The Science of The Expanse: Season 1, Episode 7

Inside the operations locker, Alex finds a sealed tube, in which is a rolled plastic sheet. On the sheet are three code words that are used to send the Martian boarding skiff on its merry way. Respect the black ops!

Respect to Alex on his pronunciation and throwing in the "Donkey Balls" for good measure. Codes like this have been a vital aspect of communication since the invention of war itself, so it's no surprise that the MCRN Corvette Class, fast-attack light frigate has a specially coded communication system, but is it unbreakable? Alex and Naomi cracked it, to be fair they almost died in the process, but they were able figure it out. 

Looking to send an unbreakable coded message? The key lies in the Diffie-Hellman key exchange method. It works like this, say Mars wants to send Earth a secret message and wants to prevent the OPA from reading it. Mars places the message in a box, puts a good lock on it, keeps the key, and sends the package to Earth. (If Mars were to separately send Earth the key, there would be a chance that the OPA could intercept both the package and the key.)

Earth has no key to the Mars' lock. So Earth puts their own lock on the box, and returns the packed locked twice. Mars gets the package, removes its own lock and then resends the box. Now Earth uses its key, opens the box, and gets the message! Praise Science!