Using and Calibrating Dynamic Lock in Windows 10 Creators Update

Posted on February 20, 2017 by Rafael Rivera in Windows 10 with 18 Comments

Last week, Paul wrote about Dynamic Lock, a Windows 10 Creators Update feature that locks your Windows PC when it’s separated from a Bluetooth-paired phone. But some readers had questions about how Dynamic Lock really works. So I naturally took it apart to answer those questions.

Let’s jump right in.

Hello and Goodbye

As you may remember, Windows Hello is a password-free sign-in experience that unlocks your Windows PC using your face, fingerprint or companion device. It can also be used to authenticate with Windows Hello-enabled apps and websites, and is available in Windows 10 today.

Conversely, in Creators Update, a new Dynamic Lock sign-out experience is coming. This feature figures out if you’re there or not and automatically locks your Windows PC in your absence.

Fun fact: This feature is internally known as Windows Goodbye and in some OEM circles as Proximity Lock. This is the name Microsoft assigned the feature at its WinHEC conference last year, but as far as we know, that name has been retired.

Dynamic Lock

As mentioned earlier, Dynamic Lock automatically locks your Windows PC when you’re not present based on the proximity of a Bluetooth-paired phone. (It may work with other devices, but the intent is phone-only right now. Consider yourself lucky if you get this feature working with a non-phone.)

If you have multiple phones paired with your PC, all of them are considered. That is, either phone will lock your PC if they stray too far. (I suspect this will be configurable via policy, if not end-user UI, in later builds.)

But How Far Is Too Far?

Dynamic Lock determines you’re missing in action by performing the following ritual several times a minute:

  1. Connect to the Bluetooth-paired phone
  2. Retrieve the average Received Signal Strength Indication (RSSI) for that connection
  3. Disconnect from the Bluetooth-paired phone
  4. Determine if the RSSI value dips below a certain threshold

Because an active connection is established every time this ritual is performed, you can bet there will be a battery life hit on both devices. It’s a win-win!

What’s The Threshold?

The threshold, by default, appears to be around -10 decibels (dB). That is, 10dB below the Golden Received Power Range — the sweet spot specific to your particular Bluetooth controller — represented at the 0dB point.

But in my apartment, a 10dB difference means walking halfway across the floorplan before my PC locks. I halved my value to shorten that distance.

To configure this delta yourself, first create the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\NaAuth key. Then create the BluetoothRssiMaxDelta DWORD registry value and set it appropriately (see below). Be sure to lock and unlock your PC to reset Dynamic Lock and commit your changes.

Figuring Out The Right Threshold Value

Setting the correct threshold value is the trickiest part of using this feature.

I couldn’t find any Windows software that retrieved RSSI values correctly, so I threw together a tool for you. It iterates the paired Bluetooth device list and retrieves the most recent signal strength value in a loop. It has a weird sounding name (based on dinosaur genera) and doesn’t handle unreachable devices correctly (thanks to iffy Windows APIs) but it gets the job done. You can find its source code on Gitlab. Pull requests are welcome.

To get the threshold value using the tool, do the following:

  1. Place your phone on your desk/near the PC
  2. Turn on Bluetooth and pair with the PC
  3. Start my tool
  4. The initial signal strength readout for the phone should be somewhere around -10dB or above. (Your mileage may vary.) Scribble this value down.
  5. Now move the phone to a distant location and wait for the app to update
  6. Let the app loop a few times then scribble down the new signal strength value.
  7. Subtract the two numbers, drop the sign, and stuff that value into the registry (see above). Be sure to lock and unlock your PC for the changes to take effect.

It’s hard to guarantee that any of this will work for you, though. There are a bunch of environmental and device specific variables that need to be accounted for. And between you and I, it’s this pickiness that has me wondering if this feature will get cut prior to the general availability of Creators Update.

I’ll keep an eye on Dynamic Lock as we get closer to the completion of Creators Update. If anything changes, I’ll let you know.

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