Unbreakable invisible ink encryption could help secure paper records

Coded messages in invisible ink might sound like something from a spy novel, but they can have important security purposes in real life. Even as electronic records advance, paper is still a common way to preserve data - potentially increasingly so, in light of wholesale state-sponsored cyber-warfare hacking of computer systems.

Conventional paper information protection mainly relies on stimuli-responsive functional materials that can display color or luminescence under external stimuli. While 'invisible ink' can hide classified economic, commercial or military information from prying eyes, many popular inks contain toxic compounds and can be easily cracked if their encryption methods are predictable, such as with the use of light, heat or chemicals.

Researchers have now developed and printed complex encoded data with normal ink and a carbon nanoparticle-based invisible ink that requires both UV light and a computer that has been taught the code in order to reveal the correct hidden message.

The ink was prepared by dissolving carbon nanoparticles in water, which has a high quantum yield and outstanding light stability and salt stability, thus ensuring the integrity of information in complex environments.

Carbon nanoparticles, which have low toxicity, can be essentially invisible under ambient lighting, but can create vibrant images when exposed to ultraviolet (UV) light - a modern take on invisible ink. In addition, advances in artificial intelligence (AI) models can ensure that messages are only decipherable on properly trained computers.

Weiwei Zhao, Kang Li, Jie Xu and colleagues trained an AI model to identify and decrypt symbols printed in a fluorescent carbon nanoparticle ink, revealing hidden messages when exposed to UV light.

The researchers made carbon nanoparticles from citric acid and cysteine, which they diluted with water to create an invisible ink that appeared blue when exposed to UV light. The team loaded the solution into an ink cartridge and printed a series of simple symbols onto paper with an inkjet printer.

The team then taught an AI model, composed of multiple algorithms, to recognise symbols illuminated by UV light and decode them using a special codebook. A five-layer convolutional neural network (one of the two mainstream architectures in today’s artificial intelligence fields) was specially trained, based on the ultraviolet light-excited symbols printed by invisible ink.

Finally, they tested the AI model's ability to decode messages printed using a combination of both regular red ink and the UV fluorescent ink.

With 100 per cent accuracy, the AI model read the regular ink symbols as 'Stop', but when a UV light was shown on the writing, the invisible ink illustrated the desired message 'Begin'. As these algorithms can notice minute modifications in symbols, this approach has the potential to encrypt messages securely using hundreds of different unpredictable symbols, the researchers said.

It was possible to design unpredictable and highly complex password books to further increase information security. This smart strategy could provide new opportunities for high-level paper information encryption and also proposes new ideas for the applications of carbon nanoparticles and artificial intelligence.

The research was funded in part by the Shenzhen Peacock Team Plan and the Bureau of Industry and Information Technology of Shenzhen through the Graphene Manufacturing Innovation Centre. The team's findings were published in the American Chemical Society journal Applied Materials & Interfaces.

Looking at the world of espionage, E&T went in search of the modern spy to find out what skills a real-world secret service agent needs and to see if the 'James Bond' legend matches the reality. Perhaps a pen containing invisible ink will become standard issue in future.