> (0x05)
>        Max encryption key size: 16
>        Initiator key distribution: EncKey IdKey Sign (0x07)
>        Responder key distribution: EncKey IdKey Sign (0x07)
> [...]
>> ACL Data RX: Handle 65 flags 0x02 dlen 11
> 
> 
> #1006 150.571137
>      SMP: Pairing Response (0x02) len 6
>        IO capability: NoInputNoOutput (0x03)
>        OOB data: Authentication data not present (0x00)
>        Authentication requirement: Bonding, No MITM, Legacy, No
> Keypresses (0x01)
>        Max encryption key size: 16
>        Initiator key distribution: EncKey (0x01)
>        Responder key distribution: EncKey (0x01)
> 
> 
> And on Linux:
> < ACL Data TX: Handle 16 flags 0x00 dlen 11               #53 [hci0]
> 132.273100
>      SMP: Pairing Request (0x01) len 6
>        IO capability: NoInputNoOutput (0x03)
>        OOB data: Authentication data not present (0x00)
>        Authentication requirement: Bonding, No MITM, SC, No Keypresses,
> CT2 (0x29)
>        Max encryption key size: 16
>        Initiator key distribution: EncKey Sign LinkKey (0x0d)
>        Responder key distribution: EncKey IdKey Sign LinkKey (0x0f)
> [...]
>> ACL Data RX: Handle 16 flags 0x02 dlen 6                #57 [hci0]
> 132.362160
>      SMP: Pairing Failed (0x05) len 1
>        Reason: Invalid parameters (0x0a)
> 
> 
> 
> So it looks like 'Bonding, MITM, Legacy, No Keypresses' used by Android
> works and 'Bonding, No MITM, SC, No Keypresses, CT2' used by the desktop
> Linux (bluez 5.62, kernel '5.6.2-050602-lowlatency' from Ubuntu) does
> not (note: I am still quite ignorant about Bluetooth stuff).
> 
> So my question is: is there any way to force using legacy pairing? Even
> if that requires bluez or kernel patching (though, I would rather avoid
> that).
> 

you can use btmgmt tool from bluez.git to force Secure Connections off. I am not sure if that sticks when starting bluetoothd, but then you need to hack it out there.

I am really not sure how your device can be a qualified Bluetooth device and fail here. The handling of the flags has actually proper test cases to ensure that this does’t happen. And I remember that even Android switched to Secure Connections support at some point.

Regards

Marcel