Protection against cracking - specifically ways to make a program harder to decompile

I'm making a commercial product that will have a client and server side开发者_开发百科. The client is totally dependent on the server , just to make it harder to crack/pirate . Problem is , even so there is a chance that someone will reverse engineer the protocol and make their own server.

I've thought about encrypting the connection either with ssl or with another algorithm so it won't be so easy to figure out the protocol just from sniffing the traffic between the client and the server.

Now the only thing I can think of that pirates would use is to decompile the program, remove the encryption and try to see the "plain text" protocol in order to reverse engineer it.

I have read previous topics and I know that it's impossible to make it impossible to crack , but what tweaks can we programmers bring to our code to make it a huge headache for crackers?

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Read how Skype did it:

• The binary is decrypted into memory at startup.
• The import table is overwritten.
• The startup code is erased from memory.
• Code integrity checks bust most debuggers: in random points in the code it computes a checksum of some other chunk of code and uses the checksum for an indirect jump to the next instruction. (Explanation: most debuggers implement breakpoints by changing the instruction at the breakpoint address. This check detects that.)
• If debugger is detected -- it scrambles the registers and jumps to a random page.
• Obfuscates code: call destination addresses are dynamically computed; dummy branches that are never executed; raises SEH where the handler sets some registers and resumes execution.

Keep in mind that these or other techniques would make reverse engineering harder, but not impossible. Also you shall never rely on any of these for security.

IMO your best option is to design your servers to provide some useful functionality (SaS). Your clients will essentially be paying for using that functionality. If your client-app is dumb enough, you won't care about it being open-source.

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One thing you need to be aware of is that most packers/cryptors cause false positives with virus scanners. And that can be pretty annoying because people complain all the time that your software contains a virus(they don't get the concept of false positives).

And for protocol-obfuscation don't use SSL. It is trivial for an attacker to intercept the plaintext when you call Send with the plain-text. Use SSL for securing the connection and obfuscate the data before sending them. The obfuscation algorithm doesn't need to be cryptographically secure.

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This might be helpful: http://www.woodmann.com/crackz/Tutorials/Protect.htm

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IMHO, it's difficult to hide the actual plain code. What most packers do is to make it difficult to patch. However, in your case, Themida could do the trick.

Here are some nice tips about writing a good protection: http://www.inner-smile.com/nocrack.phtml