The document discusses software watermarking, explaining its purposes, types (static and dynamic), and the limitations of each. It highlights the vulnerabilities to semantics-preserving transformations and the challenges of using watermarks as proof of ownership. Ultimately, the conclusion questions the effectiveness and necessity of watermarking in software protection.

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02/20/11
Software Watermarking
James Hamilton, PhD Student
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2. Software Watermarking - Motivation
0001010101010101101111 0001010101010101101111
0101001011010101101010 0101001011010101101010
1010010101010101010101 1010010101010101010101
1010010000000101010101 1010010000000101010101
1010010101011111111010 copy 1010010101011111111010
software software
company 'thief'
2

3. Software Watermarking – what is it?
0001010101010101101111 0001010101010101101111
010100MICROSOFT11010 010100MICROSOFT11010
1010010101010101010101 1010010101010101010101
1010010000000101010101 1010010000000101010101
1010010101011111111010 copy 1010010101011111111010
Allows the
software author
to prove
ownership.
software software
company 'thief'
3

4. Software Fingerprinting – what is it?
0001010101010101101111 0001010101010101101111
0101001234567890101010 0101001234567890101010
1010010101010101010101 1010010101010101010101
1010010000000101010101 1010010000000101010101
1010010101011111111010 copy 1010010101011111111010
Allows the
software author
to prove the
source of the
copied
software software. software
company 'thief'
4

5. 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).
public class HelloWorld {
public String wm = “Microsoft”;
public static void main(String[] args) {
System.out.println(“Hello World”);
Decompile }
}
Obfuscate
5

6. Static Software Watermarking
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7. 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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8. 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
8
[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.

9. 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
9
[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.

10. 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.
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[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.

11. Graph Watermarking
• Venkatesan et al. [1] encode the watermark in a CFG and 'connect'
it to the original program.
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[1] R. Venkatesan, V. Vazirani, and S. Sinha, “A graph theoretic approach to software watermarking,” Information Hiding,
Springer, 2001, p. 157–168.

12. Example of a bad watermarking algorithm
push 4
push 52
push 34
push 12
pop
pop
pop
pop
Optimiser
push 1
push 23
push 1
push 4
pop
pop
pop
pop
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13. 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
Conclusion
• Static watermarks are not good for software protection
13

14. 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
Conclusion
• Better than static watermarks, but still not great.
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15. 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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16. Program slicing
public void w() {
int a = 1;
b = a + 1;
String wm = “mywatermark”;
return b; slicing criteria
}
Program Slice [1]: An independent program guaranteed to faithfully
represent the original program within the domain of the specified subset of
behaviour
16
[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.

17. Program slicing
public void w() {
program slice shown in red
int a = 1;
b = a + 1;
String wm = “mywatermark”;
return b; slicing criteria
}
17

18. Program slicing
public void w() {
int a = 1;
b = a + 1;
String wm = “mywatermark”; slicing criteria
return b;
}
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19. Opaque predicates
• A predicate that's outcome is known a-priori
• Can be used to protect watermarks from slicing attacks
public void w() {
int a = 1;
b = a + 1;
String wm = “mywatermark”;
if(PF) {
b = wm.length();
}
return b;
}
• Introduces false dependencies to stop slicing
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20. Problems with watermarks in general
Prove that the software
is yours
judge
here's my watermark
software software
company 'thief'
20

21. Problems with watermarks in general
who can I believe?
judge
and here's
my watermark
software software
company 'thief'
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22. Problems with watermarks in general
I've examined the watermark
recogniser carefully, and believe I that
only one recogniser is genuine
independent
software
expert
judge
software software
company 'thief'
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23. Problems with watermarks in general
show me your source-code.
independent
software
expert
judge
the real author could
demonstrate ownership
by showing the source-
code of their software
(without the need for
software watermarks)
software software
company 'thief'
23

24. 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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25. 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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26. Thanks
Any comments or questions?
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http://www.gold.ac.uk/computing http://jameshamilton.eu/