# Authentication and Billing for Dynamic Wireless EV Charging in an Internet of Electric Vehicles
## 5 Proposed Authentication and Billing Scheme
### Charging Activation Phase  
After getting the hash key set from the CSC, EV \(e\) starts to communicate with \(PO_{i}\) by sending the \(N\)th hash key assigned by the CSC with the charging parameters and the ID of the CSC who generated the hash chain, in \(m_{\mathcal{T}}\{h_{N},p,C,t_{e},\mathit{param},\text{SIG}_{\mathit{gb}_{ \mathit{c}}}\}\), encrypted using \(E_{\mathit{pk}_{O_{i}}}(m_{\mathcal{T}})\). The charging parameters field, \(\mathit{param}\), includes the EV energy requirement, \(\mathit{req}\mathcal{E}\), as well as the desired charging rate, EV speed, EV charging coil model, battery type, etc. The privacy of this information is maintained as it is linked to the pseudonym \(p\) assigned to the EV, not to the real EV identity. Hence, even if this message is compromised, this information cannot be traced back to the EV.  
Upon receiving \(E_{\mathit{pk}_{PO_{i}}}(m_{\mathcal{T}})\), the PO replies by sending its ID, the unit cost of charging and the time stamp to the EV, as \(m_{\mathcal{S}}\{cost,PO_{i},t_{PO_{i}},SIG_{\mathit{gb}_{\mathit{PO_{i}}}}\}\). The PO also publishes the \(N\)th hash key shared by the EV to the charging segments, in \(m_{\mathcal{B}}\{h_{N},t_{PO_{i}}\}\). Once the EV enters the communication range of the first charging segment, it sends a message containing its pseudonym, the next hash key, \(h_{N-1}\), the CSC ID, \(C\), and the time stamp generated by EV. This is denoted as \(m_{10}\):\(\{p,h_{N-1},C,t_{e}\}\). The message is encrypted using the session key, \(k_{p}\), as \(\varepsilon_{k_{p}}(m_{10})\). \(CS_{1}\) decrypts the message and verifies the hash key by finding \(h_{N}=\mathcal{H}(h_{N-1})\), which has to be equal to the root key sent by the PO in \(m_{9}\). If verification succeeds, \(CS_{1}\) is activated and the EV starts charging. The segment also adds the received pseudonym, \(p\), and corresponding key, \(k_{p}\), to the revocation list to avoid reuse by free-riders.  
In addition, \(CS_{1}\) forwards the verified hash key received from the EV, \(h_{N-1}\), in a message, \(m_{11}\), to the next segment, \(CS_{2}\), which includes the EV pseudonym, \(p\), the segment index, \(s\), and \(h_{N-1}\), encrypted using the segment-to-segment session key, \(k_{s}\), as \(\varepsilon_{k_{s}}(p,s,h_{N-1})\). The second segment stores the hash key and waits for the incoming message from the EV with pseudonym, \(p\). As soon as the next message is received from the EV with \(h_{N-2}\), \(CS_{2}\) performs one hashing operation to verify that \(h_{N-1}=\mathcal{H}(h_{N-2})\) before activating the charging process. This process continues until EV \(e\) finishes the required number of charging segments by sending \(h_{0}\) to the last charging segment, \(CS_{N}\). The use of dedicated short range communication (DSRC) between the EVs and the CSs ensures that the message transmitted by the EV only reaches the nearest charging segment before fading out, which minimizes the interference between the different messages shared between the EVs and the CSs.  
Figure 4: Message sequence for key pre-distribution, registration and authentication messages.