## 2 Preliminaries
### Kerberos Authentication  
Kerberos employs symmetric key cryptography to provide a secure authentication mechanism for both the vehicle and Roadside Units (RSUs). The utilization of a session key for crucial procedures, including those involving the Ticket-Granting Server (TGS) and specific services, distinguishes Kerberos authentication as a notable protocol. The decryption of messages that carry session keys and other significant information is achieved through the utilization of the entities' confidential keys.  
The original authentication mechanism utilized in Kerberos V5 [18] can be outlined as the following process:  
\[RA= ID_{\mathrm{p}}||Realm_{\mathrm{p}}||ID_{TGS}||Times||Nonce_{1}||PreAuth. \tag{1}\]  
\begin{table}
\begin{tabular}{l l l} \hline \hline  & \multicolumn{2}{c}{Entities} \\ \hline Vehicular Ad-Hoc Network & & VANET \\ On-Board Unit & & ORU \\ Vehicle & & V \\ Trusted Authority Server & & TAS \\ Authentication Server & & AS \\ Ticket Granting Server & & TGS \\ Roadside Unit & & RSU \\ \hline \multicolumn{3}{c}{Keys} \\ \hline Vehicle’s secret key & & \(K_{V_{\mathrm{s}}}\) \\ TGS secret key & & \(K_{TGS_{\mathrm{s}}}\) \\ TGS session key & & \(K_{TGS_{\mathrm{s}}}\) \\ Service session key & & \(K_{\mathrm{s}_{\mathrm{s}}}\) \\ Service secret key & & \(K_{\mathrm{s}_{\mathrm{s}}}\) \\ Signature of the Client & & \(P_{\mathrm{s}}\) \\ Signature KDC & & \(P_{\mathrm{s}_{\mathrm{s}_{\mathrm{s}_{\mathrm{s}_{\mathrm{s}_{\mathrm{s}_{ \mathrm{s}_{\mathrm{s}_{\mathrm{s}_{\mathrm{s}_{\mathrm{s}_{\mathrm{s}_{ \mathrm{s}}}}}}}}}}}}}}\) \\ Certificate KDC & & \(C_{\mathrm{s}_{\mathrm{s}_{\mathrm{s}_{\mathrm{s}_{\mathrm{s}_{\mathrm{s}_{ \mathrm{s}_{\mathrm{s}_{\mathrm{s}_{\mathrm{s}_{\mathrm{s}_{\mathrm{s}}_{ \mathrm{s}}}}}}}}}}}}}\) \\ \hline \multicolumn{3}{c}{Name of Message} \\ \hline General & Request authentication & \(RA\) \\  & Reply \\  & Ticket Granting Ticket & \(TGT\) \\  & Service Ticket & \(ST\) \\ \hline Authentication msg & Vehicle auth V to TGS & \(AllV_{TGS}\) \\  & Vehicle auth V to Service & \(AllV_{VIS}\) \\  & Service auth message & \(All_{Sun}\) \\  & Service authentication Message & FA \\ \hline Attributes & Attributes AS to V & \(AT_{ASIV}\) \\  & Attributes V to TGS & \(AT_{VT_{TGS}}\) \\  & Attributes TGS to V & \(AT_{TGSV}\) \\ \hline \multicolumn{3}{c}{Message Contents} \\ \hline ID & Vehicle’s ID & \(ID_{\mathrm{s}}\) \\  & Service name/ID & \(ID_{\mathrm{s}}\) \\  & TGS name/ID & \(ID_{TGS}\) \\ \hline IP & User IP Address & \(IP_{\mathrm{s}}\) \\ \hline Req & Request lifetime for TGT & \(Req_{TGT}\) \\  & Request lifetime for ticket & \(Req_{LT}\) \\ \hline Timestamp & Timestamp attributes AS to V & \(T_{\mathrm{S}ASIV}\) \\  & Timestamp TGT & \(T_{STGT}\) \\  & Timestamp Vehicle authentication & \(T_{\mathrm{S}VA}\) \\  & Timestamp attribute TGS to V & \(T_{\mathrm{S}TGSV}\) \\  & Timestamp ST & \(T_{\mathrm{S}ST}\) \\  & Timestamp vehicle to service & \(T_{\mathrm{S}VIS}\) \\  & Timestamp service auth message & \(T_{\mathrm{S}SV}\) \\ \hline Parameter & DH parameter & \(DH_{p}\) \\ \hline Lifetime & Lifetime for TGT & \(LT_{TGT}\) \\  & Lifetime for Service Ticket & \(LT_{ST}\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Annotations and abbreviations.  
\[RP=\ ID_{v}||TGS||E(K_{\text{ps}},[K_{TGS}||Times||None_{1}||Realm_{TGS}||ID_{TGS} ]). \tag{2}\]