ZigBee® security and the smart grid


In the early 1960s, some technologically curious people discovered how to illegally route telephone calls without cost through the AT&T North American telephone switching network. What was their tool of entry? A small plastic whistle given way inside boxes of Cap’n Crunch cereal. The tone of the whistle perfectly matched the tone used to command the phone trunk lines. It seemed like harmless if subversive fun. No one died, the phone monopoly lost some revenue, and modern hacking was born.

The utility industry is on the verge of a major evolutionary change with the multiple networks used to deliver power to customers. Commonly known as the “smart grid”, this evolutionary change will create an interoperating network of networks that weave power transmission and distribution, customer communications, and command and control into a much more reliable, efficient and intelligent power delivery system. If once a continental phone network could be manipulated using a toy whistle, imagine what vulnerabilities – and liabilities – might lurk in a network of networks that powers every electronic device in today’s modern economy.

One type of smart grid network is the home area network (HAN). Utilities want their customers to use home area networks to better understand their energy consumption and to automate conservation measures such as demand response. But a vulnerable home area network connected to a secure utility network provides a potential access point into assets or information that should remain guarded. As ZigBee® wireless networking is rapidly becoming the HAN technology of choice, what kind of security does it provide to safeguard consumers, utilities, and the smart grid?

ZigBee places a strong emphasis on security mechanisms to prevent wireless tampering and eavesdropping, and to ensure high reliability and availability. For AMI systems, the ZigBee Smart Energy profile pre-defines security for the media access control, network, and application layers of the network. The profile also includes a certificate-based authentication mechanism for establishing security codes, or keys, and for setting the initial trust relationship between devices.

In addition, ZigBee provides four basic security services: authentication, message integrity, message confidentiality, and replay protection.

Authentication provides assurance about the originator of the message and prevents an attacker from modifying a hacked device to impersonate another device. ZigBee provides an authentication mechanism at both the network level and device level. Network level authentication uses a common network security key. This prevents outsider attacks while requiring very little in terms of memory. Device level authentication works by using unique security keys between pairs of devices, that mitigate insider and outsider attacks but have a higher memory cost.

Message integrity protects against some person or device modifying a message from an authorized sender while the message is in transit and the receiver is not able to detect this tampering. ZigBee provides a strong, yet flexible message integrity mechanism that allows for authentication code lengths of 0, 32, 64, or 128 bits. These options allow tradeoffs between message protection and message overhead, depending on the application.  

Message confidentiality means keeping information secret from unauthorized parties and is typically achieved by encryption. ZigBee uses 128-bit AES encryption, and this protection is available at the network level and the device level. Network level encryption uses a common network key, while device level encryption uses unique security keys between pairs of devices. Just like with message integrity, the tradeoff between network level and device level encryption balances protection with overhead costs. Notably, encryption can be turned off without impacting message integrity or authentication.

A replay attack consists of some person or device recording a legitimate message between two authorized devices and then replaying the message later. ZigBee prevents replay attacks using a counter mechanism. Essentially, ZigBee devices maintain a count of incoming and outgoing messages, and reset these counters only when a new security key is created. If a person or device tries to replay an old message once the counters have been incremented, the message will be rejected. The counters are large enough that even devices which communicate once per second will not exceed the counter limit for more than a century.

To coordinate these security services, ZigBee networks utilize the concept of a trust center. One device on the network serves as the trust center. Its tasks include allowing devices into the network, distributing security keys, and enabling end-to-end security between devices. Typically, the coordinator device on a ZigBee network functions as the trust center, and can even be a dedicated device. This trust center can scale with the size of the network, but additional memory is required for larger networks.

There is no guarantee that another dime-store gadget won’t come along to enable hacking of future home area networks. But if we’ve been paying attention since the days of Cap’n Crunch whistles, we’ll be better equipped to keep the various networks of the smart grid secure and reliable.