NIP-42
======

Authentication of clients to relays
-----------------------------------

`draft` `optional` `author:Semisol` `author:fiatjaf`

This NIP defines a way for clients to authenticate to relays by signing an ephemeral event.

## Motivation

A relay may want to require clients to authenticate to access restricted resources. For example,

  - A relay may request payment or other forms of whitelisting to publish events -- this can naïvely be achieved by limiting publication
    to events signed by the whitelisted key, but with this NIP they may choose to accept any events as long as they are published from an
    authenticated user;
  - A relay may limit access to `kind: 4` DMs to only the parties involved in the chat exchange, and for that it may require authentication
    before clients can query for that kind.
  - A relay may limit subscriptions of any kind to paying users or users whitelisted through any other means, and require authentication.

## Protocol flow

This NIP defines a new message, `AUTH`, which clients can send to relays when they want to authenticate. The message is of the following form:

```
["AUTH", <signed-event>]
```

The signed event is an ephemeral event not meant to be published or queried, it must be of `kind: 22242` and content must be set to the
WebSocket URL of the relay. `created_at` should be the current time. Example:

```json
{
  "id": "...",
  "pubkey": "...",
  "created_at": 1669695536,
  "kind": 22242,
  "tags": [],
  "content": "wss://relay.example.com/",
  "sig": "..."
}
```

The client may send an auth message right before performing an action for which it knows authentication will be required -- for example, right
before requesting `kind: 4` chat messages --, or it may do right on connection start or at some other moment it deems best.

Upon receiving a message from an unauthenticated user it can't fulfill without authentication, a relay may choose to notify the client. For
that it can use a `NOTICE` message with a standard prefix `"restricted: "` that is readable both by humans and machines, for example:

```
["NOTICE", "restricted: we can't serve DMs to unauthenticated users, does your client implement NIP-42?"]
```

or it can return an `OK` message noting the reason an event was not written using the same prefix:

```
["OK", "b1a649ebe8...", false, "restricted: we do not accept events from unauthenticated users, please sign up at https://example.com/"]
```

## Signed Event Verification

To verify `AUTH` messages, relays must ensure:

  - that the `kind` is `22242`
  - that the event was recently signed (~10 minutes, by `created_at`)
  - that the `content` field matches the relay URL
    - URL normalization techniques can be applied. For most cases just checking if the domain name is correct should be enough.