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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_afternm(ciphertext, nonce, k):
 """ Decrypts and returns the encrypted message ``ciphertext``, using the shared key ``k`` and the nonce ``nonce``. :param ciphertext: 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_BOXZEROBYTES) + ciphertext
    plaintext = ffi.new("unsigned char[]", len(padded))

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

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