Demystifying Plug & Charge Under ISO 15118-20: Certificates, PKI & TLS 1.3

Plug & Charge (PnC) lets a driver simply plug in and walk away — authentication, authorization, and billing happen automatically over a secure channel. Under ISO 15118-20, PnC becomes more robust thanks to stricter cryptographic rules and multi-contract support. Here's how it actually works.

The Certificate Hierarchy

• V2G Root CAs form the global trust anchor.
• Sub-CAs are operated by OEMs, charging-station operators, e-mobility providers, and provisioning services.
• Leaf certificates — contract certificates and OEM provisioning certificates — are provisioned into the vehicle, ideally protected by a hardware secure element such as a TPM 2.0.

The Session Flow

1. TLS handshake. EV and EVSE mutually authenticate with TLS 1.3 and exchange certificate chains.
2. Certificate installation. If the EV needs a new contract certificate, it requests one using its OEM provisioning certificate.
3. Authorization. The charging station validates the certificate — including revocation checking via OCSP/CRL — and confirms contract status with the back office.
4. Energy transfer. Once authorized, metering and billing data is signed with the EV's key for non-repudiation.

Secure Key Storage

ISO 15118-20 encourages hardware-backed key isolation, such as a TPM 2.0. Implementations without a dedicated secure element must provide equivalent protection for private keys.

Best-Practice Checklist

• Rotate contract certificates on a defined schedule rather than leaving them open-ended.
• Use OCSP stapling to keep certificate-status checks fast during the handshake.
• Pre-load multiple V2G root certificates to accommodate roaming growth.
• Keep revocation lists small enough to fit the communication budget of the powerline link.

Common Pitfalls

• Clock skew. Communication modules without a reliable real-time clock can wrongly treat valid certificates as expired — synchronize time carefully.
• Cipher-suite mismatch. Make sure both the EV and charger support the TLS 1.3 cipher suites required by the standard.

Future-Proofing

Post-quantum cryptography — for example hybrid classical-plus-lattice signatures — is being explored by CharIN and ISO for future revisions. It is not part of ISO 15118-20 today, but designing for cryptographic agility now will ease that migration later.