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Minimal Shitty Add-On with Capture The Flag Challenge

Copyright (C) 2019, Uri Shaked. Released under the MIT license.

Learn more in the blog post: Capture The Flag Shitty Add-On


The ctf-shittyaddon board includes the following hardware:

  1. ATtiny85 Processor
  2. A ShittyAddon V1 connector
  3. Reset button
  4. Red LED connected to ATtiny85's pin 6 (PB1)


You can also hack a simplified version on a breadboard:

Arduino Connection Diagram


Compile the firmware using the Arduino IDE:

  1. Install ATTiny Core
  2. Install the TinyWire Library
  3. Configure Arduino IDE as follows:
    • Board: ATtiny25/45/85
    • Processor: ATtiny85
    • Clock: Internal 8 MHz
  4. Set the fuses of the chip to enable self programming (SELFPRGEN=1). This can be done using avrdude:
    avrdude -pattiny85 -cusbtiny -e -Uefuse:w:0xfe:m -Uhfuse:w:0xdf:m -Ulfuse:w:0xe2:m

When building your own firmware, replace SECRET in the string "$FLAG:SECRET" with your secret value (the flag).

The Challenge

The firmware code implements an I2C slave listening on address 0x23.

The challenge consists of several milestones of increasing difficulty:

  1. Make the red LED light
  2. Find the secret flag value (it starts with the string "$FLAG:")
  3. Make the red LED blink by executing code directly on the ATTiny85
  4. Replace the secret flag with a new value without bricking the board

All the above milestones can be achieved through I2C communication with the chip.

For more information, check out the blog post