The resulting ciphertext is:

HLOSN$30$TDNSELCE$44$SUET!

The decryption function is performed as follows:

$-$ Compute the length of ciphertext. Let m be this length.

$-$ Let k be the number of rails $($i$.$e$.,$ key$).$ Reserve m placeholders on the k$-$rail fence in the

same way a plaintext was placed on a k$-$rail fence.

$-$ Assign the ciphertext letters to those placeholders row by row, starting from the top one.

$-$ Read off the resulting plaintext as a sequence of diagonals.

$1.$ Write an algorithm for a k$-$rail fence cipher, where k $2.$

$2.$ Show that your proposed algorithm is correct using an informal proof $($i$.$e$.,$ discussion$).$

$3.$ Give a program corresponding to your proposed algorithm, using your favorite

programming language. Your program should simulate this cipher and display the

ciphertext along with the corresponding plaintext and key.