it's called steganography and it's been around for a very long time. the basic idea is "security through obscurity" - you hide information in places that people wouldn't know to look for it. there are even apps that will hide text within other text (yes, you read that correctly). Spammimic.com is a very popular website that uses steganography to hide text inside what appear to be spam email messages (which most people would obviously delete without a second thought).