You are indeed correct, sir, it is CBC. I failed to mention that. I'm hoping I can still accomplish this without using GCM. Here is a sample of what works online: Actual cyphertext : 32334b4659307165655076412b596c4b394d36586c6e69304850694f784e6364434a4b4363506f2f4f7065514e43324367507470783735534b33744f654c2b6964306f4450336265787565577a527564765278707a4f79654471514162684f37576470524f3673444c2f4d3d Key : 3132333435363738393132333435363738393132333435363738393132333435 IV : 31323334353637383132333435363738 Expected plaintext : 7b20226c6f6164223a20223132333435363637222c2022737570706c696572223a20223938373635343332222c202263617272696572223a20224142434422207d FYI, your CBC nudge led me to another post of yours where you shared a Swift wrapper around CommonCrypto’s AES CBC support. Digging into that now. My cyphertext is in fact NOT a multiple of 16. Since the online site I used didn't seem to care, I'm wondering what their method is. Is there some safe padding that can be applied?