RFID Tags Chat Their Way to Energy Efficiency

by George Lawton

Basic RFID technology is widely used to provide unique identifiers for merchandise, pallets, animals, passports, and so on, and to enable contactless payment cards and keys. Now researchers are working on a way make to make RFID technology even smarter by using the network to store computational data.

Scientists at the University of Massachusetts, Amherst, and RSA Laboratories are developing a new architecture for computational RFID (CRFID) technology that securely offloads storage to the network. The development includes Cryptographic Computational Continuation Passing (CCCP), a technique they think will make it more efficient to store program data in the reader rather than the tag for many CRFID applications.

"These techniques make use of an unusual characteristic," said Ari Juels, chief scientist and director of RSA Laboratories. "It's cheaper for them to transmit data than store it locally. We are using this characteristic to boost [CRFID's] computational capacity. In a way, it's a sort of cloud computing in miniature. It's a way of extending a small device's capabilities by leveraging those of a large device. In this particular case, you don't have to trust the reader."

According to technology market consultancy IDTechEx, 2.35 billion basic RFID tags will be sold globally in 2009, up from 1.97 billion in 2008. The tags track pallets (225 million), apparel (200 million), animals (105 million), and tickets (350 million). CRFID and CCCP could extend the use of RFID into low-maintenance sensors and improve the security of contactless payment systems and ID cards.

Tight Constraints

RFID tags have severe resource constraints owing to their cost and energy requirements. For RFID tags to grow in popularity, they need to be cheap to mass produce. This puts significant constraints on the amount of silicon used in each tag.

Powering the RFID tag via an external reader eliminates the cost associated with a battery or the need for a power supply. But it also limits the device's capability to calculate, store data, or communicate.

In an RFID system, the reader sends out a radio signal that temporarily powers the RFID tag and transmits a signal back to the reader. Most RFID tags today can only transmit a unique ID to the reader, which can be used to look up more information about the tag's history in a database. More sophisticated tags can also perform calculations and store information locally in flash memory. But all of these operations must be done with the energy harvested from the RF signal sent by the reader.

Today the RFID tags for tracking operate in the 860–960 MHz band across a range up to 30 feet, and they cost 20–40 cents, said Craig Harmon, CEO of QED, a consultancy that focuses on automatic ID systems. In passports and payment systems, a particular class of RFID-like devices called IC cards operate at 13.56 MHz over a shorter range of only a few feet and cost between one and two dollars.

Harmon said that most IC cards used today provide only basic functionality, which has led to many reports of hackers cloning the information found in passports. He said more sophisticated tags have been developed that can use encryption to provide higher security, but these more expensive devices are not widely used.

Efforts are also underway to incorporate small batteries into the tags. The batteries could be used for computation or storage, but they increase the device costs.

Making Tags Smarter

Researchers at Intel have been working on the Wireless Identification and Sensing Platform. WISP devices are powered by the network, much like traditional RFID devices, and readers see them as a traditional RFID tag. The power from the reader is used to drive a 16-bit, general-purpose microcontroller that can perform computational tasks, sample sensors, and report back to the reader. The WISP can also write to local flash memory and perform cryptographic computations.

Each WISP device comes with 32 Kbytes of space for storing programs and 8 Kbytes for storing data. They include sensors for light, temperature, and 3D-accelerometers, and can work over a range of 10 feet, compared to 30 feet for a standard RFID tag.

But because the WISPs run on power from the network, they pose challenges to carrying out even the most basic computational or sensing functions. Many relatively simple operations take more power than is available from the energy harvested from a single RFID reader transmission. One strategy is to incorporate an energy storage capacitor into the WISP. The capacitor can accumulate the energy from several RFID reader transmissions until it has enough for the desired operation. But this adds cost.

Another strategy is to assume the device will run out of power during a process and intermittently store data so that the existing process can be resumed from the place it stopped. One way to do this is to store the data in the local flash drive but this requires energy.

CCCP's Delicate Balance

The novelty of CCCP is that it's more economical to store intermittent data on the reader between power cycles than on the RFID tag itself. CCCP gives the tag a boost in computational power as a tradeoff for local storage. The one concern about this approach is that an untrusted reader might compromise the data. The use of cryptography helps to ensure data integrity.

There is a delicate balance between the amount of power required to encrypt the data and transmit it to the reader for storage, compared with the amount of power required to store it locally. The University of Massachusetts and RSA researchers believe that it's more efficient to transmit data sizes above 64-bytes to the reader for storage.

"This approach is useful in any situation in which the tag has to perform an operation in which the reader might subvert the data," said Juels. "It's important that the tag create integrity protection for this type of data. If the tag is producing a digital signature on a sensor reading, we don't want the reader taking the data and modifying it or, worse yet, stealing the digital signature key of the tag."

To read the full details of CCCP, go to www.cs.umass.edu/~ssclark/crfid/papers/salajegheh-usenixsec09.pdf

To find out more about WISP and CRFID, go to http://seattle.intel-research.net/wisp.