The server processes the challenge and returns an activation token (response string).
You may find threads titled "Free VAG SFD calculation", but these are often community-driven efforts where a member with official access occasionally helps others for free.
What (e.g., ODIS, VCDS, OBDeleven) you are currently using. The specific year and model of your VAG vehicle. Which control module you are trying to unlock.
Never download .exe files claiming to be "Free SFD Generators" from unverified MHH Auto users with low post counts. These are often malware. Stick to web-based copy-paste calculations or open-source Python code.