Research On Program Obfuscation Under Cloud Service Outsourcing

Obfuscation is a semantic-preserving compiler that transforms a program/circuit into an obfuscated one while preserving the functionality of the original program,the run time of the obfuscated program is polynomial down,comparing with that of the original one,and it is impossible to obtain any information from the obfuscated program.Obfuscation can theoretically solve the problem of reverse-engineering to software code,and thus can be used to copy protection,watermarking,software copyright protection etc.Obfuscation as emerging research field,Barak et al.and Hada were firstly able to make this definition of obfuscation mathematically rigorous,but then Barak et al.showed that it is impossible to achieve virtual black-box for general circuit.Indeed in the same work(Barak et.al)they suggested a weaker notion called indistinguishability obfuscation(iO).In 2013,Garg et al.proposed a candidate construction of obfuscator satisfying the iO definition.Moreover,many follow-up works have been presented.In the thesis,we focus on secret information leakage from re-encryption program running on a third party(such as a cloud server),and construct an obfuscating re-encryption and design a proxy re-encryption from iO,and prove the security for obfuscator respectively.The main research contributions of our thesis are listed as follow:Firstly,we classify the definition and security of obfuscation,and study the correlation of different obfuscation by combining our results.We study obfuscations of different kinds of functions,and briefly describe cryptographic tools and prove security.Secondly,we propose an obfuscating re-encryption,and then securely obfuscated it.We prove that the obfuscated program achieves virtual black-box security.In order to ensure the privacy and confidentiality of re-encryption program,we use the technique of constant-hiding obfuscation to prevent the server from capturing the sensitive data(i.e.,secret key and underlying plaintext).The obfuscated program can be executed on an untrusted server,and even the server sets or traces the breakpoints during the program running.The malicious server cannot gain the result of the program in the middle.Finally,we design proxy re-encryption via iO,and prove its chosen ciphertext attack security in the standard model.The scheme combines iO and puncturable pseudorandom functions,and the obfuscated program can be run on any untrusted third-party proxy or cloud server.After,the obfuscated program preserves the same functionality as original program,and both of them can be performed re-encryption of the ciphertext.But it hard to leak any information due to the security of obfuscation.