One-time pads require no machines and are unbreakable in theory, though in reality they’re a pain to set up and use so people reuse keys out of laziness, making it possible to analyze and decipher encrypted messages.
Security is only as good as its weakest link, and people are morons.
Brown Bear Barrage.
Or Giant Scorpion Tsunami if you want maximum pants-shitting potential.
(I could only find one CR4 large beast, the giant coral snake from Curse of Strahd, but no good spell title came to mind.)