Commission Implementing Regulation (EU) 2016/799Show full title

12.2. VU — Motion Sensor Pairing Using Different Key Generations U.K.

As explained in section 9.2.1, the motion sensor master key and all associated keys are regularly replaced. This leads to the presence of up to three motion sensor-related AES keys K_M-WC (of consecutive key generations) in workshop cards. Similarly, in motion sensors up to three different AES-based encryptions of data (based on consecutive generations of the motion sensor master key K_M) may be present. A vehicle unit contains only one motion sensor-related key K_M-VU.

CSM_217 A second-generation VU and a second-generation motion sensor shall be paired as follows (compare Table 6 in [ISO 16844-3]): U.K.

1.

A second-generation workshop card is inserted into the VU and the VU is connected to the motion sensor.

2.

The VU reads all available K_M-WC keys from the workshop card, inspects their key version numbers and chooses the one matching the version number of the VU's K_M-VU key. If the matching K_M-WC key is not present on the workshop card, the VU aborts the pairing process and shows an appropriate error message to the workshop card holder.

3.

The VU calculates the motion sensor master key K_M from K_M-VU and K_M-WC, and the identification key K_ID from K_M, as specified in section 9.2.1.

4.

The VU sends the instruction to initiate the pairing process towards the motion sensor, as described in [ISO 16844-3], and encrypts the serial number it receives from the motion sensor with the identification key K_ID. The VU sends the encrypted serial number back to the motion sensor.

5.

The motion sensor matches the encrypted serial number consecutively with each of the encryptions of the serial number it holds internally. If it finds a match, the VU is authenticated. The motion sensor notes the generation of K_ID used by the VU and returns the matching encrypted version of its pairing key; i.e. the encryption that was created using the same generation of K_M.

6.

The VU decrypts the pairing key using K_M, generates a session key K_S, encrypts it with the pairing key and sends the result to the motion sensor. The motion sensor decrypts K_S.

7.

The VU assembles the pairing information as defined in [ISO 16844-3], encrypts the information with the pairing key, and sends the result to the motion sensor. The motion sensor decrypts the pairing information.

8.

The motion sensor encrypts the received pairing information with the received K_S and returns this to the VU. The VU verifies that the pairing information is the same information which the VU sent to the motion sensor in the previous step. If it is, this proves that the motion sensor used the same K_S as the VU and hence in step 5 sent its pairing key encrypted with the correct generation of K_M. Hence, the motion sensor is authenticated.

Note that steps 2 and 5 are different from the standard process in [ISO 16844-3]; the other steps are standard. U.K.

Example: Suppose a pairing takes place in the first year of the validity of the ERCA (3) certificate; see Figure 2 in section 9.2.1.2. Moreover

• Suppose the motion sensor was issued in the last year of the validity of the ERCA (1) certificate. It will therefore contain the following keys and data:

  • N_s[1]: its serial number encrypted with generation 1 of K_ID,

  • N_s[2]: its serial number encrypted with generation 2 of K_ID,

  • N_s[3]: its serial number encrypted with generation 3 of K_ID,

  • K_P[1]: its generation-1 pairing key(1), encrypted with generation 1 of K_M,

  • K_P[2]: its generation-2 pairing key, encrypted with generation 2 of K_M,

  • K_P[3]: its generation-3 pairing key, encrypted with generation 3 of K_M,

• Suppose that the workshop card was issued in the first year of the validity of the ERCA (3) certificate. It will therefore contain the generation 2 and generation 3 of the K_M-WC key.

• Suppose the VU is a generation-2 VU, containing the generation 2 of K_M-VU.

In this case, the following will happen in steps 2 — 5:

• Step 2: The VU reads generation 2 and generation 3 of K_M-WC from the workshop card and inspects their version numbers.

• Step 3: The VU combines the generation-2 K_M-WC with its K_M-VU to compute K_M and K_ID.

• Step 4: The VU encrypts the serial number it receives from the motion sensor with K_ID.

• Step 5: The motion sensor compares the received data with N_s[1] and doesn't find a match. Next, it compares the data with N_s[2] and finds a match. It concludes that the VU is a generation-2 VU, and therefore sends back K_P[2].

(1)

Note that the generation-1, generation-2 and generation-3 pairing keys may actually be the same key, or may be three different keys having different lengths, as explained in CSM_117.