Columnar Transposition involves writing the plaintext out in rows and then reading the ciphertext off in columns. The row length that is used is the same as the length of the keyword with the plaintext being padded to make it fit into the rectangle under the keyword. The columns are now reordered alphabetically and then the ciphertext is read off along the columns.

Please note: This implementation of the Columnar Transposition cipher pads the plaintext so that its length is a multiple of the key length.