## 1 Introduction  
The advancement of electromedical technology has led to new research topics associated with wireless body area networks (WBANs). A wireless body area network (WBAN) is formed by a medication information system and various wearable sensors attached to the patient's body. Integration of WBAN with modern cloud and sensor technologies offers huge improvement in the efficiency and functionality of medical and health care systems. For instance, after the ischemic stroke, patients would require a long-term electrocardiographic monitoring [1]. They suffer from the sleep apnea, and, consequently, require to wear a portable monitor while sleeping [2]. A WBAN-enabled environment allows patients to enjoy the same quality of life without being tangled by the sensor wires. To provide a comprehensive and real-time health assessment to the patient, sensed data may be transmitted to the clouds. A WBAN architecture is generally constituted of three layers, as shown in Figure 1. This architecture is composed of three types of nodes, first level nodes, second level nodes and a hub node. The first level node, e.g., a smartphone, acts as an intermediate node and forwards the data to the hub node. The second level nodes normally refer to the nodes or wearable devices situated in the body of human, sending the sensing information to a first level node. The hub node a local server or a remote cloud that analyses and manages the sensed data. Despite the WBANs being endowed with the simplicity and high efficiency, they suffer from low security so that the transmitted data contain the health information of the user which is typically highly sensitive. The need of finding a secure solution for the network is immediate as the security association in the 802.15.6 standard is in doubt [3]. To guarantee a secure WBAN, a secure authentication key agreement protocol should be executed in advance of the communication. We argue that this protocol still requires the user anonymity. Consider a user wearing a portable electrocardiographic monitor to keep track of his cardio health, where the cardio data are appropriately encrypted. The privacy of a known data transfer channel is compromised so that the electrocardiographic monitor has been related to a cardio problem through other users. According to the previously reported works, e.g., [3, 4], the authentication key agreement protocol of the WBAN shall provide the data secrecy, user anonymity, session unlinkability, mutual authentication, forward secrecy, resilient to online/offline dictionary attack, resilient to replay attack, and resilient to man-in-the-middle attack. Due to a few reasons, we should not use generic authentication key agreement protocols [5] or lightweight protocols for the general purpose short distance communications [6] in WBANs. Firstly, the specific architecture of the WBAN includes three tiers with multiple first level nodes whose most generic protocols are not optimized in this setting. Some first level nodes may be restricted in terms of power or computation ability so that a heavy computation is not possible. Furthermore, some generic authentication protocols may not offer the user anonymity as their protocol design requirement. However, in a WBAN, the identity of the patient should be concealed while being diagnosed with a WBAN. WBANs share some similar properties with Hierarchical Wireless Sensor Networks (HWSN). The valuable experience established in the HWSN research area has in turn led to the fast development of WBANs. [PERSON] et al. [7] has summarized some early advancement in the authentication protocol of HWSNs. However, conventional HWSNs assume a large-scale network and are more concerned about the battery power than the security and user's privacy. As of today, there has been no direct applicable of HWSN to WBAN. Recently, various authentication and key agreement protocols for WBANs have been proposed. In 2009, [PERSON] et al.