Once I had both versions of the files, I looked at them in a hex editor. Since there were some null bytes at the beginning of the video file, it wasn't hard to guess the string they were using. I'm not going to post the file themselves (for obvious reasons), but here's the first 16 bytes of the unencrypted and encrypted files (in hex and ascii, from xxd). Unencrypted:
0000000: 0000 0020 6674 7970 7174 2020 2005 0300 ... ftypqt ...
Encrypted:
0000000: 5241 4e64 2939 2623 2526 696e 6705 0300 RANd)9&#%&ing...