This was originally posted by me on stackoverflow but was deemed too generic.
It does seem like an impossible thing to do, but: you know when you have your perimeter alarm system you usually get to set two unlock codes, a normal one and a forced entry one. Both disarm the system, but the latter silently calls the Marines.
I'm looking for something similar in crypto (pgp preferred): an encrypted string decrypts to one content using key A, another content using key B. The forced-entry content can be set once and decrypt always to the same string.
I can think of several software ways of doing that, and they are probably better being quite unique, but is there an easier, out-of-the-box way of achieving that?
I'm not too tied to PGP/GPG but it would be nice. I have been suggested to try steganography and it seems the way to go as it provides flexibility in inserting/retrieving the final information (the method can be simple enough to be remembered and invoked with, say, perl -e). Yet before I actually reinvent the wheel I would like to verify that indeed there is no out-of-the-box solution.
Basic steganography is not ideal (like hiding information in a cat pic) as I need a honeypot for that particular purpose.
Information to be encoded would be no more than a couple of kB of 7bit ASCII, whereas the honeypot can be unlimited within reason (like a 5 page .docx).