# Python Casear Cipher Functions
# By Eliot Ball 05/11/2010

# Function to encrypt a message using the Casear Cipher with a specific shift
# Takes in a string Plaintext and an integer Shift as arguments
# Returns a string containing the ciphertext
def CaesarEncrypt(Plaintext, Shift):
    # Convert the Plaintext to upper case
    Plaintext = Plaintext.upper()
    # Start with a blank ciphertext
    Ciphertext = ""
    # Load the alphabet into a variable ready to use for shifting
    Alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    # Iterate through each character in the plaintext
    for Character in Plaintext:
        # Check for this being a letter (it appears in the alphabet)
        if Alphabet.find(Character) != -1:
            # Find the position in the alphabet of this letter, add the shift to that,
            # find the remainder when dividing by 26 and take that letter from the
            # alphabet and append it to the ciphertext
            Ciphertext += Alphabet[(Alphabet.find(Character) + Shift) % len(Alphabet)]
        # Otherwise it is not a letter (ie a space or punctuation etc)
        else:
            # Just append the character as it is to the ciphertext
            Ciphertext += Character
    # Return the ciphertext
    return Ciphertext

# Function to decrypt a message using the Caesar Cipher with a specific shift
# Takes in a string Ciphertext and an integer Shift as arguents
# Returns a string containing the plaintext
# Algorithm is identical to CasearEncrypt but the shift value is subtracted
# rather than added
def CaesarDecrypt(Ciphertext, Shift):
    Ciphertext = Ciphertext.upper()
    Plaintext = ""
    Alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    for Character in Ciphertext:
        if Alphabet.find(Character) != -1:
            Plaintext += Alphabet[(Alphabet.find(Character) - Shift) % len(Alphabet)]
        else:
            Plaintext += Character
    return Plaintext