In the movie Shot Caller (2017), in one of the ending scenes, Money kills The Beast, but before he unlockes his own handcuffs. He uses a key but I do not get when he got it. I think that it is related with a previous scene in which he buys matches, an inhaler, razors and another item that I can not recognize.
From the wikipedia Plot section on Shot Caller (2017):
...as Harlon is about to be taken away by the corrupt correctional officer, he removes a handcuff key and a razor blade that he had smuggled in through his anus