Ah right, another much bigger problem with Prey. I obviously didn't look very closely. :-/
I forgot when writing above, http://gondwanaland.com/mlog/2007/09/08/energy-encryption/ relevant wild speculation (or me casting doubt upon).
I forgot when writing above, http://gondwanaland.com/mlog/2007/09/08/energy-encryption/ relevant wild speculation (or me casting doubt upon).
Divide your hot and cold gas into tiny cubes. Make them small enough that you cannot detect their temperature without making either cube lukewarm (say, 1nm^3). Place them in a grid which can permute the cubes reversibly (i.e. without losing any energy, in the sense of reversible computation), starting with all the hot cubes on one side, the cold cubes on the other. Apply an encryption algorithm, using your secret key, to rearrange the positions of the cube.
If you don’t know the key, you cannot extract any energy — you don’t know which cubes are hot or cold, and you cannot find out without destroying the heat differential. As a whole it appears like the box of lukewarm gas in the first example. However, if you do know the key, you can separate the hot and cold cubes without cost, and extract the stored energy.
There are undoubtedly better ways to do this. The more general idea is having a system appear to have high entropy, but actually be a low entropy system carefully encrypted.
Mind blown.