Cryptography gets a boost with nanotechnology

How carbon nanotubes’ purity and positioning imperfections hold the key to new advances in cryptography 

This week, Nature Nanotechnology published a paper by a team of research scientists from IBM Research and the College of Nanoscale Science & Engineering at State University of New York in Albany on experimental research results that tested the use of self-assembling carbon nanotubes to make computing and communication more secure.

Silicon’s inherent physical weaknesses make even the best cryptography vulnerable. That literal leakage (from the chip baking process) gave engineers at IBM Research and State University of New York in Albany an idea; cover up those imperfections with something that doesn’t leak: self-assembling carbon nanotubes.

IBM’s manager of nanoscale science and technology Shu-Jen Han explains how this material science experiment could impact coded security.

How are silicon cryptographic keys vulnerable to security attacks?

Shu-Jen Han: Cryptography is the basis for most silicon-based computer security systems. It’s designed to protect information from unintended recipients or use. We use it every day when we swipe our ATM card, or log into a web site to make an online payment transaction. 

But the truth is, it’s easy to break in and steal information or someone’s identity. That’s because cryptographic keys, or physical unclonable function (PUF) using silicon technology, is vulnerable to counterfeiting or information leakage. Silicon PUFs rely on small process variations during chip fabrication and are very sensitive to temperature, so for example, using heat and actually baking a chip, hackers can try to discover its cryptographic key by forcing the charge that makes the device output a 1 or a 0 to leak some of this information, and then use the power consumption difference before and after the baking to estimate how many 1s and 0s were initially in the key. Relying on small mismatches also makes silicon PUFs vulnerable to voltage or temperature variations, therefore making the unreliable. 

Another vulnerability is chip tampering. It is not difficult to access a silicon cryptographic key (usually stored in memory cells) by reverse-engineering the chip – basically by physically removing materials including metals and dielectrics above memory cells, and inspecting them with microscopy or electrical testing.

What are the inherent challenges of carbon nanotubes?

SJH: Carbon nanotubes are an important material that could one day replace silicon technology – which, due to the fundamental laws of physics, will no longer be able to shrink in size past a certain point. Carbon nanotubes, allotropes of carbon with a cylindrical nanostructure, have been found to conduct electricity much faster than silicon, and use less power than silicon. But for logic technology, they also have two well-known materials issues: their purity and positioning. 

When you synthesize carbon nanotube materials they generate two distinct properties: a set of semiconducting tubes, and a set of metallic tubes. In fact, our initial solutions possess a high percentage of metallic tubes, close to a third of the overall solution. But, metal is not useful in transistors whose current flow has to be turned on and off with the help of the semiconducting channel material. Metal essentially short-circuits the transistor, so we continue to work on improving the purity of this material (our own research has resulted in a 99.99 percent pure semiconducting tube solution). The other challenge is controlling the placement, and how to orient and place these sub-one micron-long structures from the solution onto the wafer.

So while we continue to push the purity and positioning issues to enable logic technology to work, using these inherent “imperfections” of carbon nanotubes as the code actually presents a major opportunity to construct a new “perfect” cryptographic key.

What makes carbon nanotube-based technology more secure? 

SJH: Many nanomaterials exhibit some random properties; however, the randomness (or entropy) of most of them cannot be controlled. To serve as an ideal cryptographic key, the entropy has to be maximized. We are able to use the properties of carbon nanotubes to create an unclonable electronic random structure with controllable randomness.

We begin by preparing a trench structure from two different oxides, the bottom of the trench is based on hafnia (HfO2), and the sidewalls are based on silicon dioxide (SiO2), using a standard CMOS process. Then the carbon nanotubes – which are wrapped in a surfactant – are selectively attracted to the HfO2 surface (which is coated with a special monolayer). Using a form of ion exchange chemistry across the structure, the carbon nanotubes bind themselves to the monolayer on the HfO2 surface, but not on the SiO2 surface. The percentage of nanotubes successfully getting into the trenches can be precisely controlled by tuning the concentrations of the surfactant, ionic strength and the dimension of the HfO2 trenches, and the width of the trench is optimized to maximize the randomness of the nanotube placement, resulting in a higher quality of generated random bits. 

In other words, we create disorder, or randomness, out of order by using carbon nanotubes’ inherent self-assembling characteristics and attracting them into position within each trench, and then determine the connection yield and switching type of the nanotube devices to create random bit arrays.

Furthermore, carbon nanotubes are so small, 1 nm in diameter, that they’re impossible to reverse engineer without risk of destruction, which is another technique hackers use to steal information. And any attempt to bake a carbon nanotube will prove ineffective, as the material is extremely stable and insensitive to temperature-based attacks.

Schematics of random bits based on a 2D carbon nanotube array

By addressing the purity and positioning issues of carbon nanotubes, will this subsequently reduce their ability to keep a device secure?

SJH: Over time we will continue to address some of the imperfections of carbon nanotubes as a full replacement for silicon, and these will enable even better control of randomness. When you create a random key, you are controlling the size of the trench the carbon nanotubes are assembled in, as well as using other randomness caused by the mix of metallic and semiconducting tubes that reach the trench. Eliminating metallic tubes is important for purification and to enable logic. But to create a security key you don’t need to perform that process.

Random bit generation – the left image shows 64x40 random binary bits generated and the right image shows 64x40 ternary bits generated

What are the potential applications for these new kinds of cryptographic keys?

SJH: I think some of the promising areas are in IoT and connected devices, where privacy becomes important because of the constant information sharing on the Internet. With this technique you can create a more secure channel by verifying a device. This is happening today mainly at a software level but we can integrate it into the hardware. Another important application is supply chain security. Our technology can serve as a unique, permanent product identification that is nearly impossible to copy or alter. We hope that within five years this is going to be commercially ready.

How have colleagues in the security field reacted to this result?

SJH: The feedback is very positive and we are trying to expand its applications further. This is a first of its kind and it is bringing together two distinct disciplines – nanoscience and hardware security. We really hope to start a new field by blending hardware engineering with fundamental scientific research. This is not easy to do but we’re excited about the progress we have made so far.

Read more in the teams’ paper “Physically unclonable cryptographic primitives using self-assembled carbon nanotubesin Nature Nanotechnology.

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