Canon EOS 5D Instruction Manual page 204

3 Custom Function SettingsN
C.Fn IV -6
0: Off
1: On
Data for verifying whether the image is original or not is appended to the
image automatically. When the shooting information of an image
appended with the verification data is displayed (p.145), the <L> icon
will appear.
To verify whether the image is original, the Original Data Security Kit
OSK-E3 (sold separately) is required.
The images are not compatible with the image encryption/decryption
features of Original Data Security Kit OSK-E3.
204
Add original decision data