A method of processing a video stream comprises receiving a first/second set of input image frames comprising image data originating from a first/second source of input image frames; encoding the sets of input image frames into first/second sets of encoded image frames, forming a combined video stream, wherein an encoded image frame of the first set of encoded image frames exclusively references one or more other encoded image frames of the first set of encoded image frames, and an encoded image frame of the second set of encoded image frames references both an encoded image frame of the first set of encoded image frames and another encoded image frame of the second set of encoded image frames; and signing, using a first/second key, a first/second signing group comprising one or more encoded image frames of the first/second set of encoded image frames, wherein each key pertains to the respective source.