Caesar cipher (Comp. Sci. 2020)

caesar_cipher.py

# Title: Caesar Cipher Encryption and Decryption Python Functions
# Author: Samit Shaikh
# Purpose: To underestand an encryption algorithm and implement it in Python.
# Last Revision: 17/06/2020
# Note: In this revision I removed alot of the type checking because I have
#       learned that in Python that its good practice not to unlike most other
#       languages as it assumes the developer knows what they are doing with
#       the function. I also added comments to describe it.

ALPHABET = "abcdefghijklmnopqrstuvwxyz"  # The alphabet in lowercase.

# ==================== CAESAR ENCRYPTION ALGORITHM ===========================
def caesar_e(
    plainText: str,
    keyvalue: int,
    *,
    alphabet=ALPHABET,
    includeForeignChars=True,
    caseStrat="strict"
):
    # Create an empty string for the encrypted string.
    encrypted = ""
    # Loop throught the text input.
    for char in plainText:
        # Create a copy of newChar to modify.
        newChar = char
        # If the case strategy is to maintain or ignore and and foreign
        # characters are not to be included.
        if caseStrat == "maintain" or "ignore" and not includeForeignChars:
            # Make newChar lowercase.
            newChar = char.lower()
        if newChar in alphabet:
            # If the lowercased char is in the alphabet...
            toAppend = alphabet[
                # Join together the characters in the array of chars incase
                # it isn't a string already. Add the key value to the the
                # ASCII value of the char.
                ("".join(alphabet).find(newChar) + keyvalue)
                % len(alphabet)
            ]
            # Add the new character to the encrypted string.
            encrypted += (
                (toAppend.upper() if char.isupper() else toAppend)
                if caseStrat == "maintain"
                else toAppend
            )
        else:
            # Just add the new char to the encrypted string if new char
            # is not not in the alphabet and include foreign chars is
            # enabled.
            if includeForeignChars:
                encrypted += newChar
    # Return the final encrypted string.             
    return encrypted

# ==================== CAESAR DECRYPTION ALGORITHM ===========================
def caesar_d(
    ciphertext: str,
    keyvalue: int,
    *,
    alphabet=ALPHABET,
    includeForeignChars=True,
    caseStrat="strict"
):
    # Just run the encryption algorithm with a negative key value to 
    # make it decrypt instead.
    return caesar_e(
        ciphertext,
        0 - keyvalue,
        alphabet=alphabet,
        includeForeignChars=includeForeignChars,
        caseStrat=caseStrat,
    )


# ============================ FOR TESTING ===================================
if __name__ == "__main__":
    string = str(input("To encrypt: "))
    key = int(input("Key value: "))
    caseStrategy = str(input("Case strategy (maintain/ignore/strict): "))
    includeForeignChars = str(input("Include foreign characters? (t/f): "))
    includeForeignChars = True if includeForeignChars == "t" else False
    caesar = None
    while caesar is None:
        encryptQ = str(input("Encrypt or decrypt (e/d)"))
        if encryptQ == "e":
            caesar = caesar_e
        elif encryptQ == "d":
            caesar = caesar_d
        else:
            print("Incorrect input, try again...")
    print(
        caesar(
            string,
            key,
            includeForeignChars=includeForeignChars,
            caseStrat=caseStrategy,
        )
    )