Silicon Photonic Physical Unclonable Functions
Grubel, Brian Christopher
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Cryptographic systems control access to protected assets using keys stored in non-volatile digital memory which is vulnerable to tampering, substitution, and duplication. These key storage solutions require countermeasures that increase cost and complexity thus making their practical scalability to low-end systems intractable. Stored keys are usually derived from algorithms which include hash functions, random sequences, one-way permutations, and ciphers. Such approaches rely on computational asymmetry, e.g. algorithms that are easy to compute, yet difficult to reverse. However, the security of such “one-way” functions is not rigorously proven. Physical keys store private information within the physical structure of an object to protect against counterfeiting and spoofing. Recent physical key demonstrations are sufficiently complex to be believed unclonable; however, they exhibit reduced capacities for secret information, slow information generation rates, and vulnerability to emulation due to their linearity and slow speed. Here we investigate a primitive, a basic building block of a cryptographic system, based on the ultrafast and nonlinear optical interactions within an integrated silicon photonic cavity. Such a cavity can imprint a unique signature on an optical wave through the combination of a large number of spatial modes in a constrained medium. This interaction produces a highly complex and unpredictable, yet deterministic, ultrafast response that serves as a unique “fingerprint” of the cavity. This research includes methods to derive binary sequences from such waveforms that satisfy unpredictability, robustness, and entropic measures to meet security requirements for private key storage. We show that this robust physical key is unclonable, is impossible to emulate, and achieves dramatically improved capacity and throughput of secure information. We show that attempts at cloning this novel key fail, even with complete knowledge of the key design and maximally identical fabrication processes. We further demonstrate its use in authentication and communications applications and show that the device’s total information density is far beyond that of modern storage media. This innovative security measure is extremely small, inexpensive, and compatible with both conventional semiconductor integration and optical communications systems. This approach represents a revolutionary advancement in the construction of physical keys.