# A Novel Mobile Communications Authentication Scheme with Roaming Service and User Anonymity
## 1 Introduction  
Wireless networks and smartphones have undergone rapid developments, allowing the use of the same device across different networks [1; 2]. Users such as businessmen or tourists visiting a new area can use a smart card to register with their home agents. Such cards use an anonymous connection to register to the home agent from the foreign agent server [3; 4]. After validation, a temporary certificate is sent to the user. The user may use this temporary certificate for network roaming via the foreign agent server. This approach can provide billing information while maintaining anonymity [5; 6; 7; 8]. In general, an anonymous roaming scheme has three entities: mobile user (MU), foreign agent (FA), and home agent (HA) [9; 10; 11; 12; 13]. In communication, the user must remain anonymous to the other entities. This scheme consists of three phases: registration (initialization phase), authentication (first phase), and roaming (second phase) [14; 15; 16]. When an MU is anonymously roaming in a foreign network, it can use one of two methods: real-time online and offline. Real-time online means that when a user requests roaming permissions, the FA can immediately authenticate with the HA and verify the user's legitimacy. The FA is unaware of the user's true identity but only knows whether the user is legitimate. Offline means that when the user requests roaming permissions, the FA can verify the user's legitimacy directly through the information obtained during the registration phase. In other words, this method does not use a real-time connection with the HA to verify the user, and the FA does not know the user's true identity [17; 18; 19; 20; 21; 22; 23]. In this study, the mobile phone roaming anonymous login is based on the real-time online method. In 2004, [PERSON] and [PERSON] [24] first proposed an authentication scheme with anonymity for wireless environments. Although they claimed that their scheme was secure, some weaknesses remained. The attacker can obtain \(\mathrm{r}\) (\(\mathrm{r}=\mathrm{H(N}\,|\,|\,\mathrm{ID_{HA})}\oplus\mathrm{H(N}\,|\,|\, \mathrm{ID_{MU})}\oplus\mathrm{ID_{HA}}\oplus\mathrm{ID_{MU}}\)) by registering and calculating the HA's private key or by intercepting messages \(n\) of other legitimate users and then using the HA's private key exclusive--or \(\mathrm{r}\) can obtain the legitimate user's identity \(\mathrm{ID_{MU}}\) and \(\mathrm{PW_{MU}}\). In other words, this scheme does not provide anonymity. In 2006, however, [PERSON] et al. [25] showed several security flaws in [PERSON] and [PERSON]'s scheme and then improved it. Unfortunately, the HA still provided \(\mathrm{PW_{MU}}\) to the MU in the registration phase, enabling \(\mathrm{PW_{MU}}\) to be calculated. In 2008, [PERSON] et al. [13] noted the security vulnerabilities of [PERSON] et al. 's scheme and proposed an enhanced version of their scheme. However, in 2009, [PERSON] and [PERSON] [26] showed that [PERSON] et al. 's improved scheme still did not provide anonymity, as they claimed. Recently, in 2012, [PERSON] [27] proposed a more secure and efficient authentication scheme with roaming service and user anonymity for mobile communications. This scheme's main characteristic is that the MU chooses \(\mathrm{PW_{MU}}\) in the registration phase and sends it to the HA to perform other operations. Because the MU sends \(\mathrm{PW_{MU}}\) to the HA, this solves the \(\mathrm{ID_{MU}}\) and \(\mathrm{PW_{MU}}\) problems encountered in traditional schemes. [PERSON]'s scheme is more efficient in terms of performance because it uses a lightweight elliptic curve Diffie-Hellman computation compared with traditional schemes that use RSA ([PERSON], [PERSON], and [PERSON]) [28] with certificates.