This NIP defines a new command, `AUTH`, which can be sent by either the relay or the client with different meanings.
A relay may send `["AUTH", <human readable reason>, <data object>]` when it needs authentication (examples: accessing kind 4 events, whitelist only, requiring proof of authorship for `EVENT`).
The human readable reason SHOULD be prefixed with a string in the format `<short desc string>: <description>`. A list of short descriptions is listed below:
-`restricted`: This relay is restricted and requires the pubkey of the client to check if it can access the relay. (requires a whitelist, payment, etc)
-`publish`: The relay requests that the client identify who is sending an `EVENT` command.
This can be used for where only the signer of an event can publish it, or a pay-as-you-go relay allowing for you to publish others' events
-`private`: The client has attempted to access a restricted set of events (example: kind 4 DMs) and should authenticate with the relay to receive them.
-`other`: Any reason not defined here.
`data object` MUST be a JSON object. It currently has one defined field, but may be extended by amendments to this NIP or other NIPs:
-`subscription_id`: The subscription ID that triggered the `AUTH` request.
A client may send one of the following to the relay:
-`["AUTH", <signed event>]` to indicate it has accepted the request. This may also be sent without an authentication request.
-`["AUTH", null]` to indicate it has rejected the request.
A relay SHOULD send the [`OK`](https://github.com/nostr-protocol/nips/blob/master/20.md) command after they receive a
non-rejecting authentication response, and use one of the following `message` prefixes if the event sent cannot be verified:
-`too-old:`: The event signed has a too old `created_at` date.
-`invalid-url:`: The URL in `content` is not matching.
-`already-used:`: This event was already used to authenticate to this relay.
-`bad-signature:`: The event has a bad signature.
Please note that the `OK` message should only be sent as a response to other commands that the client sends instead of the `AUTH` command if the issue is not related to the authentication event being incorrectly signed (example: not on whitelist).
Relays SHOULD send [`EOSE`](https://github.com/nostr-protocol/nips/blob/master/15.md) when an authentication request is triggered by a `REQ` command, and not send stored events after the `EOSE` when authentication is completed.
Relays SHOULD send `OK` as a response when a command triggers authentication with the reason starting with `auth:`.
Clients SHOULD retry the action (resending event, resubscribing) after they authenticate if they receive an `AUTH` request.
## Signed event verification
Relays when processing `AUTH` responses SHOULD verify the following before accepting the response:
- that the `kind` is `22223`
- that the event was recently signed (~10 minutes, by `created_at`)
