Cryptography with Python - Caesar Cipher

In the last chapter, we have dealt with reverse cipher. This chapter talks about Caesar cipher in detail.

Algorithm of Caesar Cipher

The algorithm of Caesar cipher holds the following features −

• Caesar Cipher Technique is the simple and easy method of encryption technique.

• It is simple type of substitution cipher.

• Each letter of plain text is replaced by a letter with some fixed number of positions down with alphabet.

The following diagram depicts the working of Caesar cipher algorithm implementation −

Example - Caesar Cipher Algorithm Usage

The program implementation of Caesar cipher algorithm is as follows −

main.py

def encrypt(text,s):
   result = ""
   # transverse the plain text
   for i in range(len(text)):
      char = text[i]
      # Encrypt uppercase characters in plain text
      
      if (char.isupper()):
         result += chr((ord(char) + s-65) % 26 + 65)
      # Encrypt lowercase characters in plain text
      else:
         result += chr((ord(char) + s - 97) % 26 + 97)
   return result
#check the above function
text = "CEASER CIPHER DEMO"
s = 4

print ("Plain Text : ", text)
print ("Shift pattern : ", str(s))
print ("Cipher: ", encrypt(text,s))

Output

You can see the Caesar cipher, that is the output as shown in the following image −

Plain Text :  CEASER CIPHER DEMO
Shift pattern :  4
Cipher:  GIEWIVrGMTLIVrHIQS

Explanation

The plain text character is traversed one at a time.

• For each character in the given plain text, transform the given character as per the rule depending on the procedure of encryption and decryption of text.

• After the steps is followed, a new string is generated which is referred as cipher text.

Hacking of Caesar Cipher Algorithm

The cipher text can be hacked with various possibilities. One of such possibility is Brute Force Technique, which involves trying every possible decryption key. This technique does not demand much effort and is relatively simple for a hacker.

The program implementation for hacking Caesar cipher algorithm is as follows −

main.py

message = 'GIEWIVrGMTLIVrHIQS' #encrypted message
LETTERS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'

for key in range(len(LETTERS)):
   translated = ''
   for symbol in message:
      if symbol in LETTERS:
         num = LETTERS.find(symbol)
         num = num - key
         if num < 0:
            num = num + len(LETTERS)
         translated = translated + LETTERS[num]
      else:
         translated = translated + symbol
   print('Hacking key #%s: %s' % (key, translated))

Output

Consider the cipher text encrypted in the previous example. Then, the output with possible hacking methods with the key and using brute force attack technique is as follows −

Hacking key #0: GIEWIVrGMTLIVrHIQS
Hacking key #1: FHDVHUrFLSKHUrGHPR
Hacking key #2: EGCUGTrEKRJGTrFGOQ
Hacking key #3: DFBTFSrDJQIFSrEFNP
Hacking key #4: CEASERrCIPHERrDEMO
Hacking key #5: BDZRDQrBHOGDQrCDLN
Hacking key #6: ACYQCPrAGNFCPrBCKM
Hacking key #7: ZBXPBOrZFMEBOrABJL
Hacking key #8: YAWOANrYELDANrZAIK
Hacking key #9: XZVNZMrXDKCZMrYZHJ
Hacking key #10: WYUMYLrWCJBYLrXYGI
Hacking key #11: VXTLXKrVBIAXKrWXFH
Hacking key #12: UWSKWJrUAHZWJrVWEG
Hacking key #13: TVRJVIrTZGYVIrUVDF
Hacking key #14: SUQIUHrSYFXUHrTUCE
Hacking key #15: RTPHTGrRXEWTGrSTBD
Hacking key #16: QSOGSFrQWDVSFrRSAC
Hacking key #17: PRNFRErPVCURErQRZB
Hacking key #18: OQMEQDrOUBTQDrPQYA
Hacking key #19: NPLDPCrNTASPCrOPXZ
Hacking key #20: MOKCOBrMSZROBrNOWY
Hacking key #21: LNJBNArLRYQNArMNVX
Hacking key #22: KMIAMZrKQXPMZrLMUW
Hacking key #23: JLHZLYrJPWOLYrKLTV
Hacking key #24: IKGYKXrIOVNKXrJKSU
Hacking key #25: HJFXJWrHNUMJWrIJRT