// k-os chat Linux

A self-hosted Discord alternative for small friend groups. Native desktop client with real-time text chat, voice chat, video streaming, inline media, friends & DMs, end-to-end encrypted channels, and role-based permissions. Your server. Your rules. Your voice.

Built in C++20 with a custom binary protocol — no Electron, no web browser, no cloud dependencies. Spin up the server on a VPS or your own box; everyone runs the native client.

## // WHAT IT DOES //

K-os is a full chat stack that runs on machines you own.

Text chat — real-time messaging, message history, threads, replies, edits, deletes, pinning, full-text search, link previews, channel topics, typing indicators, markdown (bold, italic, ~~strike~~, ||spoiler||, `code`, ```code blocks```), emoji reactions, 32 built-in emojis plus custom server emojis, scheduled messages, polls, saved messages, and OSRS-style colour prefixes (red:hi everyone).

Voice chat — Opus codec with SDL2 capture/playback through an SFU relay. Push-to-talk, voice-activity detection, or toggle modes. RNNoise neural denoise, Opus DTX (silent participants cost zero bandwidth) and in-band FEC (single-packet losses recover without a click). Per-user volume sliders, spatial audio (drag participants on a 2D pad to position them in stereo), and a voice-effects rig with pitch shift, reverb, robot, telephone, distortion, and bitcrush — all live, with a standalone tuner that loops your mic back to your own speakers so you can dial in your sound without joining a call. Whisper hotkey routes only to selected users. Voice messages, soundboard, and personal entrance sounds round it out.

Video — screen sharing (DXGI on Windows, X11 on Linux) and webcam capture. The streamer probes for hardware H.264 (NVENC / QuickSync / AMF / VideoToolbox / VA-API) and falls back to libx264 or VP8 software so anything with a GPU made since ~2010 gets free hardware encode. Adaptive bitrate (median-based across viewers so one slow watcher doesn't drag everyone down), NACK retransmission with dedupe, server-enforced caps, and a viewer-side magnifier (mouse-wheel zoom + right-click pan).

Friends & DMs — friend requests, presence, end-to-end encrypted 1-on-1 and group DMs (X25519 ECDH + AES-256-GCM), encrypted attachments, read receipts, DM threads, DM voice calls.

End-to-end encrypted channels — the server is a blind relay. Encrypted channel keys are distributed per member; attachments auto-purge from the server once everyone has them.

Admin & moderation — role-based permissions (41 granular flags across 6 categories), kick / ban / mute, ban-list management, server-side broadcasts, audit log, user-approval queue, file-upload whitelist, custom emoji management, ownership transfer.

Bot / scripting — embedded Lua 5.4 sandbox with 65+ API functions and 30+ events. Music streaming via yt-dlp + ffmpeg + Opus into voice channels.

Notifications, themes, accessibility — unread badges, @mention highlighting, system-tray integration, snooze, 50+ themable colour fields with a live editor, theme Import/Export so you can share palettes with friends as JSON files (drop them into a themes/ folder to use them), full DPI / UI scaling (100% / 125% / 150% / 175% / 200% presets), reduce-motion toggle.

## // HOW IT WORKS //

┌──────────────┐      TCP :7770 (TLS)     ┌──────────────┐
│    Client    │◄────────────────────────►│    Server    │
│  SDL2+ImGui  │       UDP :7771          │  SQLite DB   │
│  OpenGL3     │◄────────────────────────►│  SFU Voice   │
└──────────────┘     (voice/video/Opus)   └──────────────┘

- Client — SDL2 + Dear ImGui with the OpenGL3 backend. Native, ~15 MB install. Talks to the server over TCP (control + text/media) and UDP (voice + video). - Server — single-process, multi-threaded. select()-based TCP event loop plus a dedicated UDP voice/video thread that runs as an SFU (forwards packets per channel; never mixes audio, never re-encodes video). SQLite for all persistence — messages, users, channels, attachments, audit. Self-signed TLS certs auto-generated on first run. - Protocol — custom binary, little-endian. 8-byte TCP header plus payload. 100+ message types covering auth, channels, text, media, voice, video, friends, DMs, roles, reactions, admin, and encryption. Versioned (current: v2 — peers with mismatched versions are rejected at handshake). - E2E layer — for encrypted channels and DMs, the server is treated as untrusted. X25519 ECDH between participants establishes a shared key; AES-256-GCM encrypts messages and attachments. The server stores ciphertext only; encrypted attachments auto-purge once all recipients fetch them. - Transport encryption for voice/video — a per-channel shared key (AES-256-GCM) is distributed by the server to each member, wrapped with a KEK derived from that member's voice session token via HKDF-SHA256. The SFU forwards packets byte-for-byte without decrypting — senders encrypt once, viewers decrypt. Keys rotate on join/leave with a short overlap so in-flight packets still decode cleanly.

## // GETTING STARTED //

The easy path is the Windows installer or the macOS app below — download, run, you're in. The client will guide you through connecting to a server. (The macOS build is Apple Silicon, self-contained, and unsigned — right-click → Open the first time.)

To run your own server, grab the server zip, unzip it on any Windows box with TCP 7770 + UDP 7771 reachable, and run kos_server.exe. Defaults are sensible out of the box. A Linux build is coming for the same release line.

Licensed GNU AGPL v3: same license as Mastodon, Nextcloud, and Matrix's Synapse. Anyone modifying K-os and running a public server must publish their changes. Full source release coming soon — the repository is currently private while the early 1.x line stabilizes.