ADFGVX cipher
A German field cipher from 1918 that combined substitution with transposition, and was broken by a Frenchman within months anyway.
Fritz Nebel designed it ahead of the 1918 spring offensive. Each character is first replaced by two letters from a 6×6 Polybius square, then the letters are shuffled by a columnar transposition keyed with a codeword. The grid's rows and columns were labelled A, D, F, G, V and X, chosen because they are hard to confuse in Morse. The first version, ADFGX, used a 5×5 grid without digits; the V was added in June.
Because every letter ends up split across two places in the ciphertext, frequency analysis is neutralised. The French cryptanalyst Georges Painvin nevertheless recovered a key in early June 1918, exploiting messages that began identically, and gave the Allies warning of where a German attack would fall.