As far as I’m aware, the prizes operate mostly on points – however, as Harry has mentioned in a thread, they “ask the victors for detailed accounts of how they decoded the final challenge and to furnish [them] with copies of workings out or code”.
So it seems to me that if a bunch of people tied for first, they’d have a look at the accounts, see who has used entirely their own resources and who has written their own code, see who used skill over luck to break the cipher, and work from there.
Correct me if I’m wrong!