Deciphering General-Signing (0xDF) APDUs with ECDSA in DLMS Security Suite 1

[alex_turner]

Digital Signatures for Legal Non-Repudiation in Smart Meter Billing

Under European PTB / BSI TR-03109 regulations, billing measurements must be cryptographically signed by the meter's on-chip private key using General-Signing (0xDF).

Wire Format (APDU Tag 0xDF)

DF 08 47 55 52 55 58 31 32 33 00 00 00 1A 21 00 00 00 55 ... 30 44 02 20 ... 02 20 ...

Field Breakdown

1. Tag: DF (general-signing)
2. System Title: 8 bytes (47 55 52 55 58 31 32 33)
3. Signature Length: 00 00 00 1A
4. Security Control Byte: 21 (Signed with Suite 1)
5. Transaction Counter: 4 bytes
6. Signed Plaintext APDU: Includes meter serial, OBIS code, measurement value, and timestamp.
7. ECDSA Signature (DER format): $R$ and $S$ integers (P-256 curve, 64-72 bytes).

Anyone (including consumers and third-party auditors) can verify the signature using the meter's publicly certified X.509 root key!

[elena_rostova]

In firmware, we generate ECDSA signatures inside a dedicated hardware Secure Element (SE) with side-channel attack countermeasures (DPA/SPA resistance).

[marcus_vance]

Our HES archives the signed 0xDF APDUs in immutable cold storage for 10 years to satisfy fiscal regulatory audit mandates.