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    <p>One approach would be to put the code that does the license check into a separate DLL. In the main application, load the DLL at runtime and calculate the checksum of the DLL itself. The app stores the checksum that was calculated with the DLL was built. If the checksums don't match, you have several options, show a wrong-version message - a bit obvious; Do not call the license check - less obvious but will be noticed when the attacker wonders why the license check doesn't get called; call a function with a similar name to the real license-check function.</p> <p>Think of it as using <strong>Public Key Encryption</strong>. Use a public key as part of the config and have a private key built into the app. If they mess with the public key, the digital signature of the app will be compromised in a detectable way. </p> <p>I agree with @camccann that it would help to understand the kind of attack you expect. As a last resort, split the license-check into as many parts as is feasible to make it harder to bypass by changing a single branch point. </p> <p>[EDIT]</p> <p>Another thought would be to use a <strong>State Machine</strong>. See the command structure example in the top answer to <a href="https://stackoverflow.com/questions/1647631/c-state-machine-design">this question</a>. Put the evaluation of the license check into the form of a hash lookup and a set of dummy function calls into an array along with the proper one. The decision code that evaluates the license check into a table/hash lookup for the appropriate function will not look like your typical </p> <pre><code>if(){ pass;} else { fail; } </code></pre> <p>construct.</p> <p>Two benefits,<br> 1) there isn't a boolean condition to bypass and<br> 2) they can't do a simple JMP instruction without knowing the address/name of the function to pass control to.</p> <p><a href="https://stackoverflow.com/questions/1371460/state-machines-tutorials">SO thread on a state machine turorial</a>.<br> <a href="https://stackoverflow.com/questions/133214/is-there-a-typical-state-machine-implementation-pattern">SO thread on state machine implementations</a></p>
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    1. POHow can I obfuscate a test in code to prevent tampering with response processing?
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    1. COUnfortunately, I feel this misses the area the OP was concerned about. He isn't so much worried about the actual process of checking the license. Instead, he's getting a boolean that represents a pass/fail, and needs to operate specific code based on that condition. However, this boils down to a very simple jmp in the assembly, and a simple hex editor can subvert the conditional check rather trivially if they know where it is. He wants to obfuscate the location of this jmp (if statement), not the actual license check itself.
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    2. CO@KevenK: Thank you so much. This is correct.
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    3. CO@KevenK I edited the post to cover hiding the branch.
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