# A Secure and Lightweight Group Mobility Authentication Scheme for 6 LoWPAN Networks
## 4 System Models
### The Proposed Scheme (SL_GAS)
#### 4.3.1 Registration Phase  
Every node in the network must register with the AAA to establish trust communication. Figure 3 shows registration procedures.  
* The 6L\({}_{\text{MN}}\) sends a unique identification, _IDi_, and a randomly chosen random number, \(R1\), to the AAA. As a result, a secure handshake is initiated, and each session begins with fresh inputs, thus minimizing the risk of replay attacks.
* Then, AAA picks up a random number, \(R_{2}\), and computes \(Kr=H\left(ID_{A}\right)\oplus R_{2}\), where H represents a secure hash function, and is the AAA server's unique identifier. The XOR operation ensures that the resulting \(Kr\) is tied to the server's identity and the current session. To strengthen the protocol's resilience, the session key, \(Kr\), is divided into four equal chunks, each being 64 bits in size: \(Kr1\), \(Kr2\), \(Kr3\), and \(Kr4\). The secret parameter, \(SP\), is then calculated as \(SP=Kr1\oplus Kr2\oplus Kr3\oplus Kr4\). This mechanism ensures that any modification to the key chunks will result in a different \(SP\), which assists in detecting any unauthorized modifications. It acts as a dynamic and robust shared secret, ensuring message integrity and synchronizing cryptographic operations between the AAA server and the 6L\({}_{\text{MN}}\). After this, the AAA server calculates a one-time alias identity, \(AID_{i}=H\left(ID_{i}\mid 1R_{2}\mid\mid SP\right)\), so that an attacker cannot link this alias identity with the actual \(ID_{i}\) to correlate communications or track the 6L\({}_{\text{MN}}\). \(AID_{i}=\left\{AID1,AID2,\ldots,AID_{n}\right\}\), where each \(AID_{n}\in AID_{i}\) is represented as a set of dynamically generated values for distinct sessions, ensuring privacy. The AAA server picks up the temporary ID (\(TID_{A}\)) and sends the message to 6L\({}_{\text{MN}}\): \(\left\{AID_{i}\mid TID_{A}\mid R_{2}\mid SP\right\}\). SP acts as a shared secret between the AAA server and 6 LoWPAN nodes, authenticating messages and ensuring synchronized cryptographic operations between authorized entities.
* Once both parties have stored the secret parameters in the database, a pairwise key is computed using \(K_{i}\leftrightarrow_{A}=H\left(AID_{i}\mid TID_{A}\mid R_{2}\mid SP\right)\). The 6L\({}_{\text{MN}i}\) publicly shares the \(AID_{i}\) instead of utilizing the actual identity, hence thwarting the attacker's ability to collect device information and keep track of the node.  
\begin{table}
\begin{tabular}{c l} \hline \hline
**Notations** & **Description** \\ \hline \(SP\) & Secret parameter \\ \hline \(Tc\) & Ticket \\ \hline \(Texp\) & Time for the expired ticket \\ \hline \(R_{x}\) & Random number of 64 bits \\ \hline \(N_{x}\) & Nonce \\ \hline \(K_{x+y}\) & Established session key between \(x\) and \(y\) \\ \hline \(CT_{x}\) & Ciphertext for \(x\) \\ \hline \(H\) & One-way hash function (\(HF\)) \\ \hline \(E_{k}\left(M\right)\) & Encrypted message (\(M\)) with the key (\(K\)) \\ \hline \(D_{k}\left(CT\right)\) & Decrypted ciphertext (\(CT\)) with the key (\(K\)) \\ \hline \hline \end{tabular}
\end{table}
Table 2: _Cont._