Abstract
Digital hardware Trojans are integrated circuits whose implementation differ from the specification in an arbitrary and malicious way. For example, the circuit can differ from its specified input/output behavior after some fixed number of queries (known as “time bombs”) or on some particular input (known as “cheat codes”).
To detect such Trojans, countermeasures using multiparty computation (MPC) or verifiable computation (VC) have been proposed. On a high level, to realize a circuit with specification \(\mathcal{F}\) one has more sophisticated circuits \(\mathcal{F}^\diamond \) manufactured (where \(\mathcal{F}^\diamond \) specifies a MPC or VC of \(\mathcal{F}\)), and then embeds these \(\mathcal{F}^\diamond \)’s into a master circuit which must be trusted but is relatively simple compared to \(\mathcal{F}\). Those solutions impose a significant overhead as \(\mathcal{F}^\diamond \) is much more complex than \(\mathcal{F}\), also the master circuits are not exactly trivial.
In this work, we show that in restricted settings, where \(\mathcal{F}\) has no evolving state and is queried on independent inputs, we can achieve a relaxed security notion using very simple constructions. In particular, we do not change the specification of the circuit at all (i.e., \(\mathcal{F}=\mathcal{F}^\diamond \)). Moreover the master circuit basically just queries a subset of its manufactured circuits and checks if they’re all the same.
The security we achieve guarantees that, if the manufactured circuits are initially tested on up to T inputs, the master circuit will catch Trojans that try to deviate on significantly more than a 1/T fraction of the inputs. This bound is optimal for the type of construction considered, and we provably achieve it using a construction where 12 instantiations of \(\mathcal{F}\) need to be embedded into the master. We also discuss an extremely simple construction with just 2 instantiations for which we conjecture that it already achieves the optimal bound.
T. Lizurej—Stefan Dziembowski, Małgorzata Gałązka, and Tomasz Lizurej were supported by the 2016/1/4 project carried out within the Team program of the Foundation for Polish Science co-financed by the European Union under the European Regional Development Fund.
K. Pietrzak—Suvradip and Krzysztof have received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (682815 - TOCNeT).
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Notes
- 1.
We show that a small fraction of wrong outputs must be allowed in Sect. 2.4. The iid assumption can be somewhat relaxed, but as we don’t have a clean necessary condition we will not discuss this further in this paper. Informally, a sufficient condition seems to just require that there is no (efficiently recognisable) subset of inputs which appear rarely (not more than with probability around 1/T) but can come in “bursts”, say two such inputs are consecutive with prob. \(\gg 1/T^2\).
- 2.
It’s acceptable by our construction if the inputs are iid conditioned on some secret, so the master on input x and key k can forward (k, x) to the circuits. Alternatively the key k can be hard-coded in the circuit (probably not a good idea if the manufacturer is not trusted in the first place) or, if the circuits have some storage, one can give them k after receiving the circuits from the manufacturer.
- 3.
To encrypt m sample a random r and compute the ciphertext \((r,\mathcal{F}(k,r)\oplus m)\).
- 4.
We consider much stronger \(\mathsf{M}^*,\mathsf{T}^*\) for the lower bounds compared to what we require in the constructions as discussed in Sect. 2.5.
- 5.
Let us mention that the opposite is not true (it’s possible that for some \(i\ne j\) we have \(\mathsf{F}'_i(x)=\mathsf{F}'_j(x)=1\), while \(\mathsf{F}_i(x)\ne \mathsf{F}_j(x)\)). This just captures the observation that an adversary who wants to deviate as often as possible without being detected can wlog. always deviate to the same value.
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Chakraborty, S., Dziembowski, S., Gałązka, M., Lizurej, T., Pietrzak, K., Yeo, M. (2021). Trojan-Resilience Without Cryptography. In: Nissim, K., Waters, B. (eds) Theory of Cryptography. TCC 2021. Lecture Notes in Computer Science(), vol 13043. Springer, Cham. https://doi.org/10.1007/978-3-030-90453-1_14
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