02/20/11Software WatermarkingJames Hamilton, PhD Student

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Software Watermarking - Motivation

software company

software 'thief'

00010101010101011011110101001011010101101010101001010101010101010110100100000001010101011010010101011111111010

00010101010101011011110101001011010101101010101001010101010101010110100100000001010101011010010101011111111010copy

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Software Watermarking – what is it?

software company

software 'thief'

0001010101010101101111010100MICROSOFT11010101001010101010101010110100100000001010101011010010101011111111010 copy

0001010101010101101111010100MICROSOFT11010101001010101010101010110100100000001010101011010010101011111111010

Allows the software author

to prove ownership.

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Software Fingerprinting – what is it?

software company

00010101010101011011110101001234567890101010101001010101010101010110100100000001010101011010010101011111111010 copy

Allows the software author

to prove the source of the

copied software.

00010101010101011011110101001234567890101010101001010101010101010110100100000001010101011010010101011111111010

software 'thief'

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Software Watermarking – what is it not?

• It does not prevent copying software.• It does not prevent decompilation or program understanding.• But it could be used in-conjunction with obfuscation (need to be

careful the obfuscation doesn't remove the watermark).

Obfuscate

Decompile

public class HelloWorld { public String wm = “Microsoft”; public static void main(String[] args) { System.out.println(“Hello World”); }}

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Static Software Watermarking

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Dynamic Software Watermarking

• Embeds code to generate a watermark at run-time.• Recogniser uses a debugger to extract watermark.• Should be resilient to semantics-preserving transformations.

.

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Code Replacement & Addition

• Very basic, early algorithms simply replaced sections of code, or data, with watermark code.

• Susceptible to collusive attacks if the watermarks are placed in the same location in every copy of a program.

• Monden et al. [1] encode the watermark as a sequence of bytecode sequences which replace instructions in a dummy method.

– difficult to generate code which is similar to the original program– easily remove by semantics-preserving transformations

[1] A. Monden, H. Iida, K.-ichi Matsumoto, K. Inoue, and K. Torii, “A practical method for watermarking java programs,” Computer Software and Applications Conference, 2000. COMPSAC 2000. The 24th Annual International, Washington, DC, USA: IEEE, 2002, p. 191–197.

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Code Re-Ordering

• The watermark is encoded as the nth permutation of some set

• Davidson and Myhrvold [1] encode the watermark as the nth permutation of the set of basic blocks in a method.

– higly unstealthy due to a greater ratio of goto instructions [2]

• Another option, in Java, is to re-order the constant pool (Gong et al.)

• Requires the original program from comparison

[1] R.I. Davidson and N. Myhrvold, “Method and system for generating and auditing a signature for a computer program,” US Patent 5,559,884, Sep. 1996.[2] Myles, G. et al., 2005. The evaluation of two software watermarking algorithms. Softw. Pract. Exper., 35(10), 923–938.[3] D. Gong, F. Liu, B. Lu, and P. Wang, “Hiding Informationin in Java Class File,” International Symposium on Computer Science and Computational Technology, 2008. ISCSCT ’08., IEEE Computer Society, 2008, pp. 160-164.

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Register Allocation

• the watermark is encoded in the interference graph, which is used to model the relationship between variables.

• each vertex represents a variable and an edge between the two variables indicates that their live ranges overlap.

• we colour the graph in order to minimise the number of registers and ensure that two live variables do no share a register.

• QP algorithm [1] adds edges to the graph

• QP is flawed; QPS, QPI, CC and CP followed with a similar idea.

[1] G. Qu and K. Potkonjak, “Analysis of watermarking techniques for graph coloring problem,” Computer-Aided Design, 1998. ICCAD 98. Digest of Technical Papers. 1998 IEEE/ACM International Conference on, San Jose, California, United States: IEEE, 2005, p. 190–193.

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Graph Watermarking

• Venkatesan et al. [1] encode the watermark in a CFG and 'connect' it to the original program.

[1] R. Venkatesan, V. Vazirani, and S. Sinha, “A graph theoretic approach to software watermarking,” Information Hiding, Springer, 2001, p. 157–168.

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Example of a bad watermarking algorithmpush 4push 52push 34push 12poppoppoppoppush 1push 23push 1push 4poppoppoppop

Optimiser

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Problems with static watermarks in general

• can be unstealthy, if the watermark code is compared with 'normal' code

• highly susceptible to semantics-preserving transformations– static watermarks rely on syntactic properties

• without perfect tamper-proofing techniques an attacker can apply any semantics-preserving transformation to a program

– tamper-proofing is hard & unstealthy• especially in Java

• Static watermarks are not good for software protection

Conclusion

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What about dynamic watermarks?

• in theory, should not be susceptible to semantics-preserving transformations (but some current ones are).

• but, they can only protect a complete program rather than single modules, classes, methods etc.

• can be susceptible to additive attacks

• Better than static watermarks, but still not great.

Conclusion

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Watermark Stealthiness

• Some watermarking algorithms are unstealthy– easy to find– 'strange looking' code– doesn't act like the rest of the program

• statistical analysis of instructions (e.g. ratio of goto instructions)• program slicing metrics

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Program slicing

public void w() { int a = 1; b = a + 1; String wm = “mywatermark”; return b;}

slicing criteria

[1] Weiser, M., 1981. Program slicing. In ICSE '81: Proceedings of the 5th international conference on Software engineering. Piscataway, NJ, USA: IEEE Press, p. 439―449.

Program Slice [1]: An independent program guaranteed to faithfully represent the original program within the domain of the specified subset of behaviour

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Program slicing

public void w() { int a = 1; b = a + 1; String wm = “mywatermark”; return b;}

slicing criteria

program slice shown in red

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Program slicing

public void w() { int a = 1; b = a + 1; String wm = “mywatermark”; return b;}

slicing criteria

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Opaque predicates

• A predicate that's outcome is known a-priori• Can be used to protect watermarks from slicing attacks

• Introduces false dependencies to stop slicing

public void w() { int a = 1; b = a + 1; String wm = “mywatermark”; if(PF) { b = wm.length(); } return b;}

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Problems with watermarks in general

software company

software 'thief'

judge

Prove that the software is yours

here's my watermark

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Problems with watermarks in general

software company

software 'thief'

judge

and here's my watermark

who can I believe?

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Problems with watermarks in general

software company

software 'thief'

judge

independent software expert

I've examined the watermark recogniser carefully, and believe I that

only one recogniser is genuine

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Problems with watermarks in general

software company

software 'thief'

judge

independent software expert

show me your source-code.

the real author could demonstrate ownership by showing the source-code of their software (without the need for software watermarks)

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Decompilation

• The attacker could decompile the the program to get their own source-code to demonstrate they own the code

• But it will 'look' decompiled– incoherent variable names– no comments– verbose code– extraneous instructions

• The attacker will probably have little understanding of the code and will have trouble answering trivial questions about it

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Conclusion

• There are many static watermarking algorithms but they are all susceptible to trivial semantics-preserving transformation attacks.

• Dynamic watermarks are better but can also be susceptible to attacks.

• Unstealthy watermarks give an attacker clues and allow them to remove a watermark easier.

• Program slicing may be able to give clues about watermarks, and/or help remove them.

• Maybe watermark is not actually needed or useful.

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Any comments or questions?

Thanks

http://jameshamilton.eu/http://www.gold.ac.uk/computing