You can easily add your message in video files using third party softwares.
But you can try out adding texts in videos without any encryption.
Just Open any hexadecimal editors , and append your informations.
You can add info to image files as well using this method
If you need to add with encryption , you would have to modify bits (Lsb,usb)
reference : http://www.sersc.org/journals/IJDTA/vol2_no2/2.pdf