The security here is a bit of a black box, but we can orchestrate a plan to get past it.
These eight boxes are full of letters that we can probe by firing beams into the box, as well as atoms that deflect the beam. Each box contains five atoms.
When a beam hits an atom on a corner, it is deflected away from that atom at a right angle. When it hits an atom straight on, it bounces straight back. If there are two atoms trying to deflect or reflect the beam in different directions, it bounces straight back.
When a beam exits the box, you can see the letters it has hit -- in alphabetical order, disregarding duplicate letters. If the beam exits in the same place it entered, the letters are shown there; if it exits in a different place, then the entry point shows an asterisk and the letters are shown at the exit.
If the beam is deflected by an atom on the edge before it gets into the box, no letters appear.
Spaces in the message are not detected by the beam and do not affect its path. A beam that crosses nothing but spaces will also be blank at the exit.
Some letters will be undetectable by beams -- particularly letters that are under atoms. You may be able to figure them out from context.
Each thief who completes their mission in the Aria hotel will get you the beam results from one side of several grids.