File used by Clonk, a strategy game where players control humanoid "Clonks;" saved in plain text and contains an encoded license key used for activating the software; also includes other license information such as an ID, the player name, date of the game purchase, and the software version.

C4K files are used for both offline and online access to Clonk. C4K files that are used by multiple game installations may cause online access to be terminated. NOTE: C4K files are typically delivered to a user through email after purchasing the software online. This file should be placed in the application directory in Windows to install the license key. For Mac OS X, the C4K file can be opened manually, which will start the game and register the software.