Permutation$-$cipher

Read the following description and then answer the $2$ follow$-$up questions:

In a special permutation cipher, the letters of an English plaintext are written in an r by ctable $(r$ rows and c columns$)$ in a row$-$major form $($row$-$wise$)$ and the corresponding ciphertext is the column$-$major listing $($column$-$wise$)$ of the letters. The table dimension $(r,c)$ is written as an integer $rc$ and serves as the shared secret key for the permutation cipher.

For example, using the key $34(3$ by $4$ table, that is$,3-$rows, $4-$columns$)$ the plaintext message ATTACK AT DAWN $($ignoring the spaces in between the words$)$ written as

ATTA

CKAT

DAWN

to generate the the corresponding ciphertext is:

ACDTKATAWATN

With this description and explanation of the permutation cipher, answer the following questions:

Question $15(3$ points$)$

Suppose Alice and Bob share some key for such a permutation cipher. Alice encrypts a plaintext using the key and sends the ciphertext to Bob. Explain how Bob would decrypt the ciphertext using the same key to get the plaintext.

$($You may use the ATTACKATDAWN example in the description to explain the decryption scheme.$)$

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