Writeup for "Play Fair, Punch!"

CRYPTO (EASY) — JERSEYCTF VI (2026)

challenge context

"The Ichikawa City Zoo has a well-kept secret regarding the lineage of their beloved rising star, Punch the Monkey. The only proof they have is this old punch card in the administrator's office. Can you figure out who it might be?"

THOUGHT PROCESS

We have a punch card with the flag in it somewhere. Let's start analyzing the punch card. There are two critical pieces of information:

'FORTRAN STATEMENT' written clearly at the top of the card
'IBM 5081', located at the very bottom left of the card (might be hard to see, so feel free to open the card and zoom-in)

DOING SOME DIGGING

We know that we have to read the card, so we take to the Internet to do so. Searching "Read IBM 5081 punch card" gives some results, including this old forum. There are many resources on reading punch cards, like this website from UIowa, and this website that has a table.

DECODING

Using the table from above and other resources, we can start to manually (and slowly) decode the card (or you could look up other punch card decoders like this or this).

There are also libraries and algorithms made to decode punch cards (in the same resources linked above) that might make it faster to decode. To be honest, I don't know how that works, but you might, so feel free to.

Regardless of how you decode, you should reach

OEGFDKGKRYRYOELTAGELGFPEWBFLRPLDCY

Not Done Yet!

Given the nature of the challenge context, the flag is probably the answer to the question. This string of letters is not really an answer, so we are missing something.

There are no other clues in the challenge context, but we do have the title. Play Fair, Punch…hmm…

THE PLAYFAIR CIPHER

The title alludes to the Playfair cipher! Playfair ciphers use a 5×5 grid of letters, and encrypt a message by breaking the text into pairs of letters and swapping them according to their positions in a rectangle within that grid (Wikipedia). These ciphers can also be encoded with a key that changes the grid of letters.

Using the key 'PUNCH', we can decode the string and get

SAMANDMISXSXSAMSPACEMACAQUEMONKEYS

(and when you remove the padding X's when dealing with duplicate letters)

SAMANDMISSSAMSPACEMACAQUEMONKEYS

which you would wrap in jctf{} and submit!

⚐ jctf{SAMANDMISSSAMSPACEMACAQUEMONKEYS}

Fun facts

The flag refers to Sam and Miss Sam, two Rhesus macaque monkeys trained at the School of Aviation Medicine [SAM] to test NASA's Mercury spacecraft escape systems. Sam flew on December 4, 1959, reaching 55 miles high, while Miss Sam launched on January 21, 1960, on a Little Joe rocket, completing a successful 8.5-minute suborbital test flight.

Punch the Monkey is also a macaque monkey (a Japanese macaque monkey), and they're not actually related, but I thought it would be cool to connect the two.

THANK YOU!

Thanks for taking the time to complete this challenge! This was one of two challenges I developed for my first CTF, and I have absolutely no experience with coding so I had to find workarounds (Figma FTW).

Developing this challenge made me appreciate the ease of storing information that we have now, and grateful to historians and collectors that keep this information alive + easily accessible.

If you're interested in reading the writeup for "Positively Going Bananas", you can check it out below.

'Positively Going Bananas' Writeup

Writeup for
"Play Fair, Punch!"