Skip to content
Concord

SDKs and libraries

vector_sdk

Section titled “vector_sdk”

An ergonomic Rust SDK for building bots and clients on top of vector-core. It handles keys, relays, streams, and encryption so you write handlers instead of cryptography.

[dependencies]
vector_sdk = "0.8"

An optional tor feature routes traffic through an anonymising transport, which matters more here than in most protocols: Concord hides what you say and who is in the room, but the network layer still sees that you connected to a relay.

The price-bot repository is a compact worked example of the SDK’s command handling.

Writing your own

Section titled “Writing your own”

There is no requirement to use an SDK. Concord rides on ordinary Nostr events, so any Nostr library plus NIP-44 gets you most of the way. What you have to add is specific and small in surface area, if not in care:

Key derivation. The group_key construction — HKDF into a normalized secp256k1 scalar, then the x-only pubkey and a NIP-44 self-ECDH conversation key. Every label is frozen; the table is in CORD-02 Appendix A.

The three-layer event. Wrap, seal, rumor. Note that the seal is kind 20013 when its content is encrypted and kind 20014 when it carries the rumor’s JSON verbatim, and that the choice is a fixed property of the plane, never a per-message decision.

The fold. Reducing chained editions to a converged head, identically to every other client.

Epoch handling. Querying every epoch address you hold, precomputing the next rekey address to catch rotations in real time, and verifying continuity before adopting a new key.

Reference implementations

Section titled “Reference implementations”

The three shipping clients are all open source, and reading one is the fastest route to understanding the protocol in practice. They are usefully different from each other:

Client Language Notable for
Vector Rust (Tauri) Native desktop and mobile, Tor transport, the vector_sdk foundation
Armada TypeScript (Capacitor) Full Discord-shaped feature set, multi-protocol, voice and video
Accordion TypeScript A compact browser-only implementation built on the applesauce toolkit

Accordion in particular is worth reading if you want to see how little code it takes to speak Concord once the derivations are right.

What to test first

Section titled “What to test first”

Interoperability failures in Concord are usually silent — you land at a different address and simply see an empty room rather than an error. So test in this order:

  1. Derive a known address. Take a community you can already open in an existing client and confirm your implementation computes the same control_pk and channel addresses.
  2. Decrypt one message. All three layers, ending at a rumor whose channel and epoch tags match what you expected.
  3. Fold a roster. Confirm you reach the same head as another client, including the refuse-downgrade rule.
  4. Survive a rotation. The rekey path is where implementations most often diverge, and where the failure mode is a member silently locked out.