# Body Area Networks in Healthcare: A Brief State of the Art
## 6 Cross-Layer Properties
### Common Security Solutions  
A secure communication is composed of two parts: trust establishment and communication. The first part means that the devices involved in the communication ensure their legitimacy. For this purpose, cryptography is widely used. In the case of symmetric keys, each entity has a copy of the key. This key is used to encrypt and decrypt a message and, therefore, to establish a secure communication. Another method is the use of asymmetric keys, where each device has a pair of keys: one key for encryption and the other for decryption. Asymmetric key cryptosystems consume more resources than systems that use only symmetric keys. In the case of BAN systems, they are composed of several layers. The importance of securing each layer is listed below:  
* The exchange of information between a BAN device and a smartphone must be secured because sensitive data are transmitted. Cryptography techniques are commonly used to secure this communication. * Smartphones transmit private information to the cloud in a server-client manner using wireless internet access. Wifi protected access (WPA) or WPA2 encryption are used to protect the exchange of information. * All places that store sensitive information have to be secured in order to ensure privacy. Secret sharing and code erasing have been proposed in the case of securing sensor storage. * The access of sensitive information has to be performed by authorized people. This ensures the patients' privacy. For controlling the access of sensitive data access-control policies, role-based access control and criticality-aware access rules are commonly used. There is a security protocol for BAN applications called physiological-signal-based key agreement (PSKA) that combines signal processing with cryptographic primitives. This protocol allows automatic key agreement between devices using physiological signals from the human body. This protocol consists of five main phases:  
1. Devices involved in the information exchange agree beforehand on the use of a physiological signal. 2. The physiological signal is processed and transformed into a binary string. 3. The sender device uses an arbitrary key to encrypt the message. 4. The sender hides the key using the binary string obtained from the physiological signal. Then, it sends the encrypted message and the hidden key in the same package. 5. The receiver uses the local binary string to obtain the key. After that, the device verifies the key decryption/checking the integrity of data received. After the process presented above, subsequent data exchange takes place without using a physiological signal. This protocol uses the observation of physiological signals which are unique at any time as a security method. Therefore, an external device that does not have contact with the patient cannot exchange information with other devices. PSKA protocol does not require any additional key to secure a communication between devices. This protocol also allows adding, removing or moving devices without a significant change in the network. PSKA protocol is an innovative way of ensuring a secure communication in BAN applications, but it has its challenges. The first challenge that has to be solved is to decide which physiological signal is used to secure the exchange of information. The human body produces a countless number of signals and PSKA decides which one is vital to use. The chosen signal has to be processed and this can lead to several issues explained below:  
* The physiological signal chosen has to meet the following properties: length and randomness, low latency, distinctiveness and temporal variance. * Physiological signals by nature are time-variant and the key-unhiding process needs synchronization between the receiver and the sender in order to be used. * Another aspect to be taken into account is the topographic specificity of the human body. Physiological signals apparently have similar trends but not exactly the same values when measured at different parts of the body.