Columnar Transposition Cipher

Every classical cipher does one of two things: substitute letters or move them around. Columnar transposition is the second kind, and that changes what cryptanalysis can do. Letter frequency in the ciphertext is identical to the plaintext's — it stays full of the same common letters — because no letter was swapped. What changed was the order.

The text is written in rows under a keyword and then read down the columns, in alphabetical order of the key's letters. Enter the key and the text and pick the direction: the page builds the grid, orders the columns and reads them off, or undoes the process. The text is padded with X when it does not fill the last row, so every column has the same height.

Recognising a transposition is easy precisely because letter frequency stays normal — if the ciphertext distribution looks like the language's but the text is unreadable, it is transposition. Breaking it means trying key lengths and column orders, hunting for the digrams that make sense in the language. That is why transposition usually appears combined with substitution: applying both destroys the order and the identity of the letters.