Unable to make subsequent BLE connections

Hi,

I'm connecting to a BLE oximeter device. The first couple of connects succeed. But after that the connection always fails. I have tried with both gatttool and with my own code. hcidump shows that the "LE Start Encryption" command fails with "PIN or Key Missing". If I delete the pairing (that was created by the earlier successful connection) then subsequent connects will succeed for a few times again.

I analysed the hcidumps of successful and failed connects. In the failed case, it looks to me like bluez is not sending out an SMP PairingRequest in response to an SMP SecurityRequest from the slave. I can see in the kernel code that smp_cmd_security_req() does not send out the PairingRequest if it finds an LTK. But is that correct? Should it not try to pair anyway because the remote device is requesting it and may have deleted the earlier bonding.

I can provide the detailed hcidumps if anyone needs that.

Thanks.

Alan

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Hi Alan,

According to the SMP specification "The Security Request command is used by the slave to request that the master initiates security with the requested security properties". That doesn't talk about always sending a Pairing Request but initiating security, which makes sense since there'd otherwise be no other way for the slave to request the connection to be encrypted with a shared LTK.

So sounds like your device is for whatever reason (maybe it's broken) loosing its pairing information after some time. Even though our behavior with the Security Request should be correct we could consider the reaction to the "PIN or Key Missing" error. There's a table in section 2.4.4.2 of the SMP specification (page 2321 of core spec 4.2) that explains the allowed behavior in this scenario. However, if bonding was performed (which I suspect it was) the table only gives the option of notifying the user of the failure (which is what we do right now).

Sure, but preferably with btmon instead of hcidump. Please include the original pairing procedure in addition to the failure so that we can see whether bonding was performed or not (otoh since you say that you had at least a second successful connection it sounds like you must have done bonding).

Johan

Hi Johan,

That makes sense and I agree that bluez does appear to be conforming to this aspect of the spec.

Yep, it may very well be an issue with the device.

Here's the dump of a successful connection. Then a failed one. The one thing I did notice is that the device requests no bonding. The local host then negotiates bonding in the actual pairing request. That's for the successful case. For the failed case, the device also requests no bonding. Not sure if that has any bearing (I'll comb the spec again to see what it says about this situation). Please let me know if you can draw any conclusions from the data or need anything further.

Successful Connection
===============

< HCI Command: LE Create Connection (0x08|0x000d) plen 25 [hci0] 59.296413

        Scan interval: 60.000 msec (0x0060)
        Scan window: 30.000 msec (0x0030)
        Filter policy: White list is not used (0x00)
        Peer address type: Public (0x00)
        Peer address: 00:1C:05:FF:F0:30 (OUI 00-1C-05)
        Own address type: Public (0x00)
        Min connection interval: 50.00 msec (0x0028)
        Max connection interval: 70.00 msec (0x0038)
        Connection latency: 0x0000
        Supervision timeout: 420 msec (0x002a)
        Min connection length: 0.000 msec (0x0000)
        Max connection length: 0.000 msec (0x0000)

      LE Create Connection (0x08|0x000d) ncmd 2
        Status: Success (0x00)

      LE Connection Complete (0x01)
        Status: Success (0x00)
        Handle: 16
        Role: Master (0x00)
        Peer address type: Public (0x00)
        Peer address: 00:1C:05:FF:F0:30 (OUI 00-1C-05)
        Connection interval: 67.50 msec (0x0036)
        Connection latency: 0.00 msec (0x0000)
        Supervision timeout: 420 msec (0x002a)
        Master clock accuracy: 0x00

@ Device Connected: 00:1C:05:FF:F0:30 (1) flags 0x0000

      SMP: Security Request (0x0b) len 1
        Authentication requirement: No bonding - No MITM (0x00)
< ACL Data TX: Handle 16 flags 0x00 dlen 11 [hci0] 60.078851
      SMP: Pairing Request (0x01) len 6
        IO capability: NoInputNoOutput (0x03)
        OOB data: Authentication data not present (0x00)
        Authentication requirement: Bonding - No MITM (0x01)
        Max encryption key size: 16
        Initiator key distribution: <none> (0x00)
        Responder key distribution: EncKey (0x01)

Num handles: 1
Handle: 16
Count: 1

      SMP: Pairing Response (0x02) len 6
        IO capability: NoInputNoOutput (0x03)
        OOB data: Authentication data not present (0x00)
        Authentication requirement: Bonding - No MITM (0x01)
        Max encryption key size: 16
        Initiator key distribution: <none> (0x00)
        Responder key distribution: EncKey (0x01)
< ACL Data TX: Handle 16 flags 0x00 dlen 21 [hci0] 60.213663
      SMP: Pairing Confirm (0x03) len 16
        Confim value: ccd21e4e59474336a01e180f6a65c37e

Num handles: 1
Handle: 16
Count: 1

      SMP: Pairing Confirm (0x03) len 16
        Confim value: 14c7fe2cb506ced2298be45d03f3e800
< ACL Data TX: Handle 16 flags 0x00 dlen 21 [hci0] 60.348319
      SMP: Pairing Random (0x04) len 16
        Random value: 61f4f3ca92d349923f52f0fc5fba4d9b

Num handles: 1
Handle: 16
Count: 1

      SMP: Pairing Random (0x04) len 16
        Random value: 4af6c7906f2eddd99a2a9ee94e3597ec
< HCI Command: LE Start Encryption (0x08|0x0019) plen 28 [hci0] 60.483601
        Handle: 16
        Random number: 0x0000000000000000
        Encrypted diversifier: 0x0000
        Long term key: df94c4102b030cfbbc2e12f8ebca01ba

      LE Start Encryption (0x08|0x0019) ncmd 2
        Status: Success (0x00)

        Status: Success (0x00)
        Handle: 16
        Encryption: Enabled with AES-CCM (0x01)

      SMP: Encryption Information (0x06) len 16
        Long term key: 080b10bacd048f5bd98e72fb1ae5527b

      SMP: Master Identification (0x07) len 10
        EDIV: 0x5256
        Rand: 0xa38007f54be58be6

@ New Long Term Key: 00:1C:05:FF:F0:30 (1) Master

Failed Connection
===========

< HCI Command: LE Create Connection (0x08|0x000d) plen 25 [hci0] 209.031177

        Scan interval: 60.000 msec (0x0060)
        Scan window: 30.000 msec (0x0030)
        Filter policy: White list is not used (0x00)
        Peer address type: Public (0x00)
        Peer address: 00:1C:05:FF:F0:30 (OUI 00-1C-05)
        Own address type: Public (0x00)
        Min connection interval: 50.00 msec (0x0028)
        Max connection interval: 70.00 msec (0x0038)
        Connection latency: 0x0000
        Supervision timeout: 420 msec (0x002a)
        Min connection length: 0.000 msec (0x0000)
        Max connection length: 0.000 msec (0x0000)

      LE Create Connection (0x08|0x000d) ncmd 2
        Status: Success (0x00)

      LE Connection Complete (0x01)
        Status: Success (0x00)
        Handle: 16
        Role: Master (0x00)
        Peer address type: Public (0x00)
        Peer address: 00:1C:05:FF:F0:30 (OUI 00-1C-05)
        Connection interval: 67.50 msec (0x0036)
        Connection latency: 0.00 msec (0x0000)
        Supervision timeout: 420 msec (0x002a)
        Master clock accuracy: 0x00

@ Device Connected: 00:1C:05:FF:F0:30 (1) flags 0x0000

      SMP: Security Request (0x0b) len 1
        Authentication requirement: No bonding - No MITM (0x00)
< HCI Command: LE Start Encryption (0x08|0x0019) plen 28 [hci0] 210.007802
        Handle: 16
        Random number: 0xa38007f54be58be6
        Encrypted diversifier: 0x5256
        Long term key: 080b10bacd048f5bd98e72fb1ae5527b

      LE Start Encryption (0x08|0x0019) ncmd 2
        Status: Success (0x00)

        Status: PIN or Key Missing (0x06)
        Handle: 16
        Encryption: Disabled (0x00)
< HCI Command: Disconnect (0x01|0x0006) plen 3 [hci0] 210.142677
        Handle: 16
        Reason: Authentication Failure (0x05)

      Disconnect (0x01|0x0006) ncmd 2
        Status: Success (0x00)

        Status: Success (0x00)
        Handle: 16
        Reason: Connection Terminated By Local Host (0x16)

@ Device Disconnected: 00:1C:05:FF:F0:30 (1) reason 0

Alan

______________________________________________________________________
This communication contains information which may be confidential or privileged. The information is intended solely for the use of the individual or entity named above. If you are not the intended recipient, be aware that any disclosure, copying, distribution or use of the contents of this information is prohibited. If you have received this communication in error, please notify me by telephone immediately.
______________________________________________________________________