Day 25: Combo Breaker

Functions:

transform(subject, loop_size)	Transform the subject in loop_size steps of a hard-coded algorithm.
deduce_loop_size(subject, public_key)	Deduce the loop size for public_key by transforming the subject.
deduce_encryption_key(door_public_key, ...)	Figure out the subject number.

transform(subject: int, loop_size: int) → int

Transform the subject in loop_size steps of a hard-coded algorithm.

Requires

• subject >= 0

• loop_size >= 0

Ensures

• 0 <= result < 20201227

deduce_loop_size(subject: int, public_key: int) → int

Deduce the loop size for public_key by transforming the subject.

Returns

The loop size, or -1 if no success

Requires

• subject >= 0

Ensures

• result != -1 ⇒ transform(subject, result) == public_key

deduce_encryption_key(door_public_key: int, card_public_key: int) → int

Figure out the subject number.

Requires

• card_public_key >= 0

• door_public_key >= 0