Columnar transposition

How it works

Write the message in rows as long as the keyword, then read the columns one after the other, in the alphabetical order of the key letters (equal letters from left to right). With ZEBRAS the column under A is read first, then B, E, R, S and Z. The last row can stay short (an irregular transposition) or be completed with filler letters.

To decrypt, the receiver works out the column lengths from the length of the message: when the last row is short, only its leftmost columns are one letter longer. The ciphertext is cut into columns of these lengths in key order, and the grid is read row by row.

A single transposition keeps the letter frequencies of the language and can be solved by trying column arrangements so that the rows read well (anagramming). Applying it twice with two keys, the double transposition, was a serious field cipher until the Second World War.