Public Key Signature

smart-me meters (version 2.0 and later) digitally sign all measurement data and transactions. This makes it possible to verify the authenticity of measured values at any time. For this purpose, smart-me uses the cryptographic principle of end-to-end protection by means of a digital signature based on asymmetric encryption.

smart-me distinguishes between two types of signed measured values:

Signed meter values: Signed measured values that are stored and signed every 15 minutes. Ideal for energy consumption billing.
Signed transaction: Signed transactions are intended for energy consumption over a specific period of time (e.g. charging electric vehicles). They always have a start value and an end value.

Technology / Process

Signature algorithm: ECDSA 256 Bit NIST curve (SECP256R1)

Hash algorithm: SHA256

Process: (certified) production

During production of the meter, an asymmetric key pair is generated and stored on the meter. The public key is also stored in the cloud, where it is publicly available.

Process: signing the measured values

Every 15 minutes, or when a transaction ends, a data packet containing the corresponding measured values is created.
A hash (SHA256) is computed over this data.
This hash is encrypted (ECDSA 256) and stored as the signature.
The data packet is sent to the cloud together with the signature and stored there.

Process: verifying the measured values

The customer receives the data packet with the measurement data via the app, the web or a third party.
The hash (SHA256) is computed over the data packet.
The public key for all smart-me meters is publicly available in the cloud (also via API)
The customer verifies the hash of the data packet using the public key.

Data packets

To keep the size of the data packet, and thus the amount of data required for transmission, as small as possible, smart-me uses "Google Protocol Buffers" as the data format for the measured values. The data packets are structured as follows:

Signed meter values

message CounterValue {
  optional bytes Obis = 1;
  optional int64 Value = 2;
  optional string Unit = 3;
}

message MeasurementValues
{
 optional uint32 SerialNumber = 1;
 optional uint32 TimestampUtc = 2;
 repeated CounterValue Values = 3;
}

Measurement values

SerialNumber: The serial number of the meter (e.g. 6300000) TimestampUtc: Unix timestamp (UTC) of this measurement data Values: List of all measured values (CounterValue) in this data packet.

CounterValue

Obis: OBIS code of the measured value (6 bytes). e.g. { 0x01, 0x00, 0x01, 0x08, 0x01, 0xFF } for 1-0:1.8.1*255 (active energy consumption tariff 1) Value: The measured value Unit: The unit of the measured value. At smart-me usually mWh (milliwatt hours)

Signed transactions

Transactions consist of a start value and an end value.

message CounterValue {
  optional bytes Obis = 1;
  optional int64 Value = 2;
  optional string Unit = 3;
}

message MeasurementValues
{
 optional uint32 SerialNumber = 1;
 optional uint32 TimestampUtc = 2;
 repeated CounterValue Values = 3;
}

message Transaction
{
 optional uint32 SerialNumber = 1;
 optional uint32 TransactionNumber = 2;
 optional int64 UserId = 3;
 optional MeasurementValues StartValues = 4;
 optional MeasurementValues EndValues = 5;
}

Transaction SerialNumber: The serial number of the meter (e.g. 6300000) TransactionNumber: The number of the transaction. This number is incremented with each transaction. UserId: The ID of the user who started this transaction. StartValues: The measured values at the start of the transaction (see above) EndValues: The measured values at the end of the transaction (see above)

Example

This example shows the validation of a signed transaction.

Data packet (transaction)

awicMRCF/v//DyIvEOrEhuYFGhMKBgEAAQgA/xCj/MaiDhoDbVdoGhIKBgEAAggA/xCkiPkCGgNtV2gqLxDQyIbmBRoTCgYBAAEIAP8Qq7v9ow4aA21XaBoSCgYBAAIIAP8QpIj5AhoDbVdo

oder HEX:

6b 08 9c 31 10 85 fe ff ff 0f 22 2f 10 ea c4 86 e6 05 1a 13 0a 06 01 00 01 08 00 ff 10 a3 fc c6 a2 0e 1a 03 6d 57 68 1a 12 0a 06 01 00 02 08 00 ff 10 a4 88 f9 02 1a 03 6d 57 68 2a 2f 10 d0 c8 86 e6 05 1a 13 0a 06 01 00 01 08 00 ff 10 ab bb fd a3 0e 1a 03 6d 57 68 1a 12 0a 06 01 00 02 08 00 ff 10 a4 88 f9 02 1a 03 6d 57 68

Signature

V0EGJ3gHNbnUZ8hAfdRn2ziEVbnXpZ3a5L5WtG24XOWOeCBKH687W/wikxqK5e+Zad3R/PuCnQDNqgeSfh4pow==

oder HEX:

57 41 06 27 78 07 35 b9 d4 67 c8 40 7d d4 67 db 38 84 55 b9 d7 a5 9d da e4 be 56 b4 6d b8 5c e5 8e 78 20 4a 1f af 3b 5b fc 22 93 1a 8a e5 ef 99 69 dd d1 fc fb 82 9d 00 cd aa 07 92 7e 1e 29 a3

Public Key

RUNTMSAAAAAN48gSNbwl1Uj4DDvwO1wReZj95r19F5nqvy8pTmoUtyMBtf2HgwN6jf9+Akzp/nsy+BMrzAdvrjOD5wfYDGVk

oder HEX:

45 43 53 31 20 00 00 00 0d e3 c8 12 35 bc 25 d5 48 f8 0c 3b f0 3b 5c 11 79 98 fd e6 bd 7d 17 99 ea bf 2f 29 4e 6a 14 b7 23 01 b5 fd 87 83 03 7a 8d ff 7e 02 4c e9 fe 7b 32 f8 13 2b cc 07 6f ae 33 83 e7 07 d8 0c 65 64

Calculate the hash (SHA256) of the data packet:

SHA256(data packet) = 52 2F 46 C6 26 70 17 32 B6 FD 4B 78 7E 31 5D 3B EE F0 F4 E3 42 66 4A D0 5F AB 95 74 F1 C1 3C 0C

Validate the hash with the public key using ECDSA 256

Deserialize the data packet: Together with the Protobuffer definition, the data packet can easily be deserialized (example visualized as JSON):

{
   "SerialNumber":6300,
   "TransactionNumber":4294967045,
   "UserId":0,
   "StartValues":{
      "SerialNumber":null,
      "TimeStampUtc":1556193898,
      "CounterValues":[
         {
            "Obis":"AQABCAD/",
            "Value":3830562339,
            "Unit":"mWh"
         },
         {
            "Obis":"AQACCAD/",
            "Value":6177828,
            "Unit":"mWh"
         }
      ]
   },
   "EndValues":{
      "SerialNumber":null,
      "TimeStampUtc":1556194384,
      "CounterValues":[
         {
            "Obis":"AQABCAD/",
            "Value":3833552299,
            "Unit":"mWh"
         },
         {
            "Obis":"AQACCAD/",
            "Value":6177828,
            "Unit":"mWh"
         }
      ]
   }
}

Reading aid

StartValues

TimeStampUtc 1556194384 = 25.04.2019 12:04 (UTC)

Measured value 1 Obis: AQABCAD/ = 01 00 01 08 00 ff = 1-0:1.8.0*255 (active energy consumption total)

Measured value 1: Value 3830562339 mWh = 3830.562 kWh

Measured value 2 Obis: AQACCAD/ = 01 00 02 08 00 ff = 1-0:2.8.0*255 (active energy delivery total)

Measured value 2: Value 6177828 mWh = 6.177 kWh

EndValues

TimeStampUtc 1556194384 = 25.04.2019 12:13 (UTC)

Measured value 1 Obis: AQABCAD/ = 01 00 01 08 00 ff = 1-0:1.8.0*255 (active energy consumption total)

Measured value 1: Value 3833552299 mWh = 3833.552 kWh

Measured value 2 Obis: AQACCAD/ = 01 00 02 08 00 ff = 1-0:2.8.0*255 (active energy delivery total)

Measured value 2: Value 6177828 mWh = 6.177 kWh

-> Energy consumption in this transaction: 3833.552 kWh - 3830.562 kWh = 2.99 kWh

Technology / Process​

Process: (certified) production​

Process: signing the measured values​

Process: verifying the measured values​

Data packets​

Signed transactions​

Example​