Custom Multithreaded SSH Client

A fully custom SSH client developed from scratch in C++, implementing all aspects of the SSH protocol from the ground up using sockets, cryptographic primitives, POSIX threads, and raw terminal I/O.

Check out SSH-Client-Design.txt for an overview of the design

Features

• Implements core SSH protocol (RFC 4253)
  • Algorithm Negotiation
  • Key Exchange
  • Encryption/Decryption
  • Secure Channel setup
  • Interactive shell session
• POSIX sockets for TCP/IP communication over port 22
• Cryptographic operations using OpenSSL 3.0
• Multithreaded architecture with POSIX threads to ensure a smooth user experience
• Raw terminal mode for seamless interactive shell experience
• Packet-level data management
• Minimal external dependencies

Supported Algorithms

Key Exchange (KEX)

• curve25519-sha256
• diffie-hellman-group14-sha256

Public Key Algorithms

• ssh-ed25519
• rsa-sha2-256

Encryption

• aes128-ctr
• aes256-ctr

Message Authentication Code (MAC)

• hmac-sha2-256
• hmac-sha1

Compression

• none

Demo

Connecting to a Remote Terminal

Disconnecting from a Remote Terminal

Wireshark Capture

Build and Run

Dependencies

• g++ with C++20 support
• OpenSSL 3.0 (libcrypto)
• POSIX threads (libpthread)

Install Dependencies

sudo apt install -y build-essential libssl-dev

Build

# Clone the repository
git clone https://github.com/adsouza7/SSH-Client.git
cd SSH-Client/

# Normal build
make

# Verbose build (with logs)
make verbose

Run

# Run this command
./ssh-client <username>@<hostname>

References

• RFC 4253 - SSH Transport Layer Protocol
• RFC 4251 - SSH Protocol Architecture
• SSH Message Numbers
• SSH Connection Process
• POSIX Threads Documentation
• OpenSSL Documentation