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Substitution Cipher

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  • Difficulty Level : Medium
  • Last Updated : 29 Sep, 2021
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Hiding some data is known as encryption. When plain text is encrypted it becomes unreadable and is known as ciphertext. In a Substitution cipher, any character of plain text from the given fixed set of characters is substituted by some other character from the same set depending on a key. For example with a shift of 1, A would be replaced by B, B would become C, and so on. 

Note: Special case of Substitution cipher is known as Caesar cipher where the key is taken as 3.

Mathematical representation

The encryption can be represented using modular arithmetic by first transforming the letters into numbers, according to the scheme, A = 0, B = 1,…, Z = 25. Encryption of a letter by a shift n can be described mathematically as.

E_n(x)=(x+n)mod\ 26

(Encryption Phase with shift n) 

D_n(x)=(x-n)mod\ 26

(Decryption Phase with shift n) 



Plain Text: I am studying Data Encryption
Key: 4
Output: M eq wxyhCmrk Hexe IrgvCtxmsr

Key: 4

Algorithm for Substitution Cipher: 


  • A String of both lower and upper case letters, called PlainText. 
  • An Integer denoting the required key. 


  • Create a list of all the characters. 
  • Create a dictionary to store the substitution for all characters. 
  • For each character, transform the given character as per the rule, depending on whether we’re encrypting or decrypting the text. 
  • Print the new string generated. 

Below is the implementation.


# Python program to demonstrate
# Substitution Cipher
import string
# A list containing all characters
all_letters= string.ascii_letters
create a dictionary to store the substitution
for the given alphabet in the plain text
based on the key
dict1 = {}
key = 4
for i in range(len(all_letters)):
    dict1[all_letters[i]] = all_letters[(i+key)%len(all_letters)]
plain_txt= "I am studying Data Encryption"
# loop to generate ciphertext
for char in plain_txt:
    if char in all_letters:
        temp = dict1[char]
        temp =char
cipher_txt= "".join(cipher_txt)
print("Cipher Text is: ",cipher_txt)
create a dictionary to store the substitution
for the given alphabet in the cipher
text based on the key
dict2 = {}    
for i in range(len(all_letters)):
    dict2[all_letters[i]] = all_letters[(i-key)%(len(all_letters))]
# loop to recover plain text
decrypt_txt = []
for char in cipher_txt:
    if char in all_letters:
        temp = dict2[char]
        temp = char
decrypt_txt = "".join(decrypt_txt)
print("Recovered plain text :", decrypt_txt)


Cipher Text is:  M eq wxyhCmrk Hexe IrgvCtxmsr
Recovered plain text : I am studying Data Encryption


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