Substitution Cipher is an old technique that substitutes one thing for another. For example, alphabet  messages can be ciphered by applying the following substitution.  plaintext:   abcdefghijklmnopqrstuvwxyz                     |                                               |                    V                                              V ciphertext: mnbvcxzasdfghjklpoiuytrewq  However, this type of ciphering produces patterns since the same plaintext strings produce the same  cipher text. Polyalphabetic Cipher, on the other hand mitigates this problem as plaintext characters  are not always replaced with the same cipher text character. The replacement of each character depends on a key as well as on the position of the character in the text; for example, by using a formula such as: for (int i=0; i < length of P; i++)  C[i] = P[i] + K + (i mod 3)  In essence, if the key, K, is choses as K=10, then 10 is added to characters in position 0, 3, 6, …; and  11 is added for those in positions 1, 4, 7; and 12 is added for those in 2, 5, 8. Consequently, given  K=1, a string such as THEMTHENTHEY will be encrypted as UJHNVKFPWIGB. a) Pls write an algorithm as a pseudo code that would implement the encryption  technique of Polyalphabetic Cipher, and returns the cipher text for a given plaintext input.  Determine the input and output of your algorithm.  b) Pls write an algorithm as a pseudo code that would implement the decryption  technique of Polyalphabetic Cipher, and returns the plain text for a given cipher text input.  Determine the input and output of your algorithm.  c) What is the time complexity of each of your algorithms, in terms of Big-O? What is the space complexity of each of your algorithms algorithm, in terms of Big-O?

Substitution Cipher is an old technique that substitutes one thing for another. For example, alphabet 
messages can be ciphered by applying the following substitution. 

plaintext:   abcdefghijklmnopqrstuvwxyz

                    |                                               |

                   V                                              V
ciphertext: mnbvcxzasdfghjklpoiuytrewq 

However, this type of ciphering produces patterns since the same plaintext strings produce the same 
cipher text. Polyalphabetic Cipher, on the other hand mitigates this problem as plaintext characters 
are not always replaced with the same cipher text character. The replacement of each character depends
on a key as well as on the position of the character in the text; for example, by using a formula such as:

for (int i=0; i < length of P; i++)
 C[i] = P[i] + K + (i mod 3) 

In essence, if the key, K, is choses as K=10, then 10 is added to characters in position 0, 3, 6, …; and 
11 is added for those in positions 1, 4, 7; and 12 is added for those in 2, 5, 8. Consequently, given 
K=1, a string such as THEMTHENTHEY will be encrypted as UJHNVKFPWIGB.

a) Pls write an algorithm as a pseudo code that would implement the encryption 
technique of Polyalphabetic Cipher, and returns the cipher text for a given plaintext input. 
Determine the input and output of your algorithm. 

b) Pls write an algorithm as a pseudo code that would implement the decryption 
technique of Polyalphabetic Cipher, and returns the plain text for a given cipher text input. 
Determine the input and output of your algorithm. 

c) What is the time complexity of each of your algorithms, in terms of Big-O? What is the space complexity of each of your algorithms algorithm, in terms of Big-O?