doom-captcha

A DOOM®-based CAPTCHA for the web¹

How it works

The project works by leveraging Emscripten to compile a minimal port of Doom to WebAssembly and
enable intercommunication between the C-based game runloop and the JavaScript-based CAPTCHA UI.

Some extensions were made to the game to introduce relevant events needed for its usage
in the context of a CAPTCHA.

• Started out with a minimal, SDL-based port of Doom that can be efficiently compiled to WebAssembly by Lorti.
• Tweaked the build to make it compatible with the shareware version of wad (doom1.wad) for legal use
• Introduced new unofficial process flags:
  • -nomenu to skip the main menu and jump straight into the game (m_menu.c)
  • -autoreborn to automatically rebirth the player after a 2s delay (m_player.c)
• Introduced callbacks into JS land to be used by the CAPTCHA UI:
  • onPlayerBorn when the player is born or reborn
  • onPlayerKilled when the player is killed
  • onEnemyKilled when the main player kills an enemy
• Tweaked the default process flags to make the game more challenging and skip all the menus:
  • -skill 5 sets the difficulty to “Nightmare!”
  • -fast makes it even harder
  • -warp e1m1 jumpstarts the game to where the action is
  • -nomenu doesn’t let the player trigger the main menu

Developing

• Install Emscripten
  • On macOS, brew install emscripten does the trick
• Place doom1.wad in the sdldoom-1.10 directory (Shareware version)
• Run build.sh
  • Get the DOOM® shareware WAD URL and save it to $DOOM_WAD_URL
  • During development, I recommend running watchexec -- ./build.sh to make the process automatic (install watchexec with brew install watchexec)
• Run vercel dev

Credits

• SDLDoom 1.10
• sdldoom.wasm by Lorti
• Emscripten
• Doom Shareware WAD
• DOOM® is a registered trademark of id Software LLC, a ZeniMax Media company
• Stylized as a reference to Google’s reCAPTCHA
• Prior art in spirit by vivirenremoto

¹ for educational and entertainment purposes only