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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_open(ciphertext, nonce, pk, sk):
 """ Decrypts and returns an encrypted message ``ciphertext``, using the secret key ``sk``, public key ``pk``, and the nonce ``nonce``. :param ciphertext: bytes :param nonce: bytes :param pk: bytes :param sk: bytes :rtype: bytes """

    if len(nonce) != crypto_box_NONCEBYTES:
        raise exc.ValueError("Invalid nonce size")

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

    if len(sk) != crypto_box_SECRETKEYBYTES:
        raise exc.ValueError("Invalid secret key")

    padded = (b"\x00" * crypto_box_BOXZEROBYTES) + ciphertext
    plaintext = ffi.new("unsigned char[]", len(padded))

    res = lib.crypto_box_open(plaintext, padded, len(padded), nonce, pk, sk)
    ensure(res == 0, "An error occurred trying to decrypt the message",
           raising=exc.CryptoError)

    return ffi.buffer(plaintext, len(padded))[crypto_box_ZEROBYTES:]