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Physical attacks constitute a significant threat for any cryptosystem. Among them, Side-Channel Analysis (SCA) is a common practice to stress the security of embedded devices like smartcards or secure controllers. Nowadays, it has become more than relevant on mobile and connected devices requiring a high security level. Yet, their applicability to smartphones is not obvious, as the architecture of modern System-on-Chips (SoC) is becoming ever more complex.This paper describes how a secret AES key was retrieved from the hardware cryptoprocessor of a smartphone. It is part of an attack scenario targeting the bootloader decryption. The focus is on practical realization and the challenges it brings. In particular, catching meaningful signals emitted by the cryptoprocessor embedded in the main System-on-Chip can be troublesome. Indeed, the Package-on-Package technology makes access to the die problematic and prevents straightforward near-field electromagnetic measurements. The described scenario can apply to any device whose chain-of-trust relies on firmware encryption, such as many smartphones or Internet-of-Things nodes.

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Practical side-channel attacks on recent devices may be challenging due to the poor quality of acquired signals. It can originate from different factors, such as the growing architecture complexity, especially in System-on-Chips, creating unpredictable and concurrent operation of multiple signal sources on the device. This work makes use of mixture distributions to formalize this complexity, allowing us to explain the benefit of using a technique like Scatter, where different samples of the traces are aggregated into the same distribution. Some observations of the conditional mixture distributions are made in order to model the leakage in such context. From this, we infer local coherency of information held in the distribution as a general property of side-channel leakage in mixture distributions. This leads us to introduce how spatial analysis tools, such as Moran's Index, can be used to significantly improve non-profiled attacks compared to other techniques from the state-of-the-art. Exploitation of this technique is experimentally shown very promising, as demonstrated on two AES implementations including masking and shuffling countermeasures.