## 3 RFID Security and Threats
### RFID Challenges  
As mentioned, each RFID system is composed of three main components; therefore, the challenges for each part of the system will be highlighted. In general, RFID suffers from inflexibility issues, as usually, the readers ought to be fixed in place. Moreover, and more precisely, commercial devices are expensive to re-design or reproduce since they come as a black box with limited information [7]; at this point, manufacturers restrict users' ability to make profound changes. Furthermore, message collisions can be spotlighted as another challenge that can reduce the system's efficiency, since the same communication channel is used by the diverse tags utilized. Frequency interferences, the read speed ratio, and energy wastage are some of the problems that can also be encountered. In addition, the read range and energy-gathering limitation comprise another set of challenges because the communication range relies on the device's available power [8]. The previously mentioned challenges could lead to security issues that reflect the normal system workflow and may lead to information loss, which is pivotal in some cases, for instance, healthcare or industry. To overcome this issue, several suggestions have been presented to secure RFID communications and protect the confidentiality, integrity, and privacy of users. The limitation of RFID is highly recommended to be taken into consideration while applying any sort of implementation to find a suitable solution. Consequently, cryptography-based schemes are deployed to reinforce RFID communications' security. The primary concern of cryptography is to safeguard data from being eavesdropped on or sniffed, but it needs some computational and storage abilities to be undertaken. Figure 2: Studies’ extraction and inclusion process. Resource limitations comprise the main challenge confronted by any RFID application, where heavy computational cryptosystems cannot be supported [8]. For example, passive tags' lifespan is a necessity. The latter only supports simple operations such as rotate and XOR. At this stage, conventional crypto primitives utilization might lead to an increase in heat, fast energy wastage (tag lifespan decreasing), and tag damage. As a result, ultra-lightweight schemes can be implemented to fit this type of tag. For semi-passive or semi-active tags, we can have some moderate resources that can support some advanced operations such as checksums, random number generation, hash functions, and mutual authentication. However, active tags consist of small batteries and have a good storage ability, so they are efficient enough to perform heavy computational operations without restrictions. In this regard, classical crypto-primitives can be used to offer strong security to the system, and we can mention Public Key Cryptography (PKC) schemes such as Elliptic Curve Cryptography (ECC) and Rivest, Shamir, and Adelman (RSA). In addition to symmetric key encryption, we can highlight the Advanced Encryption Standard (AES) and other computationally heavy algorithms. Besides this, communication protocol issues should be accommodated; therefore, anti-collision protocols are used to avoid tag message collisions. For instance, multi-access methods are implemented to identify exactly the tags where the signals came from and to decrease the collision number, which reflects positively on the throughput and number of transmitted bits. These methods are classified into four main categories as follows [9]:  
* **Code division multi-access:** This is built by multiplying the tag ID by a pseudo-random sequence before the data transmission. This method offers security to the communication between the reader and the tag; however, it has some high demands such as computation, along with enhancing the complexity. * **Frequency division multi-access:** This refers to the utilization of frequency ranges for the sake of recognizing tags. At such a level, each tag must belong to a specific frequency. FDMA seems expensive to implement and it is not designed for general employment.