Encrypts the plaintext message using the given `nonce` and returns
        the ciphertext encoded with the encoder.

        .. warning:: It is **VITALLY** important that the nonce is a nonce,
            i.e. it is a number used only once for any given key. If you fail
            to do this, you compromise the privacy of the messages encrypted.

        :param plaintext: [:class:`bytes`] The plaintext message to encrypt
        :param nonce: [:class:`bytes`] The nonce to use in the encryption
        :param encoder: The encoder to use to encode the ciphertext
        :rtype: [:class:`nacl.utils.EncryptedMessage`]

def encrypt(self, plaintext, nonce, encoder=encoding.RawEncoder):
        """
        Encrypts the plaintext message using the given `nonce` and returns
        the ciphertext encoded with the encoder.

        .. warning:: It is **VITALLY** important that the nonce is a nonce,
            i.e. it is a number used only once for any given key. If you fail
            to do this, you compromise the privacy of the messages encrypted.

        :param plaintext: [:class:`bytes`] The plaintext message to encrypt
        :param nonce: [:class:`bytes`] The nonce to use in the encryption
        :param encoder: The encoder to use to encode the ciphertext
        :rtype: [:class:`nacl.utils.EncryptedMessage`]
        """
        if len(nonce) != self.NONCE_SIZE:
            raise ValueError("The nonce must be exactly %s bytes long" %
                             self.NONCE_SIZE)

        ciphertext = libnacl.crypto_box_afternm(
            plaintext,
            nonce,
            self._shared_key,
        )

        encoded_nonce = encoder.encode(nonce)
        encoded_ciphertext = encoder.encode(ciphertext)

        return EncryptedMessage._from_parts(
            encoded_nonce,
            encoded_ciphertext,
            encoder.encode(nonce + ciphertext),
        )