Add NIP 43 for joining relays using a claim

Author: Jonathan Staab

43.md

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+NIP-43
+======
+
+Relay Access Requests
+---------------------
+
+`draft` `optional`
+
+This NIP defines a way for clients to request admission to relays enforcing
+authentication as defined in NIP 42 by signing an ephemeral event.
+
+## Claim Event
+
+This NIP defines kind `28934` events which are intended to allow clients to
+request admission to a relay. It MUST have a `claim` tag containing an invoice,
+invite code, or any other arbitrary string.
+
+The event's `created_at` MUST be now, plus or minus a few minutes. Clients MAY
+send a claim at any time, but MUST check for relay support via NIP 11. This is
+to avoid non-compliant relays broadcasting invites to subscribers.
+
+This event should be sent to a relay using the standard `EVENT` verb.
+
+```jsonc
+{
+  "kind": 28934,
+  "tags": [
+    ["claim", "<invoice id, invite code, or something else>"]
+  ],
+  // ...other fields
+}
+```
+
+## Relay response
+
+Upon receiving a claim, a relay MUST notify the client as to what the status
+of the claim is using an `OK` message. Failed claims SHOULD use the same
+standard `"restricted: "` prefix specified by NIP 42.
+
+Some examples:
+
+```
+["OK", <event-id>, false, "restricted: That invoice is expired."]
+["OK", <event-id>, false, "restricted: That is an unsupported claim."]
+["OK", <event-id>, true, "claim-ignored: You are already a member of this relay."]
+["OK", <event-id>, true, "claim-accepted: Welcome to wss://relay.bunk.skunk!"]
+```
+
+## Claim Request
+
+Users may request a claim string from a relay by making a request for `kind 28935`
+events. These events MUST NOT be considered valid unless signed by the same pubkey
+as exists in the relay's NIP 11 document.
+
+```jsonc
+{
+  "kind": 28935,
+  "tags": [
+    ["claim", "<invoice, invite code, or something else>"]
+  ],
+  // ...other fields
+}
+```
+
+Note that these events are in the `ephemeral` range, which means relays must explicitly
+opt-in to this behavior by generating claims on the fly when requested. This allows relays
+to improve security by issuing a different claim for each request, only issuing claims
+to certain users, or expiring claims.