SDR-Chat

Encrypted, Bidirectional SDR Chat over FSK

A lightweight Python & C applicatoin enabling reliable, two-way chat over software-defined radios (e.g. USRP B205mini). Messages are framed with CRC-16, AES-256 encrypted, and exchanged using an RTS/CTS handshake to maximize link reliability.

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Table of Contents

1. Overview
2. Features
3. Architecture & Design
4. Requirements & Dependencies
5. Installation
6. Usage
7. Encryption & Security
8. Protocol Details
9. Error Handling & Diagnostics
10. License
11. Author

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Overview

SDR-Chat turns a USRP SDR into a secure text chat device. It combines Python for control and GUI, C for performance-critical modulation/demodulation, and UHD for radio interfacing.

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Features

• FSK Modulation & Demodulation
  FSK at configurable baud rates.
• AES-256 Encryption
  Encrypting every message payload.
• CRC-16 Error Detection
  Catching transmission errors before decryption.
• RTS/CTS Handshake
  Simple link-establishment protocol to avoid collisions.
• CLI & GUI Frontends
  Lightweight terminal mode or a minimal GUI.
• Configurable Parameters
  Frequency, gain, baud rate, preamble length, and more via flags or config file.

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Architecture & Design

+-----------+      +-----------+      +------------+      +----------+
|  main.py  | <--> |  C module | <--> |    UHD     | <--> |   SDR    |
|  (Python) |      |   (FSk)   |      |   drivers  |      | hardware |
+-----------+      +-----------+      +------------+      +----------+
      ^
      |
  +--------+
  | GUI UI |
  +--------+

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Requirements & Dependencies

• Hardware
  • Ettus USRP B200mini or B205mini (other UHD-compatible radios may work).
• Software
  • Python 3.8+
  • UHD drivers & firmware
  • C compiler (GCC/Clang)

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Installation

1. Clone the repo

   git clone https://github.com/roeelupo/sdr-chat.git
   cd sdr-chat

2. Build the projet

   pip install .

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Usage

export AES_KEY=$(openssl rand -hex 16)
sdr-chat --help

CLI Mode

sdr-chat --aes-key $AES_KEY
sdr-chat --aes-key "d76df8"

GUI Mode

sdr-chat --aes-key $AES_KEY --gui

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Encryption & Security

• AES-256-CBC for payload confidentiality.
• Each message includes a random IV—prepended to the ciphertext.
• Pre-shared key or config file specifies AES key.

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Protocol Details

Field	Size	Description
Preamble	16 bits	0xAA55 pattern for sync
Header	8 bits	Payload length & flags
Payload	0–256 bytes	AES-encrypted message
CRC-16	16 bits	CRC-16 of header+payload

• FSK frequencies: mark = +5 kHz, space = –5 kHz offset by center frequency.
• RTS/CTS: short control frames before data exchange to reserve the channel.

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Error Handling & Diagnostics

• CRC Failures: Logged to console/GUI; peer will automatically retransmit.
• Timeouts: If no CTS received within 500 ms, CLI retries handshake.

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License

Distributed under the MIT License. See LICENSE for details.

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Author

Roee Lupo
– Self-Proclaimed Engineer & SDR Enthusiast
– GitHub · roeelupo@roeelupo.com