Autodesk has largely transitioned to a subscription-based model and has officially shut down activation for many legacy versions like 2013. For those seeking reliable drafting tools without the risks of piracy:
After the software is patched, the user copies a "Request Code" from the AutoCAD activation screen and pastes it into the keygen. The keygen then processes this code and generates an "Activation Code," which the user pastes back into AutoCAD to complete the "activation."
A completely free, open-source parametric 3D modeler.
If you own a legitimate copy of the software, you can activate it through official Autodesk channels: