Dissecting leakage resilient PRFs with multivariate localized EM attacks: A practical security evaluation on FPGA

In leakage-resilient symmetric cryptography, two important concepts have been proposed in order to decrease the success rate of differential side-channel attacks. The first one is to limit the attacker’s data complexity by restricting the number of observable inputs; the second one is to create correlated algorithmic noise by using parallel S-boxes with equal inputs. The latter hinders the typical divide and conquer approach of differential side-channel attacks and makes key recovery much more difficult in practice. The use of localized electromagnetic (EM) measurements has already been shown to limit the effectiveness of such measures in previous works based on PRESENT S-boxes and 90 nm FPGAs. However, it has been left for future investigation in recent publications based on AES S-boxes. We aim at providing helpful results and insights from LDA-preprocessed, multivariate, localized EM attacks against a 45 nm FPGA implementation using AES S-boxes. We show, that even in the case of densely placed S-boxes (with identical routing constraints), and even when limiting the data complexity to the minimum of only two inputs, the guessing entropy of the key is reduced to only 248, which remains well within the key enumeration capabilities of today’s adversaries. Relaxing the S-box placement constraints further reduces the guessing entropy. Also, increasing the data complexity for efficiency, decreases it down to a direct key recovery. While our results are empirical and reflective of one device and implementation, they emphasize the threat of multivariate localized EM attacks to such AES-based leakage-resilient constructions, more than currently believed

An adaptive measurement protocol for fine-grained electromagnetic side-channel analysis of cryptographic modules

An adaptive measurement protocol is presented to increase effectiveness of fine-grained electromagnetic side-channel analysis (EM SCA) attacks that attempt to extract the information that is unintentionally leaked from physical implementations of cryptographic modules. Because measured fields vary with probe parameters as well as the data being encrypted, identifying the optimal configurations requires searching among a large number of possible configurations. The proposed protocol is a multi-step acquisition that corresponds to a greedy search in a 4-D configuration space consisting of probe’s on-chip coordinates, orientation, and number of signals acquired. This 4-D space can be extended to a 6-D space by repeating the protocol for different probe sizes and heights. This approach is presented as an alternative to current fine-grained EM SCA techniques that perform exhaustive full-chip scans to isolate information leaking locations. To demonstrate the feasibility of the approach, the protocol is tested by performing EM SCA attacks for different configurations and identifying the best attack configuration for two realizations of the advanced encryption standard (AES), subject to the precision of the measurement equipment. It is found that the protocol requires ~20× to ~25× less acquisition time compared to an exhaustive search for the optimal attack configuration.Electrical and Computer Engineerin