Double transposition cipher:

• Double transportation can make the job of the cryptanalyst difficult. It designates the letters in the original plaintext message by the numbers designating their position.
• First the plaintext is written into an array of a given size and then permutation of rows and columns is done according to the specified permutations.

Plaintext: attack at four

First transportation: permute rows from (1, 2, 3) to (3, 2, 1)

Middle text: fourckatatta

Second transportation: permute columns from (1, 2, 3, 4) to (4, 2, 1, 3)

Ciphertext: ROFUTKCAATAT