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Translingual Obfuscation Translingual Obfuscation Pei Wang, Shuai Wang, Jiang Ming, Yufei Jiang, and Dinghao Wu College of Information Sciences and Technology The Pennsylvania State University fpxw172, szw175, jum310, yzj107, [email protected] Abstract—Program obfuscation is an important software pro- making it difficult to directly reverse engineer [35]. How- tection technique that prevents attackers from revealing the ever, recent work has shown that the decode-and-dispatch programming logic and design of the software. We introduce execution pattern of virtualization-based obfuscation can translingual obfuscation, a new software obfuscation scheme be a severe vulnerability leading to effective deobfusca- which makes programs obscure by “misusing” the unique tion [21], [62], implying that we are in need of obfuscation features of certain programming languages. Translingual ob- techniques based on new schemes. fuscation translates part of a program from its original lan- We propose a novel and practical obfuscation method guage to another language which has a different program- called translingual obfuscation, which possesses strong se- ming paradigm and execution model, thus increasing program curity strength and good stealth, with only modest cost. The complexity and impeding reverse engineering. In this paper, key idea is that instead of inventing brand new obfuscation we investigate the feasibility and effectiveness of translingual techniques, we can exploit some existing programming lan- obfuscation with Prolog, a logic programming language. We guages for their unique design and implementation features implement translingual obfuscation in a tool called BABEL, to achieve obfuscation effects. In general, programming which can selectively translate C functions into Prolog pred- language features are rarely proposed or developed for icates. By leveraging two important features of the Prolog obfuscation purposes; however, some of them indeed make reverse engineering much more challenging at the binary language, i.e., unification and backtracking, BABEL obfuscates level and thus can be “misused” for software protection. both the data layout and control flow of C programs, making In particular, some programming languages are designed them much more difficult to reverse engineer. Our experiments with unique paradigms and have very complicated execution show that BABEL provides effective and stealthy software models. To make use of these language features, we can obfuscation, while the cost is only modest compared to one of translate a program written in a certain language to another the most popular commercial obfuscators on the market. With language which is more “confusing”, in the sense that it BABEL, we verified the feasibility of translingual obfuscation, consists of features leading to obfuscation effects. which we consider to be a promising new direction for software In this paper, we obfuscate C programs by translat- obfuscation. ing them into Prolog, presenting a feasible example of the translingual obfuscation scheme. C is a traditional im- 1. Introduction perative programming language while Prolog is a typical logic programming language. The Prolog language has some Obfuscation is an important technique for software pro- prominent features that provide strong obfuscation effects. tection, especially for preventing reverse engineering from Programs written in Prolog are executed in a search-and- infringing software intellectual property. Generally speak- backtrack computation model which is dramatically dif- ing, obfuscation is a semantics-preserving program trans- ferent from the execution model of C and much more formation that aims to make a program more difficult to complicated. Therefore, translating C code to Prolog leads understand and reverse engineer. The idea of using obfus- to obfuscated data layouts and control flows. Especially, the cating transformations to prevent reverse engineering can complexity of Prolog’s execution model manifests mostly be traced back to Collberg et al. [17], [18], [52]. Since then in the binary form of the programs, making Prolog very many obfuscation methods have been proposed [42], [51], suitable for software protection. [54], [61], [16], [73]. Translating one language to another is usually very Currently the state-of-the-art obfuscation technique is to difficult, especially when the target and source languages incorporate with process-level virtualization. For example, have different programming paradigms. However, we made obfuscators such as VMProtect [9] and Code Virtualizer [4] an important observation that for obfuscation purposes, lan- replace the original binary code with new bytecode, and a guage translation could be conducted in a special manner. custom interpreter is attached to interpret and execute the Instead of developing a “clean” translation from C to Pro- bytecode. The result is that the original binary code does log, we propose an “obfuscating” translation scheme which not exist anymore, leaving only the bytecode and interpreter, retains part of the C memory model, in some sense making two execution models mixed together. We believe this im- Developer Side Attacker Side proves the obfuscating effect in a way that no obfuscation methods have achieved before, to the best of our knowledge. Source-Code Binary Obfuscation Obfuscation Deobfuscation Consequently in translingual obfuscation, the obfuscation Source code Binary does not only come from the obfuscating features of the in language A target language, but also from the translation itself. With this new translation scheme we manage to kill two birds with one stone, i.e., solving the technical problems in implementing translingual obfuscation and strengthening the obfuscation Source-Code Translingual Binary simultaneously. Obfuscation Obfuscation Obfuscation Deobfuscation There may be of a concern that obfuscation techniques Source code Source code Binary without solid theoretical foundations will not withstand re- in language A in language B (and A) verse engineering attacks in the long run. However, research on fundamental obfuscation theories, despite promising pro- cess made recently [43], [31], [59], is still not mature enough Figure 1. Translingual obfuscation is a new protection layer complementary to spawn practical protection techniques. There is a widely to existing obfuscation methods, pushing the frontier forward in the battle accepted consensus that no software protection scheme is with reverse engineering. resilient to skilled attackers if they inspect the software with intensive effort [19]. A recently proved theorem [12] partially supporting this claim states that, a “universally diversity. Our experiments demonstrate that BABEL effective” obfuscator does not exist, i.e., for any obfuscation provides strong protection against reverse engineer- algorithm, there always exists an program that it cannot ing with only modest cost. effectively obfuscate. Given the situation, it seems that The remainder of this paper is organized as follows. x2 developing an obfuscation scheme resilient to all reverse provides a high-level view on the insights and features of engineering threats (known or unknown) is too ambitious at our translingual obfuscation technique. x3 explains in detail this point. Hence, making reverse engineering more difficult why the Prolog programming language can be misused for (but not impossible) could be a more realistic goal to pursue. obfuscation. We summarize the technical challenges in im- We have implemented translingual obfuscation in a tool plementing translingual obfuscation in x4. x5 and x6 present called BABEL.BABEL can selectively transform a C func- our C-to-Prolog translation method and the implementation tion into semantically equivalent Prolog code and compile details of BABEL, respectively. We evaluate BABEL’s perfor- code of both languages together into the executable form. mance in x7. x8 has a discussion on some important topics Our experiment results show that translingual obfuscation about translingual obfuscation, followed by the summary of is obscure and stealthy. The execution overhead of BABEL related work in x9. x10 concludes the paper. is modest compared to a commercial obfuscator. We also show that translingual obfuscation is resilient to one of the most popular reverse engineering techniques. 2. Translingual Obfuscation In summary, we make the following contributions in this research: 2.1. Overview • We propose a new obfuscation method, namely The basic idea of translingual obfuscation is that some translingual obfuscation. Translingual obfuscation is programming languages are more difficult to reverse en- novel because it exploits exotic language features gineer than others. Intuitively, C is relatively easy to re- instead of ad-hoc program transformations to pro- verse engineer because binary code compiled from C pro- tect programs against reverse engineering. Our new grams shares the same imperative execution model with the method has a number of advantages over existing source code. For some programming languages like Prolog, obfuscation techniques, which will be discussed in however, there is a much deeper gap between the source depth later. code and the resulting binaries, since these languages have • We implement translingual obfuscation in a tool fundamentally different abstractions from the imperative called BABEL which translates C to Prolog at the execution model of the underlying hardware. Starting from scale of subroutines, i.e., from C functions to Prolog this insight,
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