Eavesdropping on wireless communications depends on the ability to determine critical information about the basic properties of the communication link, known as channel state information, or CSI. A new study suggests ways to prevent eavesdroppers from obtaining the channel state information, offering a first line of defense for wireless security.

In a recently published paper, Yingbo Hua, a professor of electrical and computer engineering in UC Riverside’s Bourns College of Engineering, describes a scenario involving a wireless network of legitimate users all equipped with full-duplex radio, which can transmit and receive on the same channel. Hua’s proposed fix allows two or more users to transmit a secret to each other without allowing eavesdroppers to discover the secret by preventing them from learning the network’s channel state information.

“Given the widespread applications of wireless communications and our virtual dependence on them, the technology for wireless security must stay ahead of all advances relevant to wireless communications, including robotics and artificial intelligence,” Hua said.

Encryption is industry’s standard for wireless security. Encryption uses public and private keys, which are random strings of 0’s and 1’s designed to scramble the content of the communication. The public key is always known to everybody and it’s difficult to get a private key from the public key. As computing technology, especially quantum computing, advances, however, finding the private key from the public key will become much less difficult. 

Another way, the symmetric key approach, avoids using a public key. Two users agree on a password to communicate securely. However, the more the users deploy the secret key, the more vulnerable it becomes because eventually listeners will be able to decode it. Security thus depends on periodically changing the password. Users can meet privately and agree on the new password, which is inconvenient. It is a big challenge to transmit a secret key wirelessly, in a mobile environment.

Researchers have been looking for ways to improve wireless security that don’t depend on encryption. Hua’s proposed technique takes advantage of newly developed hardware known as full-duplex radio, which can transmit and receive at the same time on the same frequency. By sending packets of information at the same time, Hua discovered that eavesdroppers are unable to ascertain the channel state information.

The CSI in wireless communications describes how a signal travels from transmitter to receiver, accounting for the cumulative effects of various types of interference and fading over distance. Channel state information constantly changes because of the mobility of the environment. As people walk by, as cars move around, or even as trees move with wind, the channel state changes. When the channel state information is unknown it’s very difficult for the receiver to receive information transmitted from the other end.

In order to keep track of the CSI, every data packet transmitted must have pilot symbols embedded somewhere. Based on the pilots, the receiver estimates the channel state information and acquires the contents of the communication. 

Existing standards and infrastructures, such as half-duplex radio, which must alternate sending and receiving, will always make it possible for eavesdroppers to get the channel state information because the pilot is public information and if eavesdroppers listen long enough they will eventually be able to get the channel state information. 

But Hua discovered that two full-duplex wireless transmitters can send the same pilots at the same time. The two pilots mix in the eavesdropper’s receiver, obscuring the channel state information. 

“With this way of doing it we can actually prevent the eavesdropper from getting the channel state information,” Hua said.

Hua’s method confuses the channel state information so that eavesdroppers can’t get a handle on the communications.

“That is the key idea,” said Hua. “That is something novel in this particular work. It’s very promising because it’s a first line defense, and onto that we can always add on more secretive layers. If you can do a first line of defense very well then everything else will be easier.” 

Hua’s theoretical work is based on the known constraints and potential of the technologies we already have.

“These days industry can pick up ideas very quickly. An idea published today, maybe within a year they can have it in production,” Hua noted. 

The paper, “Advanced Properties of Full-Duplex Radio for Securing Wireless Network,” is published in the January, 2019 issue of IEEE Transactions on Signal Processing. The work was funded by the Department of Defense.