So that's generally true--Babbage's solution to the Vigenere was one of my favorite things to learn in basic crypto--but amusingly I'm not sure it would be easy in this case!

In particular, while I don't know the .mov file format backwards, the last step (after lossy encoding) is probably a lossless encoder, either LZ-type or some sort of entropy coder. This means that the plaintext is going to be a very good approximation of IID coinflips; in other words, a one-time pad _key_.

"Decoding" the repeated-key XOR also decodes the one-time-pad encryption of the repeated-key interpreted as plaintext. This is, well, hard.

(mov may have enough frame structure/boilerplate that this doesn't apply; I don't know.)