Open Source QT vs Paid QT

[probine]

What if...

What if I develop a program using the Open source edition of QT and I start selling it. Well, that is not legal as far as I also deliver the source code, but how could someone know if the program was created using the Free Edition or the Paid Edition?

[wysota]

It's illegal. Period. And the library has information about the licence embedded into itself.

[elcuco]

It's illegal. Period. And the library has information about the licence embedded into itself.

Excuse me... it's illegal to sell GPL based applications...? Since when...?

[wysota]

Excuse me... it's illegal to sell GPL based applications...? Since when...?

No, it's not, as long as you publish the sources. But I think the hidden question was "Can someone find out that a closed-source application was developed using open source library?"

[pdolbey]

Interesting. But what if I write an application (say just a simple main.cpp) with the open source version and I sell the resulting exe to someone who has also been sold Qt runtimes from the commercial version by another vendor. The application will still run (I've tried it a while ago) so how can you tell which version of Qt was used to write (or link) it. And yes I Know this is still illegal - its any stamping of signatures in the executable that interests me.

Pete

[wysota]

In that case you can't see the difference. You probably could if you built Qt with a custom build key, although I'm not sure. But it's enough to have a modified version of the libraries (with some functionality cut out) and the resulting exe will not link against a different library though to rellocation errors. At least I think so.

[jacek]

its any stamping of signatures in the executable that interests me.
It's hard to construct such system because you can simply edit the executable and get rid of it. Certainly it should involve some cryptography.

I've heard about a method in which the global offset table was scrambled and only a hardware key could produce proper addresses, but it's more like just keeping crackers away long enough to produce some profit, than a real solution.

[wysota]

Some idea is to keep not the code but the data hidden (encrypted). It should be harder to modify the binary in such a way that encryption/decryption of data gets disabled. It's still a battle against time, but safes or locks share the same idea - keep the bad guy away just long enough.