General - Interesting one. Struggled to read this right and get all of the numbers/concepts (value, subject, key, ...) right. At least the parsing of the input is easy ...

Main insight was that you can calculate the next value/subject from the previous one with a loop size of 1. And this will obviously speed up the brute forcing of the loop size.

Part 1 - Brute force the card loop size and use it to calculate the door encryption key.

Part 2 - Happy Holidays!!!