4.6 Obfuscated TX(Transmission) ID
Due to pseudo-anonymity requirements, the Device ID may not be exposed for unauthorized tracking in the radio protocol. An obfuscation process is implemented to avoid exposing the Device ID.
4.6.1 Device ID obfuscation
ID obfuscation is a method for increasing device anonymity/privacy by generating a temporary ID to help prevent brute force attacks.
The Network Server key generated during registration is used to create a Device ID obfuscation key. The Device ID obfuscation key is used to generate TXID using the equation below. The TXID rotation window is negotiated during registration. The default value for the TXID rotation window is 900 s.
- TXID=HMAC(DeviceIDobfuscationKey, sha256(Time_Reference, Device ID))[0:4]
where
- DeviceIDobfuscationKey: TXID_CTRREF for counter-based procedure and TXID for GCS time-based procedure
- Time_Reference: counter or GCS time. Time size: 4 B are allocated.
- [ 0 ..4] - TXID is five LSB from AES-HMAC algorithm result.
The mapping from the TXID to the customer ID is maintained by the Application Server.
Due to possible differences between Endpoint local time and network server time (time drift), three TXIDs are generated with Time_Reference, Time_Reference+900 s and Time_Reference - 900 s which are called the Current, Next and Previous TXIDs. Three TXIDs are generated to validate the obfuscated Device ID received in the message. Packets with an obsolete TXID are discarded by the receiver.
TXIDs are calculated in both the Network Server and the Endpoint.