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Write Your Own Riddles ----The Skull CTF

by: Nov 05,2020 232 Views 0 Comments Posted in Activities

PCB. MCU GPIO hardware mods badge riddles hardware The Skull CTF

“Centuries ago, she was Queen of the Pirates.

Now, you are holding her skull in your hands.

A tiny piece of her mind remains within,

barely alive, and it holds a dark secret.

Awakened by a gentle touch, her eyes will glimmer,

and cease shortly after. They say the eyes are

windows to the soul, and this lost soul is waiting

for you to uncover her secret and set her free.

Will you take up her quest?”


Do you Love Riddles?

The Skull is a Capture the Flag (CTF) riddle that invites you to explore the internals of the AVR architecture. In order to keep your sanity intact, you will need three things: the ability to read and understand Arduino code, a willingness to spend some quality time with the ATtiny45 datasheet, and basic programming skills. You will not need any special hardware.

A 3D-printable model for these glasses will be made available in our GitHub repository. They are intended for cosmetic use only and should not be prescribed as an optometric intervention for the undead.

What can you do? 

When it’s not breaking your brain, the Skull doubles as a decorative badge that flashes its eyes when you touch a small contact panel on the back of its head.

Write your own riddles! The Skull will keep you busy for a long time as it is, but her story doesn’t have to end there. Using the optional programmer and headers, you can devise your own challenges based on her stock firmware, or modify that firmware to flash your own core riddle. The Skull’s the limit!

More interested in hardware mods? Use the programming pads to hook up a small buzzer and teach the Skull to speak. Or give her an I2C accelerometer and, with it, a sense of direction. Come to that, why not attach a Wi-Fi or a Bluetooth adapter and unleash her upon the Internet? What’s the worst that could happen?

And, if you want to share your mods with others, you could even design them as tiny PCBs with castellated mounting holes so they can be soldered directly onto the Skull’s programming pads. Then you’ll be able to call them Bone Shields or Skull Hats or…Slaughterboards?

The Programming Header

The Skull’s programming pads expose three GPIO pins—PB0, PB1, and PB2—as well as 3V (from the battery) and GND. PB0 and PB1 are currently unused by the firmware, and PB2 is used for the small touch panel on the back.

Learn more about the firmware.

KiCAD circuit schematic (all of which are spoiler-free)

Features & Specifications

  • MCU: ATtiny45
  • Power: One CR2032 Battery
  • Input: A resistive touch panel on the back
  • Outputs: Two sinister, glowing red eyes (aka 5 mm LEDs)
  • Connectors: Optional, solder-it-yourself ICSP header (not required to solve the riddle)
  • Dimensions: 100 x 75 mm
  • Weight: 26 g with battery


The Skull CTF is designed by Wokwi, and it is currently in the crowdfunding stage in CrowdSupply. After the crowdfunding is completed, as the partner of the Crowd Supply, PCBWay will manufacture this project. Go to support it if you are interested in it!

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