Warm Up

Network Security Spring 2021

The goals of this assignment are to:

  1. Refresh your TCP and UDP programming skills
  2. Build an assignment submission container image

“Packet Clapping”

The foundations module in Canvas contains a Linux x86_64 statically-linked binary named clap. This binary implements a “packet clapping” game. Analogous to hand clapping games, your goal is to write a program that follows the lead of the clap server by receiving instructions over a command channel and interpreting them to play the game.

The command channel implements the following protocol:

\[ \begin{align*} C \rightarrow S &: \mathsf{u16}(|\mathsf{identifier}|) \cdot \mathsf{identifier} \\ S \rightarrow C &: \langle \mathsf{command} \rangle \\ S \rightarrow C &: \langle \mathsf{command} \rangle \\ S \rightarrow C &: \ldots \\ S \rightarrow C &: \langle \mathsf{secret} \rangle \\ \end{align*} \]

where \(\mathsf{identifier}\) is a UTF-8 string that is your @northeastern.edu email address, \(|x|\) is the length of a byte array \(x\), \(\mathsf{u16}(x)\) encodes an integer \(x\) as two big-endian bytes, and \(\cdot\) is concatenation. After the client sends its remote ID, the server repeatedly sends one or more command messages. Each command message takes the form:

\[ \begin{align*} \langle \mathsf{command} \rangle &= \mathsf{u8}(\mathsf{type}) \cdot \mathsf{u16}(\mathsf{port}) \cdot \mathsf{u64}(\mathsf{challenge}) \\ \end{align*} \]

where \(\mathsf{u8}\) and \(\mathsf{u64}\) are 1 and 8-byte analogues of \(\mathsf{u16}\). If the client receives a type 1 command, it should listen for a TCP connection on the indicated port, send \(\mathsf{u64}(\mathsf{challenge})\) on the first connected socket, and immediately close both the client and server socket. If the client receives a type 2 command, it should connect a TCP socket to the server IP address using the indicated command port, send \(\mathsf{u64}(\mathsf{challenge})\) on the socket, and immediately close the socket. Finally, if the client receives a type 3 command, it should create a UDP socket, and send a datagram containing \(\mathsf{u64}(\mathsf{challenge})\) to the server on the indicated port.

The secret command takes the form:

\[ \begin{align*} \langle \mathsf{secret} \rangle &= \mathsf{u8}(0) \cdot \mathsf{u16}(|\mathsf{value}|) \cdot \mathsf{value} \\ \end{align*} \]

The only content your program should write to stdout is the following JSON object:

    "id": "{{identifier}}",
    "value": "{{secret}}"

Whitespace formatting doesn’t matter, only that piping the output of your program validates as JSON of the above form. Feel free to write whatever you like to stderr.

Submission Instructions

Package your solution as a gzipped TAR archive. Your solution should expand to the following directory structure.

$ tree -F clap
├── Dockerfile
└── src/

The source code to your solution should be contained in src/. Your Dockerfile should, when processed using docker, create a container image that runs your solution against a clap server given a server command channel socket address as an argument. For instance, if the clap server is running on, then your container must successfully print the secret when executed like so:

$ docker run -it --rm ${solution_image}

Submit the solution archive to Canvas.