It’s easy to encrypt data with 2 keys. One key unlocks one set of data and the other unlocks the data you actually want to keep confidential. There is NO way to prove that a second key exists.
This sort of scheme does take a larger amount of disk space to implement, but whether or not a second data set exists the total size will be padded out to provide plausible deniability.
TrueCrypt does this sort of thing when it creates a secret op system on your boot hd.
I created a small utility app a few years ago that encrypted files using randomly generated weak keys.
The idea was that you would always need to run the utility to crack the encryption using brute force. The idea I had was that people could encrypt files placed on usenet or P2P and anyone wanting to quickly and easily sift through a huge number of files would then need a supercomputer to brute force the zillions of keys.... making it not so easy to do any more. But still easy for people to use since it took only a few minutes to crack each individual key.
Well then they can ask for any object and get into anything you own. Whatever thing lets them in is ‘neutral.’
“NO PRIVACY FOR YOU!” /soup-nazi