The field of cryptography developed in the context of nation-state actors and idiots who use crypto wrong.  (Come to think of it, not much has changed.)  In WW2, a lot of crypto was broken thanks to known plaintexts -- e.g., "Today's weather is..." -- or chosen plaintexts -- e.g., "Submarine sighted at...".  This was made easier, of course, by the fact that military structures tend to use very consistent communications formats.<p>You're quite right that you can slow down a brute force attack by making the input data hard to recognize -- one trivial approach is to pad it to a large size and then apply an all-or-nothing transform -- but by the time this sort of mechanism was feasible, brute force key searches stopped being useful due to increased key lengths.