Secure Chat App

Course project of 103.2 CrySec

Objectives

• Design and implementation of a complete application with GUI and network communication
• Implementation of cryptographic methods
• Autonomous Development

Constraints

• Python
• Initially CLI then GUI
• Communication with a provided server

Project Communication Protocol

Address: vlbelintrocrypto.hevs.ch Port: 6000

Overview

1. Transport Layer: TCP & Sockets
   • Communication happens over TCP, which ensures reliable and ordered message delivery as well as error-checks
   • Sockets are used, which are implemented using network programming APIs
2. Message format (General Structure): All messages follow a standard format that starts with a header
3. Connection rule
   • If the expected message structure is not followed, the connection is terminated

Header

Overview

• 1st, 2nd and 3rd bytes : Always the string 'ISC' (used as a protocol identifier)
• 4th byte : Indicates the type of message, which can be:
  • 't' -> Text message
  • 's' -> Server-only message (not broadcasted)
  • 'i' -> Image message

Message

• Text / Server Messages ('t' or 's')
  • 5th & 6th bytes: Store the number of characters in the message (N) as a big-endian 2-byte integer
  • N * 4 bytes: The actual message, where:
    • Each char is 4 bytes long (big-endian order)
    • The string is UTF-8 encoded
  • Server-only messages ('s'): These are meant only for server communication and are not broadcast to all users
• Image messages ('i')
  • 5th byte: Image width (max 128 pixels)
  • 6th byte: Image height (max 128 pixels)
  • Following bytes: The image's pixel data in RGB format
    • Pixels are stored row by row (y-axis first, then x-axis)
    • Each pixel consists of 3 consecutive bytes(RGB-RGB-RGB)

Using the app

Run the app using uv run src/main.py

You can use the CLI or the GUI to interact with the server

Run tests using uv run pytest

Diagrams

TCP Client and payload Building

---
config:
  class:
    hideEmptyMembersBox: true
---
classDiagram
direction TB
    class Client {
        + bytes_per_char: int
        + payload_builder: PayloadBuilder
	    + connect(IP_address: str, port: int)
	    + send(message: str, msg_type: PayloadType)
	    + receive(timeout: int) Payload
	    + close()
    }

    class PayloadBuilder {
	    + create(type: PayloadType, message: str) Payload
        + parse(payload: bytes) Payload
    }

    class Payload {
        + header: bytes
        + type: bytes
        + length: bytes
        + message: bytes
        + to_bytes() bytes

    }

    Client --> PayloadBuilder

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CLI MVC and Command Pattern

The command pattern is used because this app will have a CLI and a GUI. Both do the exact same thing so we need to centralize the logic inside of commands

The MVC pattern was simplified to a MV and a base handler which parses the command and passes the parsed information to the model. Which in turns executes the desired command

---
config:
  layout: dagre
  class:
    hideEmptyMembersBox: true
---
classDiagram
direction BT
    class Main {
    }

    class CommandInvoker {
	    - command: Command
	    + set_command(command: Command)
	    + execute_command()
    }

    class Command {
    }

    class CLIHandler {
        - cli_model: CLIModel
        + execute_cli_command(cli_command: str)
    }

    class CLIView {
	    + print_header()
	    + print_available_commands(cli_commands: list[CLICommand])
    }

    class CLIModel {
	    - cli_view: CLIView
	    - command_invoker: CommandInvoker
	    + CLIModel(command_invoker: CommandInvoker)
        + get_available_commands() list[CLICommand]
    }

    CLIHandler --> CLIModel
    Main --> CLIHandler
    Main --> CommandInvoker
    CLIModel --> CommandInvoker
    CLIModel --> CLIView
    CommandInvoker --> Command

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