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Distributed Parallel

Proof of Work

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X-MAS CTF 2021

Fun challenges

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X-MAS CTF 2021

Fun challenges

r/place like challenge that uses PoW to draw pixels

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Proof of Work

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The limits

  • Can only get 5 PoW tasks per second per IP
  • Canvas is 640x480 (307200 pixels)
  • PoW is finding a nonce to a string that produces a partial md5 hash

Example goal: XXXXXXXXXXXX1234

Salt Prefix: 12345678

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The solution

CUDA

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CUDA md5 brute force projects

  • Are not well optimized
  • Don't support known prefix
  • Can only brute force one hash at a time

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The Solution

Write my own brute forcer forked from github

Supports:

  • Multiple input salt:goal
  • Hash salt in a single chunk before-hand and digest the rest

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Scaling it up

  • Backend redis queue for PoW to crack
  • Workers across many machines using proxies to mass produce PoW tasks
  • Distribute work evenly across all the GPU machines running the bulk-cracker
  • ~3000 PoW/s across the 3-4 GPU machines

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The UI

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Some Numbers

Peak drawing speed was 151 pixels/s

Server could not handle much more from all the teams

Had 3 million pixels saved up to draw with in the end

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Timelapse

https://youtu.be/ZEvfGL6PI1U?t=419

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Links

https://github.com/KristinnVikarJ/pixel-miner

https://github.com/elvar333/pixel_panel

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