add nip-42: authentication

42.md

@@ -0,0 +1,86 @@
+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.
+
+## Definitions
+
+This NIP defines a new message, `AUTH`, which relays can send when they support authentication and clients can send to relays when they want
+to authenticate. When sent by relays, the message is of the following form:
+
+```
+["AUTH", <challenge-string>]
+```
+
+And, when sent by clients, of the following form:
+
+```
+["AUTH", <signed-event-json>]
+```
+
+The signed event is an ephemeral event not meant to be published or queried, it must be of `kind: 22242` and it should have at least two tags,
+one for the relay URL and one for the challenge string as received from the relay. `created_at` should be the current time. Example:
+
+```json
+{
+  "id": "...",
+  "pubkey": "...",
+  "created_at": 1669695536,
+  "kind": 22242,
+  "tags": [
+    ["relay", "wss://relay.example.com/"],
+    ["challenge", "challengestringhere"]
+  ],
+  "content": "",
+  "sig": "..."
+}
+```
+
+## Protocol flow
+
+At any moment the relay may send an `AUTH` message to the client containing a challenge. After receiving that the client may decide to
+authenticate itself or not. The challenge is expected to be valid for the duration of the connection or until a next challenge is sent by
+the relay.
+
+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. The authentication
+is expected to last for the duration of the WebSocket connection.
+
+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` or `OK` 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", <event-id>, 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 `created_at` is close (e.g. within ~10 minutes) of the current time;
+  - that the `"challenge"` tag matches the challenge sent before;
+  - that the `"relay"` tag matches the relay URL:
+    - URL normalization techniques can be applied. For most cases just checking if the domain name is correct should be enough.

README.md

@@ -28,6 +28,7 @@ NIPs stand for **Nostr Implementation Possibilities**. They exist to document wh
 - [NIP-35: User Discovery](35.md)
 - [NIP-36: Sensitive Content](36.md)
 - [NIP-40: Expiration Timestamp](40.md)
+- [NIP-42: Authentication of clients to relays](42.md)
 
 ## Event Kinds
 
@@ -47,18 +48,20 @@ NIPs stand for **Nostr Implementation Possibilities**. They exist to document wh
 | 43          | Channel Hide Message        | [28](28.md)            |
 | 44          | Channel Mute User           | [28](28.md)            |
 | 45-49       | Public Chat Reserved        | [28](28.md)            |
+| 22242       | Client Authentication       | [42](42.md)            |
 | 10000-19999 | Replaceable Events Reserved | [16](16.md)            |
 | 20000-29999 | Ephemeral Events Reserved   | [16](16.md)            |
 
 
 ## Message types
 
 ### Client to Relay
-| type  | description                                         | NIP        |
-|-------|-----------------------------------------------------|------------|
-| EVENT | used to publish events                              | [1](01.md) |
-| REQ   | used to request events and subscribe to new updates | [1](01.md) |
-| CLOSE | used to stop previous subscriptions                 | [1](01.md) |
+| type  | description                                         | NIP         |
+|-------|-----------------------------------------------------|-------------|
+| EVENT | used to publish events                              | [1](01.md)  |
+| REQ   | used to request events and subscribe to new updates | [1](01.md)  |
+| CLOSE | used to stop previous subscriptions                 | [1](01.md)  |
+| AUTH  | used to send authentication events                  | [42](42.md) |
 
 ### Relay to Client
 | type   | description                                             | NIP         |
@@ -67,6 +70,7 @@ NIPs stand for **Nostr Implementation Possibilities**. They exist to document wh
 | NOTICE | used to send human-readable messages to clients         | [1](01.md)  |
 | EOSE   | used to notify clients all stored events have been sent | [15](15.md) |
 | OK     | used to notify clients if an EVENT was successuful      | [20](20.md) |
+| AUTH   | used to send authentication challenges                  | [42](42.md) |
 
 Please update these lists when proposing NIPs introducing new event kinds.