Columnar transposition cipher

Type a keyword and a message. The keyword's length sets how many columns the grid has, and the alphabetical order of its letters sets which column is read first. Decoding needs the same keyword.

Add a second keyword for double transposition: the result goes through a second grid. Leave the last row short, as most puzzles do, or fill it with X so every column is the same length. Only letters are carried, and anything else is listed below the output.

Take the keyword ZEBRAS. Number its letters in alphabetical order: A is 1, B is 2, E is 3, R is 4, S is 5 and Z is 6, so the columns run 6 3 2 4 1 5. Write WE ARE DISCOVERED. FLEE AT ONCE in rows of six under it: WEARED / ISCOVE / REDFLE / EATONC / E.

Now read column 1 down, then column 2, and so on. The message becomes EVLNACDTESEAROFODEECWIREE. This is the worked example in Wikipedia's article on transposition ciphers, and the tool gives the same answer.

If a letter appears twice in the keyword, the copy further left comes first. To decode, work out how long each column must be, cut the cipher text into columns in key order, and read across the rows.

Samuel Morland introduced an early form of columnar transposition in the middle of the 17th century. It became widely used in the late 19th and 20th centuries, notably by French military services, and stayed in serious use into the 1950s.

In the First World War the German army used a double columnar transposition and changed the keys infrequently. The French solved it regularly and called it Übchi, and the Germans moved to a new system on 18 November 1914. In the Second World War double transposition was used by Dutch Resistance groups, the French Maquis, the British Special Operations Executive and American OSS agents. It was thought the most complicated cipher an agent could work reliably in the field.

A transposition keeps every letter and only moves it, so the cipher text has the letter counts of ordinary English. Put it through frequency analysis: if E, T and A come out on top, it is probably a transposition and not a substitution.

A single transposition can be attacked by guessing the column length, cutting the text into columns and sliding them around until fragments of English appear. A keyword close to the right one already shows long runs of readable text, which is how computer searches find it.