# Covert Protocol
The Covert Protocol is the protocol used to communicate over the covert channel. The communication over the covert channels aims mainly on file transfers.

# Bidirectional Covert Protocol
A bidirectional communication which uses commands to manage a file transfer.

A packet is at least 2 bytes big. The first byte is the header followed by one or multiple data bytes.

```
|   1 byte   |       n bytes       |
|   header   |        data         |
```



## Header
A header contains a command and a segment number. Even if the `no operation` command is used the segment number must be counted.

```
|           8 bit           |
|           header          |

| 2 bit |  3 bit  |  3 bit  |
|  seg  |    -    |   com   |
```

### Command
Commands which indicate

```
0x0 no operation
0x1 send file name size
0x2 send file name
0x3 send data size
0x4 send data
0x5 -
0x6 cancel / reset
0x7 error
```

The sending commands must be sent in the right order: lower number to higher number
You can of course send a `no operation` or a `cancel / reset` command. If the order is not followed an `error` answer should be invoked.


#### `0x0 no operation`
No operation is the command which indicates to do nothing. It simply works as ACK to signal the packet was received.

#### `0x1 send file name size`
A file name size is a one byte number.
```
0 < file name size < 256
```

#### `0x2 send file name`
A file name is a string which can be up to 255 bytes long.
This command must be looped until the file name is fully transmitted!

#### `0x3 send data size`
A data size is a 4 byte unsigned int.
This command must be looped until the data size is fully transmitted!

#### `0x4 send data`
Data are as much bytes as communicated in `send data size`.
This command must be looped until the data is fully transmitted!

#### `0x6 cancel / reset`
This command resets the transmission. The following segment number is the segment number of the reset packet plus 1.

#### `0x7 error`
This command signals there was an error and the transmission is to be resetted. The following segment number is the segnemtn number of the error packet plus 1.


### Segment
The segment number is a counter for the packets in the covert channels. The segment number is increased on the passive server side.

An example how the packet exchange might look like.
```
A                       B
seg                   seg
=========================
1 -------------------->
  <-------------------- 2
2 -------------------->
  <-------------------- 3
3 --------- X                Packet lost

3 -------------------->      Retransmission from A
  <-------------------- 4
4 -------------------->
            X --------- 5    Packet lost

            X --------- 5    Retransmission from B
4 --------------------> 4    Retransmission from A
  <-------------------- 5
```



It might happen that a server is not possible to answer or all the answers won't be transmitted. This case should be handled with the `cancel / reset` or `error` command upon reconnection. Maybe the session is recoverable without a reset.
```
A                       B
seg                   seg
=========================
1 -------------------->
1 -------------------->
1 -------------------->
```