def decrypt(ciphertext, keyPath):
    """
    Decrypts a given message that was encrypted with the encrypt() method.

    :param ciphertext: The encrypted message (as a string).
    :param keyPath: A path to a file containing a 256-bit key (and nothing else).
    :type ciphertext: bytes
    :type keyPath: str
    :rtype: bytes

    Raises an error if ciphertext was modified
    >>> import tempfile
    >>> k = tempfile.mktemp()
    >>> with open(k, 'wb') as f:
    ...     _ = f.write(nacl.utils.random(SecretBox.KEY_SIZE))
    >>> ciphertext = encrypt("testMessage".encode('utf-8'), k)
    >>> ciphertext = b'5' + ciphertext[1:]
    >>> decrypt(ciphertext, k) # doctest: +IGNORE_EXCEPTION_DETAIL
    Traceback (most recent call last):
    ...
    CryptoError: Decryption failed. Ciphertext failed verification

    Otherwise works correctly
    >>> decrypt(encrypt("testMessage".encode('utf-8'), k), k).decode('utf-8') in (u'testMessage', b'testMessage', 'testMessage') # doctest: +ALLOW_UNICODE
    True

    >>> import os
    >>> os.remove(k)
    """
    with open(keyPath, 'rb') as f:
        key = f.read()
    if len(key) != SecretBox.KEY_SIZE:
        raise ValueError("Key is %d bytes, but must be exactly %d bytes" % (len(key),
                                                                            SecretBox.KEY_SIZE))
    sb = SecretBox(key)
    # The nonce is kept with the message.
    return sb.decrypt(ciphertext)