glitch_jpg.rb

#!/usr/bin/env ruby

SOI  = "\xff\xd8"
APP0 = "\xff\xe0"
DQT  = "\xff\xdb"
DHT  = "\xff\xc4"
SOF0 = "\xff\xc0"
SOS  = "\xff\xda"
EOI  = "\xff\xd9"

ARGF.set_encoding('BINARY')

if ARGF.read(2) != SOI
  raise 'file is not jpeg'
end

headers = []
marker = nil
while marker != SOS
  marker = ARGF.read(2)
  length = ARGF.read(2)
  content = ARGF.read(length.unpack('n')[0] - 2)

  case marker
  when DQT
    rand(10).succ.times do
      content[rand(content.length)] = rand(0xff).chr
    end
  end

  headers << marker + length + content
end

body = ARGF.read
body.sub!(/#{EOI}\Z/, '') or raise 'invalid jpeg file'

print SOI
print headers.join('')
print body
print EOI