Spintronics: Logic Lock Circuits Could Become Critical in Future Electronics

The importance of hardware security has been a topic of increasing concern for researchers. A recent paper published by a group of researchers discusses the use of spintronics in creating digital logic locking circuits that can only be activated with the correct key. These circuits may be crucial in future security systems, but there are also challenges that hardware security faces. What challenges does hardware security face, what did the researchers present, and could such circuits be critical in future security systems?

What challenges does hardware security face?

The past year has seen numerous advances in electronics, with RISC-V becoming more predominant, wearable devices becoming steadily more practical, and the push towards semiconductor sovereignty. However, the increasing dependencies on technology make modern life more vulnerable to cyberattacks than ever before. This has become apparent in the ongoing Russo-Ukraine war, with Russia launching numerous cyberattacks against Ukraine to destabilise the country and shut down its infrastructure.

Protecting devices against cyberattacks has traditionally been done with software solutions that actively monitor network connections, scan files, and block suspicious activity, but while these have worked well, they suffer from numerous difficulties. One such challenge is that software typically runs on top of other processes meaning that any malware that has managed to infect the underlying OS can defend itself against software-driven antimalware. 

To get around this challenge, engineers are increasingly turning to hardware security solutions consisting of physical circuits and processing units specifically designed to defend against malware attacks. For example, hardware keys are virtually impossible to break via software, meaning that devices can securely store encrypted data. Another solution developed by engineers is real-time encryption of RAM, whereby data in RAM is encrypted and decrypted on-the-fly inside the CPU. This protection mechanism prevents software from being able to peek into RAM locations that may contain sensitive information.

These two examples are but a few of the many that engineers have developed, and while they have all been demonstrated practically, they are still yet to become commonplace. One reason why hardware security is difficult to implement comes from the expensive nature of silicon space. Chips are designed to be as compact as possible so that more dies can fit on any given wafer, and this reduces the manufacturing cost for each die. However, integrating hardware security circuits results in a larger die size, and therefore the yield of each wafer is reduced, thereby making each device more expensive. 

Another reason why hardware circuits are difficult to integrate comes from their complex nature. True random number generators are easy enough to integrate into modern designs, but a co-processor based on AI technology that identifies malicious behaviour is highly complex to design and manufacture. Of course, such a co-processor would be excellent at stopping malware, but integrating such designs into low-end devices (such as microcontrollers) makes no sense whatsoever. 

Researchers propose digital locking circuits on spintronics

One area of electronics that continues to garner interest from researchers and engineers alike is spintronics. Spintronics relate to components that utilise electron spin and magnetic fields. Unlike traditional semiconductors, spintronic components can exhibit changes in resistance depending on the magnetic orientation of electrons. The magnetic state of electrons in such materials can often be changed either with external magnetic fields or via strong electric currents (as electric currents themselves generate magnetic fields). One particular advantage of spin devices (such as Spin FETs) is that they can retain their state once programmed with a magnetic field, enabling them to act as memory. 

While many engineers continue to explore spintronics for computational purposes, others are contemplating whether they can be used to create secure digital systems. Recently, a team of researchers published a paper regarding how spintronics can be used to create digital locking circuits that prevent circuits from operating without a particular key. 

Simply put, a digital logic lock circuit combines a logic function with a lock so that it cannot be used without a particular key. The lock itself consists of logic gates that are nested into a circuit's design, with one input being connected to the circuit being protected and the other inputs to a specialised key management system. The key management system utilises permanent on-chip memory to store the key, and a secondary user input is used to read the key provided by the user. When passed into an XOR array, a correct match enables the XOR array to provide the logic states needed to use the protected circuit. 

According to the researchers, spintronic devices (such as SpinFETs) can be used to store keys on-chip thanks to their non-volatile properties and tamper-proof design. Furthermore, the combination of SpinFETs with Magnetic Tunneling Junctions (MTJ) and digital locking circuits prevents foundries from reverse engineering circuits and copying the design due to the tamper-proof nature of spintronic components. As reverse-engineering designs are impractical, it also eliminates the ability for a manufacturer to integrate trojan hardware or backdoor systems (such as those that have been found on US equipment originating from China). 

Will digital locking systems become essential in the future? 

Currently, the implementation of locking logic circuits presents numerous challenges that make them difficult to use; one problem, in particular, is the complexity involved with adding additional logic elements into designs. Of course, this could be achieved with the use of automated software solutions that can analyse complex hardware description language files and then integrate locking circuits as needed. 

But even then, the addition of numerous locking elements not only takes up precious silicon space but can potentially impact the performance of the resulting device. The addition of logic elements increases propagation times which may not be tolerable in high-speed circuitry, and this is especially true in cutting-edge processors where protection is likely to be critical. 

Going forward, locking circuits will become a critical part in designs that need to provide a fundamental level of trust from a manufacturer's point of view. If locking circuits can be designed to carry a certificate or signature that can be validated by customers, it would be highly beneficial in applications where security is essential including communication infrastructure, power controllers, and datacentres.