Chapter 15 Unified Threat Management for FortiOS 5.0 : Data leak prevention : DLP document fingerprinting
  
DLP document fingerprinting
One of the DLP techniques to detect sensitive data is fingerprinting (also called document fingerprinting). Most DLP techniques rely on you providing a characteristic of the file you want to detect, whether it’s the file type, the file name, or part of the file contents. Fingerprinting is different in that you provide the file itself. The FortiGate unit then generates a checksum fingerprint and stores it. The FortiGate unit generates a fingerprint for all files detected in network traffic, and it is compared to all of the fingerprints stored in its fingerprint database. If a match is found, the configured action is taken.
The document fingerprint feature requires a FortiGate unit with internal storage. The document fingerprinting menu item does not appear on models without internal storage.
Any type of file can be detected by DLP fingerprinting and fingerprints can be saved for each revision of your files as they are updated.
To use fingerprinting you select the documents to be fingerprinted and then add fingerprinting filters to DLP sensors and add the sensors to firewall policies that accept the traffic to which to apply fingerprinting.