Cyber-physical systems (CPS) are envisioned to change the whole of society. New engineered systems joining physical and digital solutions are being employed in industry, education, etc. These new systems are networked by default, and private information is shared among the different components related to users, critical infrastructures, or business operations. In this context, it is essential to encrypt those communication links to protect such information. However, even most complicated schemes based on hybrid (asymmetric and symmetric) solutions, finally require physical devices to store a secret key. This approach is cryptographically weak, as any person with physical access to the device could obtain that key. Therefore, in this paper we propose the use of physical unclonable functions (PUF) to generate secret keys for lightweight encryption schemes. Using PUFs, any attempt to capture the key is changing the original secret stream, and even manufacturers are not able to build two identical PUFs. The proposed key generator is based on magnetic materials and lightweight pseudorandom number generators to meet the low-cost and small size requirements of CPS. In particular, materials with an activated exchange-bias effect are employed, together with simple copper coils. The encryption process can be based on a simple XOR gate because of the robustness of the proposed key generator. In order to evaluate the performance of the proposed technology, an experimental validation based on simulation scenarios is also provided.

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