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)):
        oldChar = ord(characters[i])
        if oldChar < LOWERCASE_INDEX:
            oldChar -= UPPERCASE_INDEX
            newChar = ( (oldChar + shift) % ALPHABET_LENGTH ) + UPPERCASE_INDEX
        else:
            oldChar -= LOWERCASE_INDEX
            newChar = ( (oldChar + shift) % ALPHABET_LENGTH ) + LOWERCASE_INDEX
        characters[i] = chr(newChar)

    return ''.join(characters)