Data obfuscations are program transformations used to complicate program understanding and conceal actual values of program variables. The possibility to hide constant values is a basic building block of several obfuscation techniques. For example, in XOR Masking a constant mask is used to encode data, but this mask must be hidden too, in order to keep the obfuscation resilient to attacks. In this paper, we present a novel technique based on the k-clique problem, which is known to be NP-complete, to generate opaque constants, i.e. values that are difficult to guess by static analysis. In our experimental assessment we show that our opaque constants are computationally cheap to generate, both at obfuscation time and at runtime. Moreover, due to the NP-completeness of the k-clique problem, our opaque constants can be proven to be hard to attack with state-of-the-art static analysis tools.
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