Question

Question) Write a decryption function decryp_scytale(ctext, key) in PYTHON that would decrypt any ciphertext produced through the scytale cipher scheme using a given key.

Note: the key refers to the diameter of the rod. It represents the number of characters which can be written in the one of the rounds around the rod (think of number of rows in a table). Also, remember that it is assumed that the rod can have an infinite length, i.e. there is no limit on the number of columns.

The function description is as follows:

def decryp_scytale(ctext, key):

# your code here

return ptext

Assume that key is a valid integer in string format. The decryption function should return the supposedly deciphered plaintext. When writing the function, watch for the scenario when the last row has missing characters.There are scenarios when the decryption process will fail. In that scenario, the function should return an empty string.

For reference: the encryption function is as follows:

def encryp_scytale(ptext, key):
r = int(key)
c = int(math.ceil(len(ptext)/key))
cipherMatrix = new_matrix(r,c,"")
counter = 0
for i in range(r):
for j in range(c):
cipherMatrix[i][j] = ptext[counter] if counter < len(ptext) else -1
counter+=1
#convert matrix into string vertically  

ctext = ""
for i in range(c):
for j in range(r):
if cipherMatrix[j][i]!=-1:
ctext+=cipherMatrix[j][i]
return ctext

Answer 1

Code for decrypting:

def decryp_scytale(ctext, key):
r = int(key)
c = int(math.ceil(float(len(ctext))/r))
cipherMatrix = new_matrix(r,c,"")
counter = 0
for i in range(c):
for j in range(r):
cipherMatrix[j][i] = ctext[counter] if counter < len(ctext) else -1
counter+=1
ptext = ""
for i in range(r):
for j in range(c):
if cipherMatrix[i][j]!='-1':
ptext+=cipherMatrix[i][j]
return ptext

Output: