KAIST develops time-locked semiconductor encryption technology

Researchers have developed a technology that keeps encrypted data locked until a set amount of time has elapsed — without relying on a clock or software to enforce the delay.

KAIST announced Wednesday that a research team led by Professor Kim Kyung-min of the Department of Materials Science and Engineering had developed a "dual-state programmable oxide transistor," or DUPOT, capable of independently controlling long-term and short-term memory within a single semiconductor device, and had used it to demonstrate time-based encryption.

Conventional encryption protects information through a decryption "key." The new technology adds a second condition: the precise moment at which the data is read.

The research team focused on the way electrical current inside a semiconductor changes over time. They engineered a single transistor to simultaneously hold long-term memory — which persists indefinitely — and short-term memory, which shifts as time passes.

Standard memory transistors store information primarily by adjusting the "threshold voltage," the baseline at which current begins to flow. The DUPOT, by contrast, can independently control both the threshold voltage and the "saturation current," which determines the magnitude of the current.

Long-term memory is achieved by trapping electrons inside an insulating layer to shift the threshold voltage. Short-term memory exploits the gradual recovery of current as oxygen ions migrate and redistribute within the device.

Using these properties, the team experimentally demonstrated two distinct time-based encryption schemes.

In "time-delay encryption," hidden information is set to appear only after a specified interval has passed. In testing, the letter "T" became readable only after 8.4 seconds had elapsed.

In "time-limited encryption," the correct information is revealed only at a specific moment. During the experiment, a false letter "K" appeared at 1.5 seconds, while the true letter "T" emerged at the target time of 37 seconds.

The same semiconductor device thus produced entirely different outputs depending on when the data was read.

The team also proposed an encryption scheme applicable to cloud environments. A ciphertext stored on an external server and a decryption key held by the user are each assigned different time conditions; the original information is restored only when both conditions are simultaneously met.

The technology is expected to find use across a range of security applications — from time-sensitive data such as exam questions and bid information to access-time controls for cloud services and smart devices. However, the research is currently at the stage of verifying feasibility with small-scale device arrays; improvements to operating voltage and reliability will be needed before commercial deployment.

"The significance lies in independently controlling persistent memory and time-varying memory within a single transistor, and applying that to time-based encryption," Kim said. "We will continue developing this into security solutions for cloud and smart devices, as well as broader information-processing technologies, by harnessing the intrinsic temporal behavior of the semiconductor itself."

The findings were published in the online edition of the international journal Advanced Materials on Sept. 10.