python-hard-way/caeser-cipher.py

import re # Import Regex tools
import string # Import string tools
import collections

ciphertext = input("Please type in your cipher text:") # Set variable cipertext to the input from user.
shiftNum = input("Please enter the shift used (Just hit enter if you don't know):") # Get input
action = input("Please enter encrypt or decrypt):") # Get input
ciphertext = re.sub (r'([^a-zA-Z ]+?)', '', ciphertext) # Remove all non-letters.
ciphertext = ciphertext.lower() # Make it all lowercase. https://www.geeksforgeeks.org/isupper-islower-lower-upper-python-applications/
letters = list(string.ascii_lowercase) # Use a list of lowercase letters. https://stackoverflow.com/questions/43918437/how-to-iterate-through-the-alphabet
letterCount = 0 # Set variable for keeping track of which letter in the ciphertext we're on.
letterPos = 0 # Set variable for keeping track of the letter's position in the alphabet.
answer = "" # The solution.
commonletter = 0

def encrypt():
    # Grab global variables.
    global ciphertext
    global shiftNum
    global letters
    global letterCount
    global letterPos
    global answer

    for c in ciphertext: # For every letter in the ciphertext
        while letterPos < 26: # While the letter's position in the alphabet is less than 26 (and thus valid), continue.

            if ciphertext[letterCount] == letters[letterPos]: # Match the letter in the ciphertext to a letter in the alphabet and once they match, continue.
                letter = int(letterPos) + int(shiftNum) # Take the position of the letter and the shift number and add them for the enciphered letter.

                if letter > 25: # If the enciphered letter's position is not valid because it is too high, fix it by rotating around the alphabet.
                    letter = letter - 26
                if letter < 0:
                    letter = letter + 26 # If the enciphered letter's position is not valid because it is too low, fix it by rotating around the alphabet.

                answer = answer + letters[letter] # Add letters together to get the enciphered text.

            letterPos = letterPos + 1 # Iterate through letter positions in the alphabet. (neccessary to find one that matches.)

        if letterPos > 25: # If the letter position is greater than 25, cycle back through the alphabet.
            letterPos = letterPos - 26

        letterCount = letterCount + 1 # Keep track of how many times we're doing this.

    print("\nYour enciphered text is: " + answer)


def decrypt():

    # Grab global variables.
    global ciphertext
    global shiftNum
    global letters
    global letterCount
    global letterPos
    global answer
    global commonletter

    for c in ciphertext: # For every letter in the ciphertext
        while letterPos < 26: # While the letter's position in the alphabet is less than 26 (and thus valid), continue.

            if not commonletter == 0:
                if commonletter == letters[letterPos]: # If the most frequent letter matches the current letter, set the shift to that letter because that is the shift for the cipher. Forgot the double equals for comparison.
                    shiftNum = letterPos - 4

            if ciphertext[letterCount] == letters[letterPos]: # Match the letter in the ciphertext to a letter in the alphabet and once they match, continue.
                letter = int(letterPos) - int(shiftNum) # Take the position of the letter and the shift number and add them for the enciphered letter.

                if letter > 25: # If the enciphered letter's position is not valid because it is too high, fix it by rotating around the alphabet.
                    letter = letter - 26
                if letter < 0:
                    letter = letter + 26 # If the enciphered letter's position is not valid because it is too low, fix it by rotating around the alphabet.

                answer = answer + letters[letter] # Add letters together to get the enciphered text.

            letterPos = letterPos + 1 # Iterate through letter positions in the alphabet. (neccessary to find one that matches.)

        if letterPos > 25: # If the letter position is greater than 25, cycle back through the alphabet.
            letterPos = letterPos - 26

        letterCount = letterCount + 1 # Keep track of how many times we're doing this.

    print("\nYour decrypted text is: " + answer)


def freq():
    # Grab global variables.
    global ciphertext
    global commonletter

    commonletter = collections.Counter(ciphertext).most_common()[0][0] # Find most common letter and thus the shift https://stackoverflow.com/questions/47251934/how-to-count-the-most-frequent-letter-in-a-string
    decrypt() # Decrypt using the frequency found in here.


if action == "encrypt":
    encrypt()
if action == "decrypt":
    if shiftNum == "":
        freq()
    else:
        decrypt()