# File lib/dragonfly/serializer.rb, line 19
    def b64_decode(string)
      padding_length = (-(string.length % 4)) % 4
      string = string.tr('+','-').tr('~/','_')
      Base64.urlsafe_decode64(string + '=' * padding_length)
    rescue ArgumentError => e
      raise BadString, "couldn't b64_decode string - got #{e}"
    end