Re-enable NFC Smart Lock on Android

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### How to re-enable NFC as a Smart Lock method on Android ###

Disclaimer: these steps describe a method that works for me as of Jan 14 2018. I cannot guarantee it will work for you, and it may result in loss of data. I will not be held responsible for any damages or losses that may result of following any part of these instructions.

Always make sure to keep backups of your important data!

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As root, edit /data/data/com.google.android.gms/shared_prefs/coffee_preferences.xml

Add the following lines between the <map> and </map> tags:

    <set name="nfc_trustlet_tech_codes">
        <string>android.nfc.tech.NfcV</string>
        <string>android.nfc.tech.NfcA</string>
    </set>
    <string name="auth_trust_agent_pref_trusted_nfc_Vb2tUD1p62CToOS9IXyywQS/T15GVEATDPtuKMoRPTQ=">{&quot;pr&quot;:0,&quot;id&quot;:&quot;Vb2tUD1p62CToOS9IXyywQS\/T15GVEATDPtuKMoRPTQ=&quot;,&quot;te&quot;:[&quot;android.nfc.tech.MifareUltralight&quot;,&quot;android.nfc.tech.NfcA&quot;],&quot;na&quot;:&quot;Test NFC Device&quot;}</string>

Save the file.

Now go to Security -> Smart Lock, there should be a device named "Test NFC Device" under Trusted Devices.
You may have to force-stop Google Play Services to get the changes to apply. If not, reboot your device.

When you see the "Test NFC Device" under Trusted Devices, tap it, and add your own NFC device. If you see an empty white screen, reboot your device.

Once you have authorized your own NFC device, you must remove the test device. Do not keep the test device authorized, as it may pose a security risk.

If NFC Smart Unlock appears to be enabled but doesn't work after removing the test device, edit again coffee_preferences.xml and check if the tech.NfcV and tech.NfcA lines have been removed, if so, re-add them. Do not re-add the Test NFC device line.

Keep at least one NFC device authorized, or you will be unable to access NFC unlock again.

Good luck!