UPPERCASE_INDEX = 65
LOWERCASE_INDEX = 97
ALPHABET_LENGTH = 26

class NonIntegerShiftError(ValueError): pass
class NonStringInputError(ValueError): pass

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

    if not isinstance(plaintext, str):
        raise NonStringInputError('Can only encrypt a string.')
    if not isinstance(shift, int):
        raise NonIntegerShiftError('Can only shift by an integral value.')
    characters = list(plaintext)

    for i in range(len(characters)):
        if characters[i].isalpha():
            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)

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

    if not isinstance(ciphertext, str):
        raise NonStringInputError('Can only encrypt a string.')
    if not isinstance(shift, int):
        raise NonIntegerShiftError('Can only shift by an integral value.')
    characters = list(ciphertext)

    for i in range(len(characters)):
        if characters[i].isalpha():
            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)