drastically simplify @semisol's auth NIP.

42.md

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+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
+
+```
+["NOTICE", "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.