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Activating electrical prepayment systems in homes

20 Jul 2015  | Hamed Sanogo

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Figure 6 shows how meter readings are communicated back to a collection point or cloud over the public wireless data or cellular network. Alerts, consumption data, disconnect/reconnect, meter fraud-checking commands are all transferred between the smart meter and the HDU. Just like the HDU, the prepaid phone app allows customers to access their prepaid balance, usage history, and other account information in real time. The information which flows from the utility back office to the HDU includes alerts to notify customers of low balance, disconnect, and other account information.

Figure 6: Prepayment system framework uses standard wireless PLC to communicate between the HDU and utility office.

This approach offers utilities a seamless, scalable method for implementing a prepaid service program with a widely available wireless network. Eventually, this setup can be migrated to a full AMI deployment.

Securing the energy prepay transaction
Securing these financial transactions requires the MAX66242, which is the key electronic component inside the HDU. This tag authenticator IC combines a wireless NFC/RFID interface with an I2C interface. A 32B SRAM buffer facilitates fast data transactions over the I2C interface. The IC has a crypto engine and user memory with protection modes that make it the best and most secure solution for hiding information on the HDU. The MAX66242 also has built-in hardware protection features, including proprietary die-level physical techniques, circuits, and crypto methods that isolate the HDU and protect it from tampering or being compromised by hackers or malicious attackers.

The MAX66242 essentially plays the role of a secure element inside the HDU. The integrated SHA-256 crypto engine provides symmetric challenge-and-response authentication (figure 7) based on a secret key shared between the utility company's back-office servers (as master) and the HDU (as slave). While the secure server at the utility's back office implements the M-Secret function of this system, the S-Secret resides on the HDU at the customer's home inside the MAX66242. The same secret is computed by the utility's servers from the M-Secret that they maintained.

The HDU has the S-Secret key in this authentication protocol. It is assumed here that the server derives the HDU's secret using data from the customer's profile. This SHA-256 hash algorithm is based on a secure hashing standard, publication FIPS PUB 180-4, defined by the National Institute of Standards and Technology (NIST), and it makes for a strong anti-counterfeiting or anti-cloning tool.

The HDU platform is illustrated in figure 7. There are four data bytes in the HDU's (MAX66242) user memory which store the maximum kilowatt hour (kWh) value of the energy meter. We refer to this as the maximum meter value (MMV). Unlike a scheme where credit is loaded and then depleted as energy is consumed, the scheme used in this case is additive. When the customer buys (prepays) new credits, the total overall credit amount increases by the new amount. So, this MMV is the highest kilowatt hour reading allowed by the prepaid credits and, when reached, power is disconnected. The scheme used here computes its SHA-256 operation using a memory page in the MAX66242 which has been set to the authentication protected mode. (This is one of many memory protection modes supported by the IC.) The scheme also allows each HDU to have its own unique S-Secret based on the M-Secret placed on the server. Again, the smartphone is only a path, a conduit of information, in this scheme.

Figure 7: Block diagram of the HDU. The server computes the slave secret (S-Secret) from the master secret (M-Secret) using the customer-specific profile data that contains the HUD's ROM ID.

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