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International audience; To compensate for the poor reliability of Physical Unclonable Function (PUF) primitives, some low complexity solutions not requiring error-correcting codes (ECC) have been proposed. One simple method is to discard less reliable bits, which are indicated in the helper data stored inside the PUF. To avoid discarding bits, the Two-metric Helper Data (TMH) method, which particularly applies to oscillation-based PUFs, allows to keep all bits by using different metrics when deriving the PUF response. However, oscillation-based PUFs are sensitive to side-channel analysis (SCA) since the frequencies of the oscillations can be observed by current or electromagnetic measurements. This paper studies the security of PUFs using TMH in order to obtain both reliable and robust PUF responses. We show that PUFs using TMH are sensitive to SCA, but can be greatly improved by using temporal masking and adapted extraction metrics. In case of public helper data, an efficient protection requires the randomization of the measurement order. We study two different solutions, providing interesting insights into trade-offs between security and complexity.

For asymmetric ciphers, such as RSA and ECC, side-channel attacks on the underlying exponentiation are mitigated by countermeasures like constant-time implementation and blinding. This restricts an attacker to a single side-channel trace for an attack as a different representation of the private key is used for each exponentiation. In this work, we propose an unsupervised machine learning framework for side-channel attacks on asymmetric cryptography that analyzes leakage in multiple side-channel traces, identifying the best trace for key retrieval. We apply Principal Component Analysis (PCA) preprocessing followed by a classification step that assigns segments of traces to elementary operations of the Square and Multiply exponentiation of RSA. In order to estimate the attack complexity for each trace in terms of key enumeration effort, we introduce two new metrics: The Entropy-based Cost Function (EBCF) is used to select a trace for the attack as well as bits which have to be brute-forced if not all bits can be determined correctly from this single trace. To reduce brute-force complexity further, we introduce Illegal Sequence Detection (ISD) to remove brute-force candidates which do not fit to the Square-and-Multiply scheme. We first provide a proof of concept for 320-bit key length traces and, moving towards a more realistic scenario, retrieve the key from a 1024-bit RSA implementation protected by message and exponent blinding. We are able to select the trace with the least remaining brute-force complexity from 1000 power measurements of the signature generation with randomized inputs and blinding values on a 32-bit ARM Cortex-M4 microcontroller.

International audience; Physical Unclonable Functions (PUFs) provide means to gen-erate chip individual keys, especially for low-cost applications such as theInternet of Things (IoT). They are intrinsically robust against reverseengineering, and more cost-effective than non-volatile memory (NVM).For several PUF primitives, countermeasures have been proposed to mit-igate side-channel weaknesses. However, most mitigation techniques re-quire substantial design effort and/or complexity overhead, which can-not be tolerated in low-cost IoT scenarios. In this paper, we first ana-lyze side-channel vulnerabilities of the Loop PUF, an area efficient PUFimplementation with a configurable delay path based on a single ringoscillator (RO). We provide side-channel analysis (SCA) results frompower and electromagnetic measurements. We confirm that oscillationfrequencies are easily observable and distinguishable, breaking the se-curity of unprotected Loop PUF implementations. Second, we presenta low-cost countermeasure based on temporal masking to thwart SCAthat requires only one bit of randomness per PUF response bit. The ran-domness is extracted from the PUF itself creating aself-secured PUF.The concept is highly effective regarding security, low complexity, andlow design constraints making it ideal for applications like IoT. Finally,we discuss trade-offs of side-channel resistance, reliability, and latency aswell as the transfer of the countermeasure to other RO-based PUFs

Physical Unclonable Functions (PUFs) have the potential to provide a higher level of security for key storage than traditional Non-Volatile Memory (NVM). However, the susceptibility of the PUF primitives to non-invasive Side-Channel Analysis (SCA) is largely unexplored. While resistance to SCA was indicated for the Transient Effect Ring Oscillator (TERO) PUF, it was not backed by an actual assessment. To investigate the physical security of the TERO PUF, we first discuss and study the conceptual behavior of the PUF primitive to identify possible weaknesses. We support our claims by conducting an EM-analysis of a TERO design on an FPGA. When measuring TERO cells with an oscilloscope in the time domain, a Short Time Fourier Transform (STFT) based approach allows to extract the relevant information in the frequency domain. By applying this method we significantly reduce the entropy of the PUF. Our analysis shows the vulnerability of not only the originally suggested TERO PUF implementation but also the impact on TERO designs in general. We discuss enhancements of the design that potentially prevent the TERO PUF from exposing the secret and point out that regarding security the TERO PUF is similar to the more area-efficient Ring Oscillator PUF.

Physical Unclonable Functions (PUFs) provide a cost-efficient way to store a secure key on a device. But the noisy secret from a PUF must be corrected to generate a stable key. Since the error correction processes secret material, it is a target of attacks. Previous work has shown that single bits of a key can be extracted using a power side-channel attacks. This work enhances the attack idea. Non-invasive measurement of electromagnetic radiation together with a differential power analysis is shown to be sufficient to extract not only single bits but even the complete key from an error correction used for PUF-based key generation. The efficiency of the basic attack is significantly improved over state of the art using public available preknowledge on the PUF, an advanced correlation method, and parallel manipulation of helper data. The attack is practically demonstrated on an FPGA implementation with concatenated BCH and repetition codes. The results show that, compared to state of the art, a significant improvement by a factor of more than 100 in terms of trace reduction can be achieved.