Print single-sided. Otherwise there's a risk that nominally invisible bleed through from the other side can be enhanced. It's better to just convert a PDF to images directly and redact that.
Flatten everything to a set of just images.
Have normal human staff draw black boxes over anything to be redacted.
Compose a new 'PDF' that's a set of 'scanned' images.
The image was in PPM format, which stores the color components of the pixels as ASCII text (so a white pixel is stored as "255 255 255" and a black one is "0 0 0"). To redact the image, the code replaced every digit of the numbers with '0', so white became "000 000 000" and black stayed as "0 0 0". Both are black and indistinguishable if you're viewing the image, but you can tell them apart by looking at the file text.
Sadly the UCC homepage seems to have vanished, but I found this account from the author: http://notanumber.net/archives/54/underhanded-c-the-leaky-re...