# Lightweight NFC Protocol for Privacy Protection in Mobile IoT
## 2 Relative Works
### System Model  
The IoT network contains many authentication systems. The traditional radio frequency identification authentication system in IoT consists of three parts: the tag, reader, and server. Readers authenticate tags. The cloud server is an essential part of the authentication system. As a trusted third party, it verifies or decrypts the authentication messages [26]. The cloud server also stores authentication information, such as the device keys and device identities. There may be a large number of tags in a system, but the number of readers and servers is small. Such a single system is in a kind of pyramid structure. When it comes to mobile IoT, the structure changes. For the NFC mobile IoT network, the system contains a cloud server and multiple NFC devices, such as NFC tags, NFC phones, NFC watches, special NFC readers, and many other NFC smart devices. Here we have to point out that NFC mobile phones are very special. The NFC mobile phone has three working modes that it can work in: it can work in the card mode as a tag, in the reader mode, and it can also support peer-to-peer file sharing between phones. There may be authentication requirements between devices so the relationship becomes confusing. Such a system is a kind of reticular structure. In order to achieve the interconnection of the devices, the mobile IoT must consider how to achieve mutual authentication between the devices while providing security services. There are many studies focus on the authentication model for IoT clouds. They consider the authentication from the cloud point of view, such as the authentication model for IoT clouds that [PERSON] has proposed. [PERSON]'s work is a perfect summary of the previous IoT authentication protocols and it will also be of great help to future research [27].  
We notice that although the relationship is complex, devices in IoT can be divided into different groups that rely on their authentication systems. They have clustering characteristics, which means that they belong to a system and provide the same service. Let us take the smart home as an example. In such a typical mobile IoT network, all the NFC devices in the house only serve that particular family. Any device from outside is illegal in the network. Figure 2 shows the clustering characteristics of the mobile authentication system in the smart home.  
A system with clustering characteristics has three features. Firstly, devices outside the clustering system are considered illegal if they attempt to participate in authentication. Secondly, only when the authentication begins can we know the role of the devices. For example, when the mobile phone wants to authenticate the tag, the mobile phone acts as a reader. When mobile phone X authenticates mobile phone Y, phone X acts as a reader and phone Y acts as a tag. Thirdly, a legal device in the group cannot forge the identity of other devices. Figure 3 shows the detail of the system model.  
In the next section, we propose a group-based lightweight authentication protocol from the perspective of the relationship between tags and mobile devices in the mobile IoT. The clustering characteristics in the IoT system give us inspiration. Using the group-based access control, the protocol can be well protected against replay attacks, tag forgery, and other security issues. Meanwhile, devices in the group are anonymous to each other, which protects the privacy of the devices. Our lightweight protocol does not use strongly encrypted arithmetic units, but still provides a high level of security.  
Figure 3: The authentication in the mobile IoT network.  
Figure 2: The groups in the mobile Internet of Things (IoT) network. NFC: near field communication.