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Journal of Cryptology

, Volume 27, Issue 3, pp 480–505 | Cite as

On Best-Possible Obfuscation

  • Shafi Goldwasser
  • Guy N. Rothblum
Article

Abstract

An obfuscator is a compiler that transforms any program (which we will view in this work as a boolean circuit) into an obfuscated program (also a circuit) that has the same input-output functionality as the original program, but is “unintelligible”. Obfuscation has applications for cryptography and for software protection. Barak et al. (CRYPTO 2001, pp. 1–18, 2001) initiated a theoretical study of obfuscation, which focused on black-box obfuscation, where the obfuscated circuit should leak no information except for its (black-box) input-output functionality. A family of functionalities that cannot be obfuscated was demonstrated. Subsequent research has showed further negative results as well as positive results for obfuscating very specific families of circuits, all with respect to black box obfuscation. This work is a study of a new notion of obfuscation, which we call best-possible obfuscation. Best possible obfuscation makes the relaxed requirement that the obfuscated program leaks as little information as any other program with the same functionality (and of similar size). In particular, this definition allows the program to leak information that cannot be obtained from a black box. Best-possible obfuscation guarantees that any information that is not hidden by the obfuscated program is also not hidden by any other similar-size program computing the same functionality, and thus the obfuscation is (literally) the best possible. In this work we study best-possible obfuscation and its relationship to previously studied definitions. Our main results are: (1) A separation between black-box and best-possible obfuscation. We show a natural obfuscation task that can be achieved under the best-possible definition, but cannot be achieved under the black-box definition. (2) A hardness result for best-possible obfuscation, showing that strong (information-theoretic) best-possible obfuscation implies a collapse in the Polynomial-Time Hierarchy. (3) An impossibility result for efficient best-possible (and black-box) obfuscation in the presence of random oracles. This impossibility result uses a random oracle to construct hard-to-obfuscate circuits, and thus it does not imply impossibility in the standard model.

Key words

Obfuscation 

Notes

Acknowledgements

We thank Yael Tauman Kalai and Tali Kaufman for helpful and enjoyable discussions. Thanks also to the anonymous TCC reviewers for their insightful comments which much improved (or so we hope) the presentation. We particularly thank an anonymous reviewer for suggesting a simplification to construction of un-obfuscatable circuits in the random oracle model and the proof of Theorem 5.2. Finally, we are very grateful to Oded Goldreich for his many helpful and insightful comments on this work and its presentation.

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Copyright information

© International Association for Cryptologic Research 2013

Authors and Affiliations

  1. 1.Weizmann Institute of ScienceRehovotIsrael
  2. 2.CSAILMITCambridgeUSA
  3. 3.Microsoft ResearchSilicon Valley, Mountain ViewUSA

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