Can you tell the difference between these two animated gifs? They are different. Open each in a hex editor if you don't believe me or... use this program to decode the secret message in the second one!
make giffy
Encode a message:
$ ./giffy.exe e [input-image-file] [output-filename] [message-string]
Decode a message:
$ ./giffy.exe d [input-image-file]
Encode:
$ ./giffy.exe e /input/path/to/file /desired/output/my_filename 'hey Lindsay, what is new with you?'
Decode:
$ ./giffy.exe d ~/my_filename
hey Lindsay, what is new with you?
It's just like passing notes in class.