4.2 Security Settings for Verifier and Prover Mode

The secure ranging requires the following parameters, which must be synchronized for consecutive ranging sessions:

  • Table with permutation indexes
  • Table with Trivium Cipher Initialization Vector (IV) and Trivium Cipher Key (TCK)
  • Table index for the Trivium Cipher Key (TCK) to update the key

The verifier and the prover device have the same internal tables for these parameters. The table for the permutation indexes has a total size of 48 bytes. The initialization vector for the Trivium cipher and the initial key uses a table with a total size of 2 x 10 bytes = 20 bytes. The actual TCK data is stored in a table with 16 x 10 bytes = 160 bytes.

The following figure illustrates the Trivium cipher table including the bytes for the initialization vector (IV) and the initial key (TCK). The key value is typically updated while the IV remains the same for each coding step.
Figure 4-1. Security Data Tables

The TCK is used to select the new Trivium cipher key with a size of 10 bytes. The actual TCK is selected out of the TCK table with the index.

idx = (SC + RC) AND 0x0F

The selection and synchronization of the Trivium cipher key is using the encrypted counter values SC and RC from the data communication packet. This links the data communication packet to the ranging procedure and allows the synchronization between the verifier and the prover.

The sequence counter is incremented after each successful data communication transfer. If the communication fails, which is detected when analyzing the received RNRv and RNRp data at the verifier and prover, the RC is incremented in the communication data packet while the SC remains unchanged.