No, it is not secret.

I am using CGZiplibrary.dll. This library and the source code (VB6) are freely available at http://www.codeguru.com/vb/gen/vb_graphics/fileformats/article.php/c6743 .

There is a hint: you have to change a flag in the source code do make the library save relative path, not full path. If it saves the full path the .kmz will not work in GE. If you need more information let me know.