open it destructively and make a new envelope? this is probably the easiest option for a 3-letter agency.
soak it in a liquid that doesn't dissolve the ink, photograph the layers of text, and decipher it later?
shine a really bright light on it, using a color that isn't well absorbed by the paper but is absorbed by the ink?
If there's a way to harden paper mail against even a moderately prepared adversary I'm totally interested in learning about it...
http://www.wired.com/2013/12/better-data-security-nail-polis...
Or mail a postcard with a message encrypted by one-time-pad.
open it destructively and make a new envelope? this is probably the easiest option for a 3-letter agency.
soak it in a liquid that doesn't dissolve the ink, photograph the layers of text, and decipher it later?
shine a really bright light on it, using a color that isn't well absorbed by the paper but is absorbed by the ink?
If there's a way to harden paper mail against even a moderately prepared adversary I'm totally interested in learning about it...