18/03/2007 Obfuscation 1

Software protection

Mariano Ceccato

FBK - Fondazione Bruno Kessler

ceccato@fbk.eu

18/03/2007 Obfuscation 2

Traditional computer security

Most computer security research: Protect the integrity of a benign host (and its data) from attacks by malicious client

programs Basis of the Java security model

Downloaded applet or virus infested application Restrict the actions that the client is allowed to perform

Software isolation A program is not able to write outside of a designated area (sandbox)

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More recent computer security

Interest in mobile agents changed the view of computer security Benign client code being threatened by host on which it has downloaded/installed

Defend a client is much more difficult than defend a host. To defend the host all is needed is to restrict the client Once the client code is in the host, the host can use any technique to violate its integrity.

Software piracy Reverse Engineering Software tampering

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Problem 1:Malicious Reverse Engineering

Valuable piece of code is extracted from an application and incorporated into competitor’s code.

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Problem 2:Software piracy

Illegal copy ad resale of applications 12 billion $ per year, major concern for everyone who sells

software Solution used in the past:

Dongle (it is weak and it annoys customers)

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Problem 3:Software tampering

E-commerce application programs contain encryption keys or other secret information. Pirates who are able to extract, modify, or otherwise tamper with this information can incur significant financial losses to the intellectual property owner.

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Problem 1:Malicious Reverse Engineering

Valuable piece of code is extracted from an application and incorporated into competitor’s code.

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Scenario

Customer Charles

Pirate Bob

Author Alice

IP

Pro

gram

Social ToolsAdvertising

Legal ToolsDMCA

TechnologicalTools

ObfuscationWatermarking

Tamperproofing

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IP In A Program

public class Fibonacci {Hashtable memo = new Hashtable();public int fib ( int n ) {

if ( !memo.contains(n) )if ( n <= 2 )

memo.put(n,1);else

memo.put(n, fib( n - 1 ) + fib( n - 2 ));return memo.get(n);

}

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Obfuscation

Obfuscation transforms a program into a new program which: Has the same semantics Is harder to reverse engineer

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Example

public class Fibonacci {

public int fib ( int n ) {if ( n <= 2 )

return 1;else

return fib( n - 1 ) + fib( n - 2 );}

}

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Example: Obfuscation

public class x {public int x ( int x ) {

return x <=2 ? 1 : x(x-1)+x(x-2);}}

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Problem 2:Software piracy

Illegal copy ad resale of applications 12 billion $ per year, major concern for everyone who sells

software Solution used in the past:

Dongle (it is weak and it annoys customers)

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Watermarking

ID

Watermarking transforms a program into a new program which: Has the same semantics Contains some additional robust identifier

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Watermarking

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Example: Watermarking

public class Fibonacci {String watermark = “Authored by Alice”;public int fibonacci ( int n ) {

if ( false )println ( “Authored by Alice” );

if ( n<=2 ) return 1;

else return fib ( n - 1 ) + fib ( n - 2 );

}

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Example: Watermarking

public class Fibonacci {

public int fib ( int n ) {if ( opaque predicate )

println ( “Authored by Alice” );if ( n<=2 )

return 1;else

return fib ( n - 1 ) + fib ( n - 2 );}

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Watermarking

Embed a structure W into a program p such that: W is easy to locate and extract from P Embedding W in P does not affect performances (cheap) Embedding W does not change statistical properties of P (static/dynamic

stealth) W has a mathematical property that allow to argue that its presence in P is the

result of a deliberate action (e.g. product of two prime numbers)

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Additive attack:

Add a second watermark to program P. Attack is effective if it is impossible to recover

temporal precedence between watermarks.

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Distortive attack:

applying semantic-preserving transformations such that: W can not be recognized P is still useful for the attacker

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Collusive attack:

Attacker buys sever copy of program P, each one with a different fingerprint.

By comparing the different copy of P, fingerprint is located Fingerprint can removed/modified

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Problem 3:Software tampering

E-commerce application programs contain encryption keys or other secret information. Pirates who are able to extract, modify, or otherwise tamper with this information can incur significant financial losses to the intellectual property owner.

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Tamper-proofing

trigger

Tamper-proofing transforms a program into a new program which:

Has the same semantics on expected input “Explodes” on when even slightly modified or on unexpected input

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Example: Tamper-proofing

public class Fibonacci {public int fibonacci ( int n ) {

String encrypted = “0x10 0x21 0x11 0xa2 0x22 0x91 0x21 0x13 0xaf 0xff 0xef 0x48 0x12 0xa2 0x22 0x00…”;

int key = “mykey”;Method decrypted = D (encrypted, key);return decrypted.invoke( n );}

}