nc 4239

Let’s connect to the server and see what’s going on.

It seems the server wants to send us some data. It offers 8-1-1 even parity to prevent us from getting the data incorretly. We will use 8-1-1 even parity to check whether the byte sent is correct. If it is correct we will send ‘1’ to notify the server that we got the byte, or else we will send ‘0’ to ask for retransmission.

The server is sending 11 bits as follows:

1 – start bit
8 – data bits
1 – parity bit
1 – stop bit

The start bit must be zero, and the stop bit must be one. Also, the number of ones in the 9 bits [data:parity] must be even.

Here is a python script which extracts the data.

Let’s run the script.

The flag is flag{@[email protected]@_circuit-term1nating_3quipment}.