Sending Secrets Securely

Hank Hogan

If you want to tell a secret, it is best to whisper. However, your secret is safer when people nearby are talking loudly. That is the basis for a secure communication technique developed by Bernard B. Wu and Evgenii E. Narimanov, both of Princeton University in New Jersey, who showed that using a spread-spectrum approach within a public fiber optic network enables covert communication.

By spreading a covert signal over time and transmitting it at a low rate over a network with a public signal, it is possible to create secure communications. Only the proper decoder can recover the original secret signal. EDFA = erbium-doped fiber amplifier; OCDMA = optical code division multiple access. Courtesy of Evgenii E. Narimanov.

Spread-spectrum techniques have been used for decades in military wireless applications to prevent eavesdropping. As the name implies, the method spreads a signal out — across a wavelength or over time — dropping what would be an easily detectable signal below the noise threshold. A receiver who knows how the signal was dispersed can recreate it.

However, the spread-spectrum approach is not secure if the communication is performed at a fixed bit rate. In that case, an eavesdropper can perform correlation analysis to reveal the presence of the disguised signal.

The researchers showed that spread-spectrum signals can be concealed from snoops if they are sent via a fiber optic network that is already carrying a public signal. “The public signal in the network is very important,” said Narimanov, an assistant electrical engineering professor.

Hidden in public signal

The ratio of bit rates between the covert and public signals must be an integer between 100 and 1000, with the secret signal having the slower rate. Under that condition, any correlations of the hidden signal that might have been discovered by eavesdroppers are masked by those of the public waveform.

The technique makes use of optical-code-division multiple-access multiplexing and requires that the sender and receiver agree on an encoding phase mask and a decoding filter beforehand. Narimanov noted, however, that any hardware-based scheme faces a similar information exchange requirement.

Besides hiding spread-spectrum signals in the noise, using public networks provides another security advantage: Eavesdroppers would not know which network carried covert information, making interception harder. The same cannot be said for the alternative. “A secure network will always attract unwanted attention,” Narimanov said.

Optics Express, May 2006, pp. 3738-3751.