# Relay: Join Session

Announce a party (DEVICE / BACKUP / AUREA) into an MPC ceremony session.

## Overview

The relay is a thin message-broker layer used by every MPC ceremony — DKG, sign, and the self-custody exit. It holds **no key material**; it only forwards opaque protocol envelopes between parties identified by `moniker`. Each party announces itself with its long-lived public key so the others can authenticate inbound messages.

### `POST /v1/mpc/relay/session/{id}/join`

Authentication: bearer token required.

Registers a party in the session. Idempotent — re-joining returns 204.

**Path parameters**

| Name | Type | Required | Description |
| --- | --- | --- | --- |
| `id` | string | yes | Relay session id (from /dkg/start or /sign/start) |

**Request body**

| Name | Type | Required | Description |
| --- | --- | --- | --- |
| `moniker` | string | yes | Party label — DEVICE, BACKUP, or AUREA |
| `key` | string | yes | Hex-encoded party-long-term public key (for envelope authentication) |

**Responses**

`204` Joined

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Web version: https://docs.aureahub.com/#mpc-relay-join