This paper defines Spook: a sponge-based authenticated encryption with associated data algorithm. It is primarily designed to provide security against side-channel attacks at a low energy cost. For this purpose, Spook is mixing a leakageresistant mode of operation with bitslice ciphers enabling efficient and low latency implementations. The leakage-resistant mode of operation leverages a re-keying function to prevent differential side-channel analysis, a duplex sponge construction to efficiently process the data, and a tag verification based on a Tweakable Block Cipher (TBC) providing strong data integrity guarantees in the presence of leakages. The underlying bitslice ciphers are optimized for the masking countermeasures against side-channel attacks. Spook is an efficient single-pass algorithm. It ensures state-of-the-art black box security with several prominent features: (i) nonce misuse-resilience, (ii) beyond-birthday security with respect to the TBC block size, and (iii) multiuser security at minimum cost with a public tweak. Besides the specifications and design rationale, we provide first software and hardware implementation results of (unprotected) Spook which confirm the limited overheads that the use of two primitives sharing internal components imply. We also show that the integrity of Spook with leakage, so far analyzed with unbounded leakages for the duplex sponge and a strongly protected TBC modeled as leak-free, can be proven with a much weaker unpredictability assumption for the TBC. We finally discuss external cryptanalysis results and tweaks to improve both the security margins and efficiency of Spook.

At CCS 2015, Pereira et al. introduced a pragmatic model enabling the study of leakage-resilient symmetric cryptographic primitives based on the minimal use of a leak-free component. This model was recently used to prove the good integrity and confidentiality properties of an authenticated encryption scheme called DTE when the adversary is only given encryption leakages. In this paper, we extend this work by analyzing the case where decryption leakages are also available. We first exhibit attacks exploiting such leakages against the integrity of DTE (and variants) and show how to mitigate them. We then consider message confidentiality in a context where an adversary can observe decryption leakages but not the corresponding messages. The latter is motivated by applications such as secure bootloading and bitstream decryption. We finally formalize the confidentiality requirements that can be achieved in this case and propose a new construction satisfying them, while providing integrity properties with leakage that are as good as those of DTE.

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Hard learning problems have recently attracted significant attention within the cryptographic community, both as a versatile assumption on which to build various protocols, and as a potentially sound basis for lightweight (possibly side-channel and fault resistant) implementations. Yet, in this second case, a recurrent drawback of primitives based on the Learning Parity with Noise and Learning With Errors problems is their additional randomness requirements to generate noise or errors. In parallel, the move towards nanoscale devices renders modern implementations increasingly prone to various types of errors. As a result, inexact computing has emerged as a new paradigm to efficiently deal with the challenges raised by such erroneous computations, and mitigate the cost and power consumption overheads they cause. In this paper, we show that these cryptographic and electronic challenges can actually be turned into new opportunities, and provide an elegant solution one to the other. That is, we show that inexact implementations of inner product computations lead to a natural way to define new Learning with Physical Noise or Error assumptions, paving the way to more efficient and physically secure implementations, with potential interest for securing emerging Internet of Things applications.