NIP-42

Authentication of clients to relays

draft optional

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. Relays MUST exclude kind: 22242 events from being broadcasted to any client. created_at should be the current time. Example:

{
  "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.

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