crc32_collider.rb

require 'zlib'

# A fairly naive generator of payloads which, having the same size, will produce an identical CRC32 checksum
def produce_bytes_at_iteration(bytebag_size, random_seed, iter)
  rng = Random.new(random_seed)
  iter.times do
    rng.bytes(bytebag_size)
  end
  rng.bytes(bytebag_size)
end

def collider(bytebag_size, random_seed)
  rng = Random.new(random_seed)
  computed = {}
  10_000_000.times do |iter|
    bytes = rng.bytes(bytebag_size)
    len_before = computed.length
    crc = Zlib.crc32(bytes)
    if previous = computed[crc]
      # Reproduce the first colliding payload
      previous_colliding = produce_bytes_at_iteration(bytebag_size, random_seed, previous)
      File.open("crc_#{crc}_collision_payload_a.bin", "wb") {|f| f << previous_colliding }
      File.open("crc_#{crc}_collision_payload_b.bin", "wb") {|f| f << bytes }
    else
      computed[crc] = iter
    end
  end
end

collider(3 * 1024, 123)