Let's say you need to send a private message to a group of people, but you're afraid one of them will leak the message elsewhere, and you won't know who. Fast Forward Labs has a rough-and-ready solution that will expose anyone who publicly copies and pastes your message, without letting them know they have been caught.
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Identify elements of your memo that can change without anyone noticing: Whether you use double or single quotation marks; whether you write out numbers such as three or use numerals such as 3; whether you use semicolons or commas in a list. The subtler the difference, the better. If you want to get a little sneakier, you can replace certain letters with similar-looking Unicode characters, but as FF Labs points out, these special characters might be exposed if the message is converted into plain text.
Now, instead of BCCing everyone on one email, send each recipient their own BCC. As long as your recipients are used to receiving group messages via BCC, they won't know the difference. On each BCC, make a different subtle change. FF Labs suggests combining different changes to increase your possible unique messages: One recipient gets "three"; one gets "3"; one gets 'three'; one gets '3'.
Now if anyone reveals your message to the public, you can check it against your outgoing emails and identify the leaker. If your changes are subtle enough, they might never know how you caught them.