Steganography is a fascinating/unnerving subject – hiding private messages in plain sight. It smacks of something from the Cold War and Microdots, or making invisible ink from lemon juice. But it’s very real – and an example of how it was used recently to find a leak is this article on ars technica. In this case, the NSA leaker included a scanned document, and the printer printed its serial number in very small light yellow dots on the page. The eye missed it but the NSA could use it to track the printer and even time of day it was printed, leading to an arrest.

So if you send printed letters, you might as well sign them – your printer is going to anyways…