So you want to encrypt digital media, then share the media and a decryption key from two different sources. Neither on its own is the source material.
Except, M' is based on the source material and could not have been created without it.
The goods you mention (images, books, software, music, movies, etc) are funny things. Some might be spewed out very quickly with little effort on the part of the creator, while others may be a lifelong labor.
Establishing a timeline on which one can derive sole profit for such a good is difficult as a result of the difference in time it takes to create it.
Then, you add in the wrinkle of corporations owning copyrights rather than individuals. Corporations don't die, and even if they disincorporate, the copyrights are tangible assets that are sold off.
My initial response is copyrights should last no longer than the lifetime of the copyright holder. In the case of corporations holding copyrights, they should last no more than 20 years.
This view was put in place for me by JRR Tolkien's forward in a copy of
Lord of The Rings that was printed back in the 70's. Evidently another publishing house (Ace books) found a loophole in international copyright and started printing his books. I forget the exact wording of the forward, but the part that hit me was along the lines of:
...Those who believe in courtesy to living (at least) authors will buy this edition, and no other.
In my mind, the creator of M' (who also either created R or used R as a random string of bits for an encryption key) is the copyright violator. And anyone who utilizes M' and R to re-create M.