UPPERCASE_INDEX = 65
LOWERCASE_INDEX = 97
ALPHABET_LENGTH = 26

def encrypt(plaintext, shift):
    '''encrypt a string using the caesar cipher'''

    characters = list(plaintext)

    for i in range(len(characters)):
        if characters[i].isalpha():
            oldChar = ord(characters[i])
            if oldChar < LOWERCASE_INDEX:
                oldChar -= UPPERCASE_INDEX
                shiftedChar = oldChar + shift
                if shiftedChar >= 0:
                    newChar = ( shiftedChar % ALPHABET_LENGTH ) + UPPERCASE_INDEX
                else:
                    newChar = shiftedChar + ALPHABET_LENGTH * -(shiftedChar // ALPHABET_LENGTH) + UPPERCASE_INDEX
            else:
                oldChar -= LOWERCASE_INDEX
                shiftedChar = oldChar + shift
                if shiftedChar >= 0:
                    newChar = ( shiftedChar % ALPHABET_LENGTH ) + LOWERCASE_INDEX
                else:
                    newChar = shiftedChar + ALPHABET_LENGTH * -(shiftedChar // ALPHABET_LENGTH) + LOWERCASE_INDEX
            characters[i] = chr(newChar)

    return ''.join(characters)

def decrypt(ciphertext, shift):
    '''decrypt a string using the caesar cipher'''

    characters = list(ciphertext)

    for i in range(len(characters)):
        if characters[i].isalpha():
            oldChar = ord(characters[i])
            if oldChar < LOWERCASE_INDEX:
                oldChar -= UPPERCASE_INDEX
                shiftedChar = oldChar - shift
                if shiftedChar >= 0:
                    newChar = ( shiftedChar % ALPHABET_LENGTH ) + UPPERCASE_INDEX
                else:
                    newChar = shiftedChar + ALPHABET_LENGTH * -(shiftedChar // ALPHABET_LENGTH) + UPPERCASE_INDEX
            else:
                oldChar -= LOWERCASE_INDEX
                shiftedChar = oldChar - shift
                if shiftedChar >= 0:
                    newChar = ( shiftedChar % ALPHABET_LENGTH ) + LOWERCASE_INDEX
                else:
                    newChar = shiftedChar + ALPHABET_LENGTH * -(shiftedChar // ALPHABET_LENGTH) + LOWERCASE_INDEX
            characters[i] = chr(newChar)

    return ''.join(characters)