Cryptography involves communication between two users through coded message. Cryptography is an art and science of concealing the information to introduce privacy and secrecy between the intended users. The three basic goals of security are Confidentiality, Integrity and Availability. Cryptography involves three basic mechanisms

• Symmetric-Key Encipherment or Private Key
• Asymmetric-Key Encipherment or Public Key
• Hashing

Symmetric key encipherment

A single secret key is used for both encryption and decryption. In addition, the encryption and decryption algorithms are inverses of each other. The original message known as plain text is encrypted using a secret key to obtain the unintelligible message called cipher text. This cipher text is transmitted through the channel to the intended receiver. At the receiver, the cipher text is decrypted using the same secret key to obtain the original plain text.Traditional Symmetric Ciphers are the foundation to modern ciphers which are classified into two broad categories

• Substitution Cipher

  A substitution cipher replaces one symbol with another stored in the Character list along with index.

• Transposition Cipher

  A transposition cipher does not substitute one symbol for another, instead it changes or transposes the location of the symbols.

Implementation of Substitution/Caesar Cipher

Algorithm

• Enter the default Character List

   import string
   all_letters= string.ascii_letters
   digits="0123456789"
   all_char=all_letters+digits
   print('Character List :', all_char)
  
   #>Character List: abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789 
  

• Enter the plain text and key

   x = input('Enter The String to be Encrypted :')
   k = int(input('Enter the key value :'))
  
   #=> Enter The String to be Encrypted : Hello from the other Side
   #=> Enter the key value : 6
  

• Obtain cipher text as follows.

  C= (P+K) mod n where

  • C -> Ciphered Character
  • P -> Plain Text Character
  • K -> Key
  • n -> Length of the character list

   word=''
   for letter in x:
     if (letter in all_char):
       z= all_char.find(letter)
       y=all_char[(z+k)%len(all_char)]
       word+=y
     else:
       word+= letter
  
   print('Plain Text : ', x)
   print('Encrypted Data : ', word)
   print('Key Value : ',k)
  
   #=> Plain Text : Hello from the other Side
   #=> Encrypted Data : Nkrru lxus znk uznkx Yojk
   #=> Key Value :  6
  

• Obtain deciphered text by reversing the logic.

   word2=''
   for letter in word:
     if (letter in all_char):
       u= all_char.find(letter)
       v=all_char[(u-k)%len(all_char)]
       word2+=v
     else:
       word2+= letter
        
   print('Encrypted Data : ', word)
   print('Plain Text : ', word2)
   print('Key Value : ',k)
        
   #=> Encrypted Data : ZgtsgE-98
   #=> Plain Text : Tanmay-32
   #=> Key Value : 6
  

Transposition Cipher and Reverse Cipher

Algoritm

• Enter the plain text and the positions by which the text has to be transposed

  x1 = input('Enter The String to be Encrypted :')
  k1= int(input('Enter the key value :'))
  
  #=> Enter The String to be Encrypted : Tanmay
  #=> Enter the key value : 3
  

• Apply the circular shift logic to obtain the Transposition Cipher

  word3=''
  
  for index,char in enumerate(x1):
    v1=x1[((index + k1)% len(x1))]
    word3 +=v1
  
  print('')
  print('Encrypted Data : ', word3)
  print('Plain Text : ', x1)
  print('Key Value : ',k1)
  
  #=> Encrypted Data :  mayTan
  #=> Plain Text :  Tanmay
  #=> Key Value :  3
  

• Apply reverse logic to obtain the plain text back

Python Code hosted on Google Collab