# Key Recovery for Content Protection Using Ternary PUFs Designed with Pre-Formed ReRAM
###### Abstract  
Physical unclonable functions, embedded in terminal devices, can be used as part of the recovery process of session keys that protect digital files. Such an approach is only valuable when the physical element offers sufficient tamper resistance. Otherwise, error correcting codes should be able to handle any variations arising from aging, and environmentally induced drifts of the terminal devices. The ternary cryptographic protocols presented in this paper, leverage the physical properties of resistive random-access memories operating at extremely low power in the pre-forming range to create an additional level of security, while masking the most unstable cells during key generation cycles. The objective is to reach bit error rates below the \(10^{-3}\) range from elements subjected to drifts and environmental effects. We propose replacing the error correcting codes with light search engines, that use ciphertexts as helper data to reduce information leakage. The tamper-resistant schemes discussed in the paper include: (i) a cell-pairing differential method to hide the physical parameters; (ii) an attack detection system and a low power self-destruct mode; (iii) a multi-factor authentication, information control, and a one-time read-only function. In the experimental section, we describe how prototypes were fabricated to test and quantify the performance of the suggested methods, using static random access memory devices as the benchmark. key recovery; content protection; cybersecurity; unclonable devices; network of IoT; tamper resistance; error correction +
Footnote †: journal: Applied Physics and Material Science, College of Engineering Informatics and Applied Sciences, Northern Arizona University, Flagstaff, AZ 86011, USA; $[EMAIL_ADDRESS]  
Footnote †: journal: Applied Physics and Material Science, College of Engineering Informatics and Applied Sciences, Northern Arizona University, Flagstaff, AZ 86011, USA; $799@nau. whether any of the components have been tampered with. If the output signal indicates that one of the components has been tampered with, the PUF corrects and mitigates the loss of secret information. Finally, [4] describes a system for recording digitally signed files using an authorization token. In this study, our interest is in terminal devices that are part of networks of the internet of things (IoTs), and which interact with a service provider. We propose end-to-end solutions that are based on the recovery of cryptographic keys from tamper-resistant PUFs, as a means of protecting the delivery and storage of digital files in a network of IoTs and terminal devices, driven by a service provider. This paper is organized as follows:  
[Section 2] In this section we present a generic description of one-way unclonable functions and physical unclonable functions. We also present how the use of ternary states has the potential to reduce the bit error rates in the part per million range (ppm). [Section 3] The architecture allowing the secure key recovery from the ternary PUFs, is shown in Section 3. The replacement of mainstream error correcting codes (ECC) by a search engine such as response-based cryptography (RBC) is suggested. We explain how the helper data needed by ECC is replaced by a message digest of the key that does not leak information. [Section 4] The overall architecture allowing the encryption and decryption of the stored digital files with the session key, is shown in Section 4. We present variations that incorporate public key infrastructures (PKIs). [Section 5] In this section we detail how the ternary PUFs can be implemented with resistive random-access memories (ReRAM) operating in the pre-forming range. We suggest methods to exploit their physical properties to enhance tamper resistance, to sense certain attacks, and to self-destruct the device, at low power, when needed. [Section 6] In Section 6, the experimental work conducted to validate the concept is presented.