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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def crypto_box_seal(message, pk):
 """ Encrypts and returns a message ``message`` using an ephemeral secret key and the public key ``pk``. The ephemeral public key, which is embedded in the sealed box, is also used, in combination with ``pk``, to derive the nonce needed for the underlying box construct. :param message: bytes :param pk: bytes :rtype: bytes .. versionadded:: 1.2 """

    ensure(isinstance(message, bytes),
           "input message must be bytes",
           raising=TypeError)

    ensure(isinstance(pk, bytes),
           "public key must be bytes",
           raising=TypeError)

    if len(pk) != crypto_box_PUBLICKEYBYTES:
        raise exc.ValueError("Invalid public key")

    _mlen = len(message)
    _clen = crypto_box_SEALBYTES + _mlen

    ciphertext = ffi.new("unsigned char[]", _clen)

    rc = lib.crypto_box_seal(ciphertext, message, _mlen, pk)
    ensure(rc == 0,
           'Unexpected library error',
           raising=exc.RuntimeError)

    return ffi.buffer(ciphertext, _clen)[:]