DIGITAL SIGNATURE (VALID ONLY FOR WM3M4C)
User’s Manual WM3M4 &WM3M4C
6.3 Energy meter cryptographic functions explanation
Energy meter has HW based cryptographic unit for digital signing of billing dataset.
6.3.1 Generation of private/public key par
This is one-time procedure made at production of energy meter. Generation of key pair is HW based
with dedicated crypto chip. Private key is stored internally within the crypto chip and there is no way
of reading it.
6.3.2 Public Key as QR-code on front of enclosure and readable
via MODBUS
Public key is available to end user for verification of digital signature. Therefore, public key is readable
through MODBUS communication and printed with QR code on front of the meter.
6.3.3 Generation of billing dataset using internal energy meter
value
Energy meter has MODBUS registers to store users billing dataset. Main EV charger SW must write
billing dataset to energy meter. Energy meter will fill in measured energy and timestamp to complete
billing information. Billing dataset is compatible with OCMF 1.0.
6.3.4 Generation of hash (SHA256) for billing dataset
After completing billing dataset, meter calculates hash of complete message with SHA-256 algorithm
documented in the following site: http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.180-4.pdf. Hash is
32 bytes long identification of message and is used as an input for signature generation.
6.3.5 Generation of signature for billing dataset
Signing of previously prepared hash is cryptographic procedure with ECDSA NIST P256 prime curve.
Crypto chip generates signature in less than a second. Algorithm is documented in:
FIPS 186-4 specification http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-4.pdf
6.3.6 Exporting billing dataset including signature
Complete billing dataset and digital signature are available for readout via MODBUS communication.
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