The issue with cryptography is that a muggle who sees your encrypted message knows that there is a hidden message. They may not know what it says, but it can and will arouse suspicion. With enough suspicion, and enough time, a muggle can decode any encrypted message. For instance:
Rps ipqgisw vu lvz uxgbxgc rps iqips bsksgb jvds vg lvzd wnxmmw xg wrscqgvcdqkpl dqrpsd rpqg wvmtxgc idlkrvcdqjw mxns rpxw. Rps uqmws gvdrpxgc xw gvdrp uxurl bscdssw, resgrl-uxts
jxgzrsw kvxgr gxgs. Rps uqmws eswrxgc iqg fs uvzgb qr vgs pzgbdsb uvzd bscdssw, rpxdrl-uxts jxgzrsw kvxgr rev. Rpqr'w ozxrs zwsmsww xwg'r xr? Wxgis lvz wksgr rps rxjs wvmtxgc rpxw zwsmsww kzyyms, X'mm cxts lvz q pxgr. Lvz qds mvvnxgc uvd q bdsqbsb gqgv jqnxgc zws vu xrw jqcgsrxi kdvksdrxsw.
On the other hand, if you master the art of steganography, your message is safely hidden in some kind of host, and the typical muggle won't look twice at it, because it is so ordinary. But to someone in on the secret, the message is easily found hiding in plain sight.
