class BloatPNG

Public Class Methods

bloat(input, output, size=256) click to toggle source

“Bloats” a PNG file by adding random data in the form of text elements

# File lib/bloatpng.rb, line 8
def self.bloat(input, output, size=256)
  if(size < 20)
    raise NeedsMoreBloatException
  end
  n = size - 32 - (Math.log(size) / Math.log(2)).to_i
  key = SecureRandom.hex(15)
  text = SecureRandom.hex(n)[1..n]
  self.insert_text_element(input, output, key, text)
end

Private Class Methods

add_chunk_to_png(chunk, png) click to toggle source

# File lib/bloatpng.rb, line 41
def self.add_chunk_to_png(chunk, png)
  len = png.length
  return png[0,len-12] + chunk + png[len-12,12]
end

create_text_element(key, text) click to toggle source

# File lib/bloatpng.rb, line 32
def self.create_text_element(key, text)
  raise KeySizeError if key.length > 79
  chunk_type = "tEXt"
  chunk_data = key.gsub("\0", " ") + "\0" + text.gsub("\0", " ") # null-separated
  crc = [Digest::CRC32.checksum(chunk_type + chunk_data)].pack('N')
  len = [chunk_data.length].pack('N')
  return len + chunk_type + chunk_data + crc
end

insert_text_element(input_file, output_file, key, text) click to toggle source

see: www.w3.org/TR/PNG/#11tEXt

# File lib/bloatpng.rb, line 21
def self.insert_text_element(input_file, output_file, key, text)
  png = ""
  File.open(input_file, 'rb') do |f|
    png = f.read
  end
  chunk = create_text_element(key, text)
  File.open(output_file, 'wb') do |f|
    f.write(add_chunk_to_png(chunk, png))
  end
end