This is ZXX:

ZXXCredit: Walker Art Center

It's a font designed to be difficult for machines to read. Several different techniques are used, including camouflage patterns drawn from nature, crowding the letters with digital noise, and simply crossing out each letter. 

morning!Credit: Walker Art Center

The font is named after the Library of Congress code, ZXX, which labels a document as containing "no linguistic content." The goal is to make the contents of a document unreadable by text scanning software while still being intelligible to a human reader.

Here's how the font designer, Sang Mun, explains his project

As a former contractor with the US National Security Agency (NSA), these issues hit especially close to home. During my service in the Korean military, I worked for two years as special intelligence personnel for the NSA, learning first-hand how to extract information from defense targets. Our ability to gather vital SIGINT (Signal Intelligence) information was absolutely easy. But, these skills were only applied outwards for national security and defense purposes—not for overseeing American citizens. It appears that this has changed. Now, as a designer, I am influenced by these experiences and I have become dedicated to researching ways to “articulate our unfreedom” and to continue the evolution of my own thinking about censorship, surveillance, and a free society.

It's part awareness-raising art project, part useful tool. (Though how big a part optical text recognition plays in government spying is unclear. Because, you know, the whole thing is quite secret.)

(UPDATE: Since there seems to be some confusion on this point, I've tweaked the headline and I just want to highlight the sentences above. This would be most useful for attachments or other items that can be transmitted via email as images.)

Download the .zip file with the font here.

And don't forget to read Ronald Bailey's other tips for how to keep the government from spying on you.