Solutions to identify users in video file on online courses?

Hello to the whole community !

I'm trying to find a solution to "secure" online video courses on a new online platform. The goals are the following :

• Prevent easy download of video - OK
• Be able to identify which connected user downloaded the video if it happens - NOK

The server will be a Debian machine running PHP 7 and the "encryption" of the video will be done "on the stream" for each user.

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My question is for the second point. I'm unable to find a good solution for it. Here is what i thought of so far and the downsides :

1. Add a watermark with the user's name/email with "ffmpeg" on the whole video (in a corner)
   • Can be blurred and therefore useless
2. Add an invisible digital watermark (inside the video file) with a tool (which i have not found yet)
   • Screen can be recorded and the invisible digital watermark lost.
3. Add a watermark with the user's name/email on 2 or 3 specific frames of the video while streaming using "ffmpeg"
   • Best solution from my point of view but no technical solution found so far.

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Do you have any recommendations on how to do this or a completely different approach to the problem ?

I'm open to any third party video tool such as Vimeo or any other available.

Thank you in advance for your help ! :)