Decrypting Caeser Cipher in Python

Question

I am trying to make a decrypt script for a Caeser cipher script to encrypt but can't get it to work. This is my encryption script:

def encrypt(text,s): 

    result = "" 

    # traverse text 
    for i in range(len(text)): 
        char = text[i] 

        # Encrypt uppercase characters 
        if (char.isupper()): 
            result += chr((ord(char) + s-65) % 26 + 65) 

        # Encrypt lowercase characters 
        else: 
            result += chr((ord(char) + s - 97) % 26 + 97) 
            
    return result 
#def decrypt(ciphertext, s):
   

text = "0123456789"
s = 4
Cipher=encrypt(text,s)

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

I need help with creating a decrypt script the same way. Thanks in advance!

score 0 · Answer 1 · answered Jul 27 '20 at 08:27

Here is some code of mine for a function used to decrypt a ceaser cipher. The approach used when the shift is not known is simply to get every possible value, then pick the one with more then half the decoded words being in the English dictionary. For non alphabetic characters, there is a placeholder character used.

There are some extra parts required, such as the dictionary or extra functions / lists so I will include them in the below links.

A note, it does only work for on lowercase letters however, I do hope it will help you to understand the logic a bit better.

def Decode(message, shift=-1):
    """
    Decodes a message from a caesar cipher

    Params:
     - message (str) : Message to decode
    Optional Params:
     - shift (int) : If shift is known

    Returns:
     - decodedMessage (str) : The decoded message
    """
    decodedMessage = ''
    message = message.lower() # lowercase is easier to work with

    # If the shift is known it is simple to decode
    if shift != -1:
        for letterIndex in range(len(message)):
            if message[letterIndex] in [' ', '!', '?', '.', '-', ',', '_']:
                decodedMessage += message[letterIndex]
            else:
                try:
                    index = ALPHABET.index(message[letterIndex]) - shift

                    # If the index is smaller then ALPHABET, handle it
                    while index < 0:
                        index += len(ALPHABET)

                    decodedMessage += ALPHABET[index]
                except Exception as e:
                    print("A problem occured:", e)
                    decodedMessage += '?'

        return decodedMessage
    else: #If shift is not known, figure it out thru brute force
        data = read_json('words_dictionary')
        for i in range(len(ALPHABET)):
            decodedMessage = Decode(message, i+1)
            wordList = decodedMessage.split(" ")
            try:
                # Loop over words counting english words
                count = 0
                for word in wordList:
                    if word in data.keys():
                        count += 1

                    # More accurate this way compared to only one word checks
                    if count > len(wordList) / 2:
                        return decodedMessage
            except KeyError:
                continue

Full code here

Word dictionary here

let me try this with the rest of my script but thanks a ton! — user14001760, Jul 27 '20 at 08:44

Decrypting Caeser Cipher in Python

1 Answers1