# A Secure and Lightweight Group Mobility Authentication Scheme for 6 LoWPAN Networks
## 4 System Models
### The Proposed Scheme (SL_GAS)
#### 4.3.3 Group Handover Phase  
Several related group handover schemes (GHSs) need authentication procedures to be performed each time a 6L-MN is attached to a new 6L-MAG, increasing handover latency and workload. Through the use of ticket and secret parameter mechanisms, the proposed SL_GAS scheme facilitates secure group handover authentication while reducing handover delays. The group handover phase is shown in Figure 5.  
Figure 4: Initial authentication phase. * In the event that the prior 6L\({}_{\rm MAG}\)\({}^{\rm old}\) detects that a 6L\({}_{\rm MNGL}\) has left it as a domain, it will send to LMA a de-registration proxy binding update (De-PBU). This group of data packets is buffered. * Prior to entering the domain of 6L\({}_{\rm MAG}\)\({}^{\rm new}\), the 6L\({}_{\rm MNGL}\) needs to collect \(T_{C}\)'s \(T_{exp}\) stored in its memory. If \(T_{\rm C}\) still has not expired, 6L\({}_{\rm MNGL}\) will pick \(T_{GL}\) as the timestamp. Then, for each node, the handover information is collected as \(Tagi=MAC_{Ki+A}\) (\(ID_{6G}\), \(AID_{i}\), \(T_{i}\), and \(R_{2\ i}\)\({}^{\prime}\)), where \(j\) denotes the \(j\)th handover of this group. When the 6L\({}_{\rm MNGL}\) attaches to a 6L\({}_{\rm MAG}\)\({}^{\rm new}\), an RS message {\(T_{C}\) \(\mid AID_{GL}\) \(\mid T_{GL}\) \(\mid TID_{6G}\) \(\mid CT_{A}=E_{KGL}\)\(\leftrightarrow\)\(A\) (\(T_{C}\) \(\mid AID_{GL}\) \(\mid T_{GL}\) \(\mid TID_{6G}\) \(\mid 6 GLM\)) is dispatched, where the group handover information is 6\(GLM=\{\)(\(AID_{1}\), \(Tag_{1}\)),..., (\(AID_{N}\), \(Tag_{N}\)), (\(AID_{GL}\), \(Tag_{GL}\)). * Once the message is received, 6L\({}_{\rm MAG}\)\({}^{\rm new}\) (\(NG\)) forwards a proxy binding update (PBU) message {\(T_{C}\) \(\mid\) \(T_{\rm MC}\) \(\mid\) \(\mid\) \(ID_{\rm NC}\) \(\mid\) \(\mid TID_{6G}\) \(\mid\) \(CT_{A}\) \(\mid\) \(\mid\) \(M_{\rm NG}\) = \(E_{\rm KNG}\)\(\leftrightarrow\)\({}_{\rm LMA}\) (\(ID_{\rm NC}\) \(\mid\) \(\mid\) \(T_{\rm NC}\) \(\mid\) \(AID_{GL}\) \(\mid\) \(\mid\) \(TID_{6G}\) \(\mid\) \(\mid\) \(CT_{A}\)) to inform the LMA that a group of 6L\({}_{\rm MNG}\) are currently roaming within its domain under the temporary group identifier \(TID_{6G}\). After verifying the \(TID_{6G}\) value of the received PBU message, the LMA forwards a message {\(T_{C}\) \(\mid\) \(\mid\) \(ID_{\rm LMA}\) \(\mid\) \(\mid\) \(T_{\rm LMA}\) \(\mid\) \(\mid\) \(AID_{\rm GL}\) \(\mid\) \(\mid\) \(TID_{6G}\) \(\mid\) \(CT_{A}\) \(\mid\) \(\mid\) \(M_{\rm LMA}\) = \(E_{\rm LKMA}\)\(\leftrightarrow\)\(A\) (\(ID_{\rm LMA}\) \(\mid\) \(\mid\) \(TID_{\rm GL}\) \(\mid\) \(CT_{A}\)) to the AAA for further verification. * Upon receiving the verification information, the AAA server computes the ticket using data stored in its database. It calculates the \(T_{C}\)\({}^{\prime}\) = (\(ID_{6G}\)\(\oplus\)\(N_{2}\)\(\oplus\)\(N_{3}\)\(\oplus\)\(SP\)) to verify its consistency with the received \(T_{\rm C}\). If the calculated \(T_{\rm C}\) differs from the received value, the AAA server sends a warning message to both the 6L\({}_{\rm MAG}\)\({}^{\rm new}\) and the LMA, indicating potential illegality within the mobile group. If the values match, message {\(TID_{A}\) \(\mid\) \(\mid\) \(T_{A}\) \(\mid\) \(TID_{6G}\) \(\mid\) \(CT_{LMA}\) \(\mid\) \(CT_{NG}\) \(\mid\) \(\mid\) \(CT_{GL}\)} will be sent to the LMA, where \(CT_{LMA}\) = \(E_{\rm LKMA}\)\(\leftrightarrow\)\(A\)((TID_{A}\) \(\mid\) \(T_{A}\) \(\mid\) \(1 ID_{6G}\)), \(CT_{NG}\) = \(E_{\rm KNG}\)\(\leftrightarrow\)\(A\) (\(TID_{A}\) \(\mid\) \(T_{A}\) \(\mid\) \(TID_{6G}\) \(\mid\) \(\mid\) \(N_{4}\)), \(CT_{GL}\) = \(E_{KGL}\)\(\leftrightarrow\)\(A\)((TID_{A}\) \(\mid\) \(T_{A}\) \(\mid\) \(1\)\(T_{A}\) \(\mid\) \(1\)\(N_{4}\)\(\oplus\)\(H\) (\(AID_{GL}\), \(R_{2G}^{\) (\(\oplus\) 1)), and \(N_{4}\) is a random number. * If the ciphertext, \(CT_{LMA}\), is successfully decrypted, the LMA updates the binding cache entry to record the 6L\({}_{\rm MAG}\)\({}^{\rm new}\).