Some information, such as exam questions or bid prices, must remain unreadable until an agreed time, even if it has already been delivered. KAIST researchers have used changes in the electric current inside a semiconductor to develop encryption in which information is revealed only after a set amount of time has passed, or can be read correctly only at a specific time. In addition to the key needed to restore the original information, the technology adds a second condition: when the information is read.

KAIST (President Choongsik Bae) announced on October 14 that a research team led by Professor Kyung Min Kim from the Department of Materials Science and Engineering has developed a dual-state programmable oxide transistor (DUPOT), a single device in which long-term and short-term memory can be controlled independently. The team then used the transistor to implement time-based cryptography.

A transistor is a small semiconductor component that controls the flow of electric current. The DUPOT is made from oxide materials, and its two electrical states can be independently set by applying appropriate voltages. The term “programmable” refers to this ability to set the device to a desired state.

Encryption methods that use time as a condition for accessing information have been studied before. What sets this study apart is that the time at which information can be read is determined by physical changes within the semiconductor itself, rather than by external clocks or software alone.

To achieve this, the researchers implemented two types of memory in a single transistor: long-term memory, which is retained for a long period, and short-term memory, which changes over time. In this context, memory refers to a device’s ability to retain a state that has been changed by an applied voltage. The team used the long-lasting state to set the reference for the information and the time-varying state to control when the target information appears.

Conventional memory transistors store information mainly by changing the threshold voltage, the voltage at which current begins to flow. The DUPOT, by contrast, can independently control both the threshold voltage and the saturation current, the level of current maintained under fixed operating conditions. As a result, the voltage at which current begins to flow and the amount of current that flows can be set separately.

The two states are produced by different mechanisms. When a positive voltage is applied to the gate, the terminal that controls the current, electrons become trapped in hafnium oxide (HfO₂), an insulating layer that does not easily conduct electricity. This shifts the threshold voltage. Because this state is retained for a long time, it serves as long-term memory.

When a negative voltage is applied to the gate, oxygen ions (electrically charged oxygen particles) move to the interface between the insulating layer and the channel through which current flows, which reduces the saturation current. Once the voltage is removed, the oxygen ions gradually redistribute and the current recovers. The team used this change as short-term memory.

Using several transistors, the researchers configured how the current changed over time and set a readout criterion for each device to demonstrate two encryption methods.

In time-release encryption, information appears only after a certain amount of time has passed. In the experiment, the difference in current between the target pattern and the background grew over time, and the letter “T” became readable after 8.4 seconds.

In time-bound encryption, the correct information can be read only within a specified time window. In the experiment, a decoy pattern, the letter “K,” appeared at 1.5 seconds, while the target letter “T” appeared at the designated time of 37 seconds. The same array therefore displays different information depending on when it is read. These time points were chosen for the experiment.

The team also proposed an encryption method for cloud environments, in which data are stored on remote servers accessed over the internet. In this method, different time conditions are applied to the ciphertext stored in the cloud and to the decryption key held by the user Ciphertext is data that has been converted into a form in which the original information cannot be recognized, and a decryption key is the information needed to convert it back. In a demonstration using the device array, the researchers restored the original information by combining the ciphertext and the key that matched each time condition.

The study shows that a semiconductor can do more than store information: it can also determine when that information is read. The technology could be used to protect information that must not be disclosed before a certain time or that must be readable only under specific time conditions. However, the technology cannot revoke access to copies of information that have already been obtained.

“The significance of this work is that long-lasting memory and time-varying memory can each be controlled independently within a single transistor and used for time-based encryption,” said Professor Kyung Min Kim. Kim added that the team plans to build on the time-dependent changes of the semiconductor itself to develop the technology further for cloud and smart-device security, as well as for a range of information-processing technologies.

The current study has demonstrated the feasibility of time-based encryption at the array level. The team plans to lower the operating voltage and improve device-to-device uniformity and consistency over repeated operating cycles, so that the devices can work stably in a variety of environments. The researchers then aim to extend the technology to time-based security and to neuromorphic devices, which mimic how neurons in the brain work and connect.

The study, with Ph.D. candidate Huisu Noh of the Department of Materials Science and Engineering as first author and Professor Kyung Min Kim as corresponding author, was published online in Advanced Materials on September 10, 2026.
※ Paper title: Dual-State Programmable Oxide Transistor for Time-Based Cryptography, DOI: 10.1002/adma.74951
This research was supported by the Next-Generation Intelligent Semiconductor Technology Development Program of the National Research Foundation of Korea (NRF) (RS-2025-02433006).