The key file extension is frequently used by various programs for files that hold various encrypted data or keys, such as program licenses.These files are not meant to be opened directly. Programs access them to verify legally purchased licenses and such. For example antivirus programs like Avira or Kaspersky use (.key) files to store personal licenses. These files are not meant to be opened directly. Programs access them to verify legally purchased licenses and such. For example antivirus programs like Avira or Kaspersky use (.key) files to store personal licenses