# MAC-Based Compression Ratio Improvement for CAN Security
## 1 Introduction
In [19], if the value of the delta (difference) exceeds the length of the assigned delta field, the current CAN message is transmitted rather than the delta compressed version of the message. Therefore, two message IDs are used to distinguish between the compressed data and the uncompressed data. In [20], to use a single ID, the first bit of the data field is assigned as a data reduction code (DRC) indicating whether data reduction is used or not. By using data length code (DLC) in CAN frame format, the enhanced data reduction (EDR) algorithm [21] eliminates the difficulties in the identification of compressed messages, such as the use of the reserved bit [22]. In the boundary of the fifteen compression (BFC) algorithm, if the current value of a CAN signal has changed within the maximum compression rage of \(\pm 15\), then the CAN signal can be compressed [23]. The compression efficiency of PMDV-based methods depends on the accuracy of the PMDV chosen for the application. The best compression efficiency is achieved when the PMDV adapts to changes in the vehicle operating environment [24]. However, it is difficult to continuously maximize compression efficiency since PMDV is difficult to adjust once determined for an application. In CAS map-based algorithms, a compression area selection map eliminates the need to predict the maximum difference value as described in Section 2.3. In [24], the difference between the previous and the current data field is calculated and each difference value is represented using a modified sign-magnitude number which denotes the sign by the least significant bit. In [25], the bit-wise exclusive-OR values between successive data fields are computed instead of calculating the difference values. Thus, the use of a sign bit is not necessary. In CAS map-based algorithms, the 64-bit CAN data field is arranged as three signals having 24, 24, and 16 bits. In [26], a systematic way is presented to place the CAN data bits using multi-level arrangement maps to obtain the best compression efficiency. In CAS map-based algorithms, the size of the CAS map varies according to vehicle driving conditions. Since the change of the CAS map corresponds to the change in the PMDV, the compression algorithms based on the CAS map show better compression efficiency than the PMDV-based algorithms. Recently, CAN ID shuffling (hopping or obfuscation) techniques have been proposed to prevent DoS attacks and replay attacks. [PERSON] et al. proposed a DoS defense technique using CAN ID hopping [27]. This method proposes a CAN ID hopping technique using the offset generated by the trusted party when detecting a DoS attack. [PERSON] et al. proposed a CAN ID obfuscation scheme [28]. In this method, a CAN ID obfuscation technique is used, which uses a different CAN ID system for each car, even for cars of the same model. While these methods are effective in not increasing the amount of data to be transmitted, they have the drawback that they cannot prevent impersonation and replay attacks. In this paper, we propose an algorithm that significantly reduces the amount of transmission data of the Triple ID method by combining CAS map and MAC. By the proposed method, up to 15 bits can be further reduced from the compressed data obtained by the Triple ID method. Using the proposed method, we show that the occurrence of compressed messages exceeding 4 bytes is reduced by up to 99.57% compared to the Triple ID method. Section 2 introduces existing data authentication methods for CAN security. A new compression efficiency improvement method is proposed in Section 3. Simulation results using vehicles are presented in Section 4. Finally, a brief conclusion is provided in Section 5.