# Relay: Post Message

Forward a ceremony envelope to another party (or broadcast to all).

## Overview

Every protocol message produced by the engine — DKG round, signing share, etc. — is wrapped in an **envelope** and posted to the relay. The `wire` field is base64-encoded opaque protocol bytes; the relay treats it as a black box. Recipients pick up envelopes via `/inbox`.

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

Authentication: bearer token required.

Posts one envelope. The relay never inspects the wire payload.

**Path parameters**

| Name | Type | Required | Description |
| --- | --- | --- | --- |
| `id` | string | yes | Relay session id |

**Request body**

| Name | Type | Required | Description |
| --- | --- | --- | --- |
| `from` | string | yes | Sender moniker |
| `to` | string | no | Recipient moniker — omit if broadcast=true |
| `broadcast` | boolean | no | If true, delivered to every other party (default: false) |
| `wire` | string | yes | Base64-encoded protocol envelope |

**Responses**

`204` Accepted

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