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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_afternm(message, nonce, k):
 """ Encrypts and returns the message ``message`` using the shared key ``k`` and the nonce ``nonce``. :param message: bytes :param nonce: bytes :param k: bytes :rtype: bytes """

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

    if len(k) != crypto_box_BEFORENMBYTES:
        raise exc.ValueError("Invalid shared key")

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

    rc = lib.crypto_box_afternm(ciphertext, padded, len(padded), nonce, k)
    ensure(rc == 0,
           'Unexpected library error',
           raising=exc.RuntimeError)

    return ffi.buffer(ciphertext, len(padded))[crypto_box_BOXZEROBYTES:]