  Visual adversarial examples have so far been restricted to pixel-level image
manipulations in the digital world, or have required sophisticated equipment
such as 2D or 3D printers to be produced in the physical real world. We present
the first ever method of generating human-producible adversarial examples for
the real world that requires nothing more complicated than a marker pen. We
call them $\textbf{adversarial tags}$. First, building on top of differential
rendering, we demonstrate that it is possible to build potent adversarial
examples with just lines. We find that by drawing just $4$ lines we can disrupt
a YOLO-based model in $54.8\%$ of cases; increasing this to $9$ lines disrupts
$81.8\%$ of the cases tested. Next, we devise an improved method for line
placement to be invariant to human drawing error. We evaluate our system
thoroughly in both digital and analogue worlds and demonstrate that our tags
can be applied by untrained humans. We demonstrate the effectiveness of our
method for producing real-world adversarial examples by conducting a user study
where participants were asked to draw over printed images using digital
equivalents as guides. We further evaluate the effectiveness of both targeted
and untargeted attacks, and discuss various trade-offs and method limitations,
as well as the practical and ethical implications of our work. The source code
will be released publicly.
