In regular uncompressed air in a container, the position of
all
the air molecules within the container is fairly random. You
can compress the air by pushing all the air molecules into,
for
example, the lower half of the container.
In encrypted compressed air, the container is divided into
a
huge number of cells, which contain an average of 1 air
molecule each, but sometimes more or less. The
distribution
of all the air molecules between the cells appears fairly
random, but it's actually generated by a cryptographic
random
number generator, from a secret seed/key.
Someone with the key can open all the cells with more air
molecules than average (while leaving the empty cells
closed), and get compressed air out. But if someone steals
the
containers as they are being shipped from the manufacturer
to the buyer, without the key, it is effectively just full of
regular air.
If it's too hard to exactly control the exact number of
molecules in each cell, the manufacturer can just expose
some cells (while open) to compressed air, and others to a
vacuum. This will give an approximate distribution of the
molecules, and still allow the receiver to extract
compressed
air.