In a special case of a permutation cipher, we take a message, M , and write its letters in an s × t table, in a row-major fashion, and then let the ciphertext be a column-major listing of the entries...

In a special case of a permutation cipher, we take a message,
M, and write its letters in an
s


× t
table, in a row-major fashion, and then let the ciphertext be a column-major listing of the entries in the table. For example, to encrypt the message ATTACKATDAWN, using a 3
×
4 table, we would write the message as ATTA CKAT DAWN and then write down the ciphertext as ACDTKATAWATN. The secret key in this cryptosystem is the pair (s,
t). How is decryption done in this cryptosystem? Also, how hard would it be to attack this cryptosystem using a ciphertext-only attack?