How to prove, recognize the authorship of the video?

Question

YouTube

It is necessary to organize a video contest (participants will have to send a video), according to the terms of the contest, the video may be without sound.
Question: how can you at least indirectly check that the video was really made by the user and not downloaded from YouTube, for example. Any ideas and suggestions are accepted. File metadata? Does the tuba video have any special properties?
If you upload a video to YouTube, can you check for matches?
At the beginning of the video ask people to take down the newspaper with the date?

Answer 1

Legally, it is possible to fix in the contract (offer) or the rules of the competition their obligation to provide only the author's video and responsibility.

Answer 2

The simplest thing is to ask the participants to do something specific, what it will be depends on the conditions of the competition. All metadata can be destroyed by transcoding the video. Matches are only searched for members of the Content ID.