A double transposition cipher can be made much stronger by using the following approach. First, the plaintext is put into an n x m array, as described in the text. Next, permute the columns, and then...

A double transposition cipher can be made much stronger by using the following approach. First, the plaintext is put into an n x m array, as described in the text. Next, permute the columns, and then write out the intermediate ciphertext column by column. That is, column 1 gives the first n ciphertext letters, column 2 gives the next n, and so on. Then repeat the process, that is, put the intermediate ciphertext into annx m array, permute the columns, and write out the ciphertext column by column. Use this approach, with a 3 x 4 array, and permutations (2,3,1,4) and (4, 2,1,3) to encrypt the plaintext attackatdawn.