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Ryan Palo
Ryan Palo

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Advent of Code 2020 Solution Megathread - Day 25: Combo Breaker

Christmas Christmas Christmas Christmas Christmas!

The Puzzle

In today’s puzzle, we finally made it! But we have to hack our way into our room by spoofing the key card’s cryptographic handshake.

Thank you so very much for working through these with me. It’s been really fun seeing everybody’s solutions and working together on these. Whatever you are celebrating or not celebrating this season, I wish you a very blessed Christmas from our family and I hope your 2021 is filled with lots of love and joy. Thanks for being an amazing community and merry coding!

The Leaderboards

As always, this is the spot where I’ll plug any leaderboard codes shared from the community.

Ryan's Leaderboard: 224198-25048a19
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If you want to generate your own leaderboard and signal boost it a little bit, send it to me either in a DEV message or in a comment on one of these posts and I'll add it to the list above.

Yesterday’s Languages

TBD (on mobile now!)

Merry Coding.

Top comments (4)

bgaster profile image
Benedict Gaster

Happy Holidays! Sad that it's over! Pleased that it was a quick one today as I was thinking I'd have to do it tomorrow... Anyway all pretty straightforward in any language and so my choice Haskell:

card = 2069194
cardSize = size card 1 0

door = 16426071
doorSize = size 7 door 1 0

step s c = c * s `mod` 20201227

size s k c n | k == c = n
             | otherwise = size s k (step s c) (n+1)

private s c 0 = c
private s c n = private s (step s c) (n-1)

main = print (private card 1 doorSize)
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rpalo profile image
Ryan Palo

I'm not ashamed to say that I set this one up and let it run. It seemed like it ran more slowly than I expected, but at this point, it is what it is, and I'm good with it.

#include "Day25.h"

/// Day 25: Combo Breaker
/// "Break" into your hotel room by brute-forcing the shared key between
/// key card and door electronics.

#include <stdio.h>

#define PUB_KEY_SUBJECT 7 ///< The number used to generate public keys

/// Transfor a number (subject) based on a number of loops through a
/// simple hashing algorithm.
static int transform(long subject, long loop_size) {
  long long value = 1;
  for (long i = 0; i < loop_size; i++) {
    value *= subject;
    value %= 20201227;
  return value;

/// A handshake consists of public keys from door and card that,
/// when looped by the other entity, come out to the same number.
typedef struct {
  long door_key;
  long card_key;
  long door_loop_size;
  long card_loop_size;
} RFIDHandshake;

/// Parse the input file, which is just two lines with one integer
/// per line.
static RFIDHandshake parse(const char* filename) {
  FILE* fp;
  fp = fopen(filename, "r");
  if (fp == NULL) {
    printf("Couldn't open input file.\n");
  RFIDHandshake key;
  fscanf(fp, "%ld\n%ld", &key.card_key, &key.door_key);
  return key;

/// Get the loop size from a public key by guessing and checking each
/// loop size until it matches.
static long loop_size(long pub_key) {
  for (long loop_size = 0; loop_size < 1000000; loop_size++) {
    if (transform(PUB_KEY_SUBJECT, loop_size) == pub_key) {
      return loop_size;
  return -1;

/// Figure out what the shared final key is between door and card.
long part1(const char* filename) {
  RFIDHandshake keys = parse(filename);
  keys.card_loop_size = loop_size(keys.card_key);
  if (keys.door_loop_size == -1) {
    printf("Couldn't get door loop size.\n");
  return transform(keys.door_key, keys.card_loop_size);

int day25() {
  printf("====== Day 25 ======\n");
  printf("Part 1: %ld\n", part1("data/day25.txt"));
  return EXIT_SUCCESS;
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thibpat profile image
Thibaut Patel

My JavaScript video walkthrough:

Thanks for putting this solution thread together Ryan!

neilgall profile image
Neil Gall

Thankfully easy enough, we've all got better things to do on Christmas Day. It was fun - see you all again next year.