"Pixelize" tool leaking private data

[boltgolt]

In the Shutter editor there is an option to "pixelize" data, it even adds you can use it to "hide private data". However, you can still uncover what was supposed to be covered up by simply upping the contrast on the editor output image. The pixelation seems to be semi-transparent.

I did not notice this for quite a while, unknowingly sending private information to people. A fix would be greatly appreciated, as other people might also be leaking data without knowing it.

Shutter version: 0.90.1 (latest release as far as i know)
System version: Ubuntu 14.04 LTS