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Puzzle Hunt: Space Invaders
With a group of roughly 40 friends, we’re doing sort-of-annual weekends away where we hang out, watch movies and play board games in a rented group accommodation. I figured it would be fun to set up a puzzle game for this year’s iteration. Starting half a year in advance, I had plenty of time to think it over, and it kind of snowballed from there.
In this write-up I’ll walk through the puzzles and how they panned out. The game was played in Dutch and is heavy on in-jokes and group culture, but I’ll translate where relevant. The iconic alien from the video game Space Invaders has become the de-facto logo of the group; I’ve used it as an icon throughout the puzzle.
Space Invaders have landed
Arriving an hour before the group, four accomplices set out to hide 44 acrylate-cut Space Invaders throughout the building. Each space invader was engraved with a sequence number and two letters.
Six hardboard poster boards in various rooms hinted that there was something brewing..
As people started to trickle in, they initially ignored the boards in the commotion or reassured each other that their meaning would surely be explained shortly. I had no such intentions, of course! One conspicuously placed space invader attached to a stray screw in the ceiling started to draw attention, and when someone happened upon another, it was clear: the game was afoot.
The ‘landing parties’ described on the posters seems to have made people wonder whether it was a team-based game; I assured the initial group of enthusiasts that it was fully cooperative, and to get more people involved. Before long, dozens of space invaders started turning up.
Anagrams
The invaders were quickly placed near their respective boards, but the letters remained a mystery. Will it spell a sentence? Are they initials? An anagram..?
Several hours into the hunt with a dozen space invaders to go, one of the landing parties was completed and the first anagram was solved!
[E] ‘VRAAG HOTEMETOOT’
Landing party E spelled the sentence ‘VRAAG AAN HOTEMETOOT’ (‘Ask head honcho’). Each day, one of the organizers is the ‘hotemetoot’; the main point of contact for any questions. Naturally, I had slipped them an envelope and asked them to only open it when asked.
A card in the envelope suggested that the players needed to find the evaluation of $f(0)$, where $f$ is some polynomial over the finite field $GF(97)$. While a number of eyes rightfully glazed over, the group contains several professional cryptographers that immediately recognized Shamir’s secret sharing.
The QR code lead to a web page where the players were confronted with a login screen. Several months earlier I had set up an instance of an identity management system for the group. This ensured most players had accounts already, and people could not log in twice.
After logging in, each player was presented with another evaluation of $f$. But they would need 25 such points to uniquely define the polynomial and find $f(0)$!
The envelope held another card, which listed several playing card values:
Earlier, as people were settling into their rooms and making their beds, several playing cards had surfaced from underneath mattresses. Uncertain what to do with them (or whether they were even related to the puzzle), the cards had been collected on the bar. Each card was marked with a letter on its back and a sequence of cards had seemed to spell a fragment of a sentence, but there was not much to go on. Now they had a purpose!
At this point it was well past midnight, but people were literally coming back from bed when they read about the progress in the group chat. The hype was, as they say, real.
It would take until morning until sufficient evaluations of $f$ had been collected to find that $f(0) = 7$, but enough playing cards had been scraped together to decode the sentence “BEKIJK DEZE FATA MORGANA IN EEN ANDER LICHT” (‘Look at this Fata Morgana in a different light’).
Solving a 1000-piece puzzle is a staple weekend activity, and this weekend would be no different!
[F] ‘JE SLAAPT EROP’
The second anagram was solved fairly shortly after the first, spelling ‘JE SLAAPT EROP’ (‘You’re sleeping on it’). This hint obviously referred to the playing cards that had already been found, but helped confirm that they were indeed hidden beneath the mattresses rather than simply anywhere in the rooms.
As the cards had already been used to find the Fata Morgana puzzle, this branch of the puzzle was left untouched for a while. Still, people wondered why two of the playing cards were marked with dashed lines rather than regular letters..
[A] ‘STIEKEM GEWIJZIGD’
The anagram ‘STIEKEM GEWIJZIGD’ (‘Secretly changed’) was solved with seven out of eight invaders, but what had been changed?
In preparation for this puzzle, I had asked the administrator to temporarily disable notifications that announce changes to the group wiki via e-mail and our IRC channel. As it dawned on him that this was why, he successfully kept a straight face and it took the group several more minutes to figure it out.
The wiki page used to organize this weekend contained a space invader emoji (👾) that took the players to a flappy-bird-inspired mini game. Players had to click or tap to make their space invader jump and hover over the finish flag. As multiple people opened the game, they all connected to the same instance. Sufficiently many invaders had to hover over the finish flag at the same time.
Winning the game presented the players with the number 4.
[C] ‘VRAAG AAN IRC BOT’
The anagram ‘VRAAG AAN IRC BOT’ (‘Ask IRC bot’) required players to ask our community IRC bot a question. Several questions would lead to the correct solution here; asking the bot about space invaders presented the players with a link to an audio fragment.
The audio fragment spelled out a sentence in morse code: “HET GETAL IS VIJF” (‘The number is five’). The solution for this branch of the puzzle was 5.
[B] ‘ZIE VORIG JAAR’
With just the ‘ZI’ missing, the anagram ‘ZIE VORIG JAAR’ (‘See last year’) was solved. At this point people were really heading to bed, though, and work on this branch of the puzzle had to wait until morning.
Browsing through photo albums, one of the system administrators found I had uploaded a photo to a seemingly unrelated album. Indeed, this had been an artifact of a test for the real puzzle, where I had managed to leave less traces..
Several of the photos in the album of the group weekend in November of last year now also contained space invaders, edited into the background.
Through a combination of manually thumbing through the pages and some digital forensics, the players were able to identify fourteen doctored photos. The space invaders on these photos spelled out characters of the braille alphabet. Altogether, they formed the sentence ‘IK ZIE EEN DRIE’ (‘I see a three’); the number we were looking for here was 3.
[D] ‘MAAND MINSTE JARIG’
The final anagram proved difficult, not in the least because the ‘J’ was initially misread as an ‘I’ and one of the space invaders (‘MA’) was still missing; this stumped even the most thorough dictionary searches.
When the anagram was finally solved to be ‘MAAND MINSTE JARIG’ (‘Month of least birthdays’), discussion kicked off who was to be included in the count. All group members? Everyone in the world, statistically? All people present? People drew their phones to look at their calendars, and hypothesized that January was notably empty..
Several months ago I set up a shared address book for the group. While this helped me prepare the puzzle, I could not allow the players the same benefit, of course – I replaced the calendar’s web page with a test card image, and sabotaged the CardDav integration.
In response to the creation of the shared calendar, one of the group members had equipped our IRC bot with a script that congratulated people on their birthdays. As I sabotaged the CardDav feed, I hoped to also cripple the script for the duration of the puzzle. Unfortunately it was well-engineered, and, while it did crash because of missing data, it also contained debug logs that listed all birthdays. Using these logs, the players confirmed that January was indeed the correct answer.
As January is the first month, the number we were looking for here was 1.
Anagram ‘F’, revisited
While work continued on the 1000-piece puzzle, the only other remaining lead was the fact that two of the playing cards had a dashed letter. People were convinced the riddle hinted at using a UV light to read a hidden message on the puzzle, but the cards would not budge.
Finally, after the puzzle was finished and everyone was focussed on the single task at hand, the cards were sorted from 2 to A and clubs to spades ascending.
The cards revealed the message “BIJ DEZE PUZZELHULP KOMT DRIE VOOR TWEE EN VANDAAG NA MORGEN” (‘In this puzzle help, three comes before two and today after tomorrow’), with the dashed C and F at the end; these were not needed to form this riddle, but were used as part of the ‘Fata Morgana’ sentence in the earlier card-based puzzle.
The solution to the riddle is a dictionary! Sprinting to the central bookcase, the players quickly pulled out a hefty ‘dictionary for puzzle-solving’.
The long-awaited blacklights! Everyone gathered in anticipation as the room was dimmed and the lights were aimed at the solved 1000-piece puzzle..
Coordinates! Slotting in the numbers from earlier riddles in place of the letters pointed to a location just outside the house, in a communal playground..
.. where a little wooden space invader marked the spot.
Treasure! The players dug up a chest containing Space Invader pins, marking the end of the puzzle.