Detailed explanation of the technical working principle of PnC Plug&Charge charging service -1

1、 Trust building phase between vehicles and charging stations

Identity verification: The charging station verifies its identity through the SECC (Supply Equipment Communication Controller) certificate, while the vehicle uses the factory preset V2G root certificate (Trust Anchor) for cross verification. To prevent certificate forgery attacks, the system has introduced OCSP (Online Certificate Status Protocol). In the latest ISO15118-20 standard, real-time revocation verification is mandatory for all certificates (including Sub CA and Leaf certificates) to ensure that every communication link is under strict compliance monitoring.

At this stage, the vehicle (EV) and charging station (EVSE) shake hands based on the ISO 15118 protocol. After completing the gun insertion operation and building a digital communication link, both parties will negotiate to determine the application version of the Transport Layer Security Protocol (TLS):

• TLS 1.2 is used in the ISO 15118-2 standard;
• When using the ISO 15118-20 standard, TLS 1.3 is adopted;

1. The charging station presents its certificate chain to the electric vehicle, including the Supply Equipment Communication Controller (SECC) certificate and the Charge Operator (CPO) Intermediate Certificate Authority (Sub CA) certificate, to complete the trusted identity authentication of the vehicle.
2. Electric vehicles use pre installed trust anchors (i.e. V2G root certificates pre-set before vehicle delivery to users) to verify the complete certificate chain submitted by charging stations.
3. Electric vehicles will undergo certificate revocation status verification: Under the ISO 15118-2 standard, SECC certificates are designed with a short-term validity period (2-3 months), and the revocation status of CPO Sub CA certificates is verified through the Online Certificate Status Protocol (OCSP); The ISO 15118-20 standard requires all certificates to undergo revocation status verification through OCSP.
4. After all verification steps have been passed, the electric vehicle can confirm the trusted identity of the charging station, and the subsequent full communication data will be transmitted using encrypted methods.

2、 Identity recognition method negotiation stage

The mutually exclusive logic between EIM and PnC: The protocol strictly prohibits performing “dual identity recognition” on users. If it is detected that the user has manually started through RFID or App (EIM mode), the charging station must automatically terminate the PnC triggering process to avoid the business risk of duplicate billing.

The ISO 15118 standard defines two charging session identity recognition mechanisms:

• 1. External Identification Means (EIM): covering Radio Frequency Identification (RFID) cards, credit cards, mobile applications, QR codes, etc;
• 2. Plug&Charge: A fully automated authentication mechanism based on certificates.

Core constraint: Charging stations shall not perform dual identity recognition on users. If the user completes authentication through external means such as RFID card before connecting the charging gun, the charging station should terminate the plug and play function triggering; Otherwise, it may result in the vehicle being repeatedly certified through the contract certificate, leading to the risk of duplicate billing.

3、 Vehicle identity verification stage

After the system selects the plug and play charging mode, the electric vehicle submits the contract certificate and the supporting Mobile Service Provider (MSP) Sub CA certificate to the charging station. The above certificates are integrated into a complete certificate chain, and the core identity verification process is officially launched. The backend will synchronously perform multiple security checks to comprehensively ensure the legality of charging sessions, the validity of billing accounts, and prevent the risk of vehicle identity forgery.

To prevent replay attacks (i.e. hackers intercepting authorization messages and sending them repeatedly), the system generates a 128 bit random challenge code. The vehicle must use a private key to sign the challenge code, and the charging station will perform millisecond level signature verification locally using the contract certificate public key. This asymmetric encryption ensures the uniqueness and non repudiation of instant authorization.

1. Validity verification of charging piles on certificate chains

The charging station or CPO backend system needs to complete the following two core verifications:

• Validity verification: Confirm that all certificates in the certificate chain are within their validity period;
• Authenticity verification: Verify the validity of the full chain digital signature from the contract certificate to the root certificate.

The prerequisite for implementing local verification is that the charging station has pre-set the corresponding root certificate – usually the V2G root CA certificate provided by the PKI operator, and in some cases, the MSP’s own root CA certificate (with a relatively low application proportion).

2. Online verification of certificate revocation status

Part of the verification process relies on online interaction, and certificate revocation status verification is one of the core processes. The charging station will initiate a query to the pre-set OCSP responder in the certificate to confirm that the following two types of certificates have not been revoked:

• CPO Sub CA Certificate (applicable to both ISO 15118-2 and ISO 15118-20 standards);
• Contract certificate (applicable only to ISO 15118-20 standard);

3. Billing account status verification

After the charging is completed, the system automatically completes the process from data exchange to fund receipt: After verifying the validity of the certificate, CPO will confirm the validity status of the user’s billing contract. The system will extract the service provider ID from the vehicle networking account identifier (EMAID), and CPMS will associate the corresponding MSP with this ID to verify that the account is not in a suspended or cancelled state.

4. Charging process started

After confirming the identity of the electric vehicle, the vehicle and charging station will follow the message exchange process of ISO 15118 standard, negotiate charging parameters and timing, and officially start the charging service.

To complete the PnC (Plug&Charge) plug and charge function, it is necessary to pre-set, store, and verify the contract certificate, and confirm the encrypted material system deployed internally in the vehicle end (EV) and pile end (EVSE).

As mentioned above, in the TLS protocol system: TLS 1.2: adapted to ISO 15118-2; TLS 1.3: Compatible with ISO 15118-20; Both communication parties need to have two types of core data: Identity materials and trust materials. Among them, the key repository stores the private key of the device and the corresponding leaf certificate, which is used to prove the “legitimacy of identity”; Trust inventory stores trusted CA certificates (trust anchors), which are used to verify the validity of the other party’s certificates and answer the “trust boundary” question.