Project Nayuki

Overwriting confidential handwritten text

tl;dr: Suppose you handwrote a password on paper, and now you want to destroy the information securely. Instead of blunt methods like shredding, you can simply overwrite the text several times with random garbage characters, until it’s infeasible to tell apart the original text from garbage.

(Problem scenario) Sometimes we handwrite short pieces of text that should be destroyed after it had served its useful purpose. It could be a password, an address, a name, etc. It could be something you don’t want your roommate to ask questions about, or something you don’t want a random dumpster diver to collect.

(Poor alternatives) There are a number of possible ways to purge this sensitive information, but many are unsatisfactory. Crossing out the text with a pen is among the lamest, as the information is readily recoverable. Manually tearing the paper into bits is messy, imprecise, and possible to reconstruct. Cutting with scissors is okay but still cumbersome. Shredding with an actual office shredder is good but not always available; furthermore cross-cuts are much better than strip cuts. Physical destruction with fire/water/chemicals is possible but very messy.

(Overwriting technique) In fact there is a simple way to destroy handwritten text – simply overwrite it with more handwritten text until it’s no longer possible to distinguish what text is true and what text is nonsense. It’s helpful to overwrite with the same kind of plausible content – for example overwriting numbers with random numbers, overwriting names with names, overwriting English sentences with general phrases. If the same pen is used for the original writing and the overwriting, then it’s sufficient to add 2 to 4 layers of overwritten text. If the pen is not the same, then it’s best to use a few (say 3) different pens of similar colors to do the overwriting, so that it’s difficult to tell which pen wrote the real information and which pen wrote the bullshit.

(Limitations and caveats) Note that this technique of overwriting cannot safely destroy printed material, because the look of handwritten text is too distinct from typefaces, and pen stroke ink looks very different from laser printer toner or offset printing ink. Printed materials still need to be shredded properly; there’s no shortcut for that. Also for handwriting, avoid overwriting with simple patterns like long straight lines, periodic loops, etc. You should overwrite with text that actually imitates the sensitive text, because this is what makes it hard to separate the content from fiction. Finally note that overwriting is tedious for more than a paragraph of text, so bulk destruction of paper should still seek a proper cross-cut shredder.