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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:
 async def _auth_cram_md5(self, username, password):
 """ Performs an authentication attemps using the CRAM-MD5 mechanism. Protocol: 1. Send 'AUTH CRAM-MD5' to server ; 2. If the server replies with a 334 return code, we can go on: 1) The challenge (sent by the server) is base64-decoded ; 2) The decoded challenge is hashed using HMAC-MD5 and the user password as key (shared secret) ; 3) The hashed challenge is converted to a string of lowercase hexadecimal digits ; 4) The username and a space character are prepended to the hex digits ; 5) The concatenation is base64-encoded and sent to the server. 6) If the server replies with a return code of 235, user is authenticated. Args: username (str):
 Identifier of the user trying to authenticate. password (str):
 Password for the user. Raises: ConnectionResetError: If the connection with the server is unexpectedely lost. SMTPAuthenticationError: If the authentication attempt fails. Returns: (int, str):
 A (code, message) 2-tuple containing the server response. """

        mechanism = "CRAM-MD5"

        code, message = await self.do_cmd("AUTH", mechanism, success=(334,))

        decoded_challenge = base64.b64decode(message)

        challenge_hash = hmac.new(
            key=password.encode("utf-8"), msg=decoded_challenge, digestmod="md5"
        )

        hex_hash = challenge_hash.hexdigest()
        response = "{} {}".format(username, hex_hash)
        encoded_response = SMTP.b64enc(response)

        try:
            code, message = await self.do_cmd(encoded_response, success=(235, 503))
        except SMTPCommandFailedError as e:
            raise SMTPAuthenticationError(e.code, e.message, mechanism)

        return code, message