Re: BlueZ health device interface, problems with link security level?

Hello Elvis, James

2011/3/22 [off-list ref]

Hi,

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I've gotten the device to correctly connect and exchange data if I
modify the call to mcap_create_instance to use BT_IO_SEC_MEDIUM
instead, so the MITM protection isn't required.  I'm not sure if I'm
doing something else wrong and BT_IO_SEC_HIGH should work, or if there
is indeed a problem in the BlueZ code (and if so what the correct
solution is).

Does anyone else have the unmodified BlueZ code working correctly with
health devices?

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Another known workaround is to disable SSP mode with hciconfig
(hciconfig hciX sspmode 0).

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There has been discussion whether BlueZ HDP is correct or not in this
respect. The HDP specification says that devices SHOULD require authenticated
and encrypted connections (which maps to SEC_HIGH) while some devices are
known not to use authentication (SEC_MEDIUM). But the word in spec is 'SHOULD',
not 'SHALL'.

An "authenticated" connection has a slightly ambiguous meaning in
Bluetooth since 2.1+EDR, since you can have an authenticated link that
does not have any MITM protection.

I think the correct behavior is that HDP should be using "Level 2"
(from GAP in the Core specification), where HDP wants the strongest
level of security it can achieve with a device, but it does not want to
exclude devices that do not have the capability to support input/output.

There does seem to be a slight discrepancy between SEC_MEDIUM in BlueZ
and Security Level 2 in GAP.  I believe that the intention of Level 2
is to propose that MITM protection is needed - however it will happily
accept security where no MITM protection has been achieved (this being
the difference between Level 2 and Level 3).  BlueZ however does not
seem to propose MITM protection for SEC_MEDIUM - which would be important
for HDP (at least in the BlueZ <-> BlueZ case).

I remember that issue with we were developing HDP and MCAP. We set
SEC_HIGH in HDP to get encrypted channels and to pass Bluetooth PTS.
The problem doing that is related with devices that don't support MITM
protection (In fact if they don't have any
user input capabilities). This may be a good opportunity to go back
about this issue. What do you think?

In any case, such as Elvis has said, you can disable sspmode to get
HDP working without modify BlueZ code.

Regards
Santiago