We've captured this encrypted file being smuggled into the country. All we know is that they rolled their own custom CBC mode algorithm - its probably terrible.

HINT: take a second look at the file for elements needed for the crypto

https://ctf.isis.poly.edu/static/uploads/0790d21939ae55d991a8b7adfa5f59c9/bricks_of_gold_40d12e05cd6d67ed51d29a6da39d6878