NIP-01
======

Basic protocol flow description
-------------------------------

`draft` `mandatory`

This NIP defines the basic protocol that should be implemented by everybody. New NIPs may add new optional (or mandatory) fields and messages and features to the structures and flows described here.

## Events and signatures

Each user has a keypair. Signatures, public key, and encodings are done according to the [Schnorr signatures standard for the curve `secp256k1`](https://bips.xyz/340).

The only object type that exists is the `event`, which has the following format on the wire:

```jsonc
{
  "id": <32-bytes lowercase hex-encoded sha256 of the serialized event data>,
  "pubkey": <32-bytes lowercase hex-encoded public key of the event creator>,
  "created_at": <unix timestamp in seconds>,
  "kind": <integer between 0 and 65535>,
  "tags": [
    [<arbitrary string>...],
    // ...
  ],
  "content": <arbitrary string>,
  "sig": <64-bytes lowercase hex of the signature of the sha256 hash of the serialized event data, which is the same as the "id" field>
}
```

To obtain the `event.id`, we `sha256` the serialized event. The serialization is done over the UTF-8 JSON-serialized string (which is described below) of the following structure:

```
[
  0,
  <pubkey, as a lowercase hex string>,
  <created_at, as a number>,
  <kind, as a number>,
  <tags, as an array of arrays of non-null strings>,
  <content, as a string>
]
```

To prevent implementation differences from creating a different event ID for the same event, the following rules MUST be followed while serializing:
- UTF-8 should be used for encoding.
- Whitespace, line breaks or other unnecessary formatting should not be included in the output JSON.
- The following characters in the content field must be escaped as shown, and all other characters must be included verbatim:
  - A line break (`0x0A`), use `\n`
  - A double quote (`0x22`), use `\"`
  - A backslash (`0x5C`), use `\\`
  - A carriage return (`0x0D`), use `\r`
  - A tab character (`0x09`), use `\t`
  - A backspace, (`0x08`), use `\b`
  - A form feed, (`0x0C`), use `\f`

### Tags

Each tag is an array of one or more strings, with some conventions around them. Take a look at the example below:

```jsonc
{
  "tags": [
    ["e", "5c83da77af1dec6d7289834998ad7aafbd9e2191396d75ec3cc27f5a77226f36", "wss://nostr.example.com"],
    ["p", "f7234bd4c1394dda46d09f35bd384dd30cc552ad5541990f98844fb06676e9ca"],
    ["a", "30023:f7234bd4c1394dda46d09f35bd384dd30cc552ad5541990f98844fb06676e9ca:abcd", "wss://nostr.example.com"],
    ["alt", "reply"],
    // ...
  ],
  // ...
}
```

The first element of the tag array is referred to as the tag _name_ or _key_ and the second as the tag _value_. So we can safely say that the event above has an `e` tag set to `"5c83da77af1dec6d7289834998ad7aafbd9e2191396d75ec3cc27f5a77226f36"`, an `alt` tag set to `"reply"` and so on. All elements after the second do not have a conventional name.

This NIP defines 3 standard tags that can be used across all event kinds with the same meaning. They are as follows:

- The `e` tag, used to refer to an event: `["e", <32-bytes lowercase hex of the id of another event>, <recommended relay URL, optional>]`
- The `p` tag, used to refer to another user: `["p", <32-bytes lowercase hex of a pubkey>, <recommended relay URL, optional>]`
- The `a` tag, used to refer to a (maybe parameterized) replaceable event
    - for a parameterized replaceable event: `["a", <kind integer>:<32-bytes lowercase hex of a pubkey>:<d tag value>, <recommended relay URL, optional>]`
    - for a non-parameterized replaceable event: `["a", <kind integer>:<32-bytes lowercase hex of a pubkey>:, <recommended relay URL, optional>]`

As a convention, all single-letter (only english alphabet letters: a-z, A-Z) key tags are expected to be indexed by relays, such that it is possible, for example, to query or subscribe to events that reference the event `"5c83da77af1dec6d7289834998ad7aafbd9e2191396d75ec3cc27f5a77226f36"` by using the `{"#e": ["5c83da77af1dec6d7289834998ad7aafbd9e2191396d75ec3cc27f5a77226f36"]}` filter.

### Kinds

Kinds specify how clients should interpret the meaning of each event and the other fields of each event (e.g. an `"r"` tag may have a meaning in an event of kind 1 and an entirely different meaning in an event of kind 10002). Each NIP may define the meaning of a set of kinds that weren't defined elsewhere. This NIP defines two basic kinds:

- `0`: **user metadata**: the `content` is set to a stringified JSON object `{name: <username>, about: <string>, picture: <url, string>}` describing the user who created the event. [Extra metadata fields](24.md#kind-0) may be set. A relay may delete older events once it gets a new one for the same pubkey.
- `1`: **text note**: the `content` is set to the **plaintext** content of a note (anything the user wants to say). Content that must be parsed, such as Markdown and HTML, should not be used. Clients should also not parse content as those.

And also a convention for kind ranges that allow for easier experimentation and flexibility of relay implementation:

- for kind `n` such that `1000 <= n < 10000`, events are **regular**, which means they're all expected to be stored by relays.
- for kind `n` such that `10000 <= n < 20000 || n == 0 || n == 3`, events are **replaceable**, which means that, for each combination of `pubkey` and `kind`, only the latest event MUST be stored by relays, older versions MAY be discarded.
- for kind `n` such that `20000 <= n < 30000`, events are **ephemeral**, which means they are not expected to be stored by relays.
- for kind `n` such that `30000 <= n < 40000`, events are **parameterized replaceable**, which means that, for each combination of `pubkey`, `kind` and the `d` tag's first value, only the latest event MUST be stored by relays, older versions MAY be discarded.

In case of replaceable events with the same timestamp, the event with the lowest id (first in lexical order) should be retained, and the other discarded.

When answering to `REQ` messages for replaceable events such as `{"kinds":[0],"authors":[<hex-key>]}`, even if the relay has more than one version stored, it SHOULD return just the latest one.

These are just conventions and relay implementations may differ.

## Communication between clients and relays

Relays expose a websocket endpoint to which clients can connect. Clients SHOULD open a single websocket connection to each relay and use it for all their subscriptions. Relays MAY limit number of connections from specific IP/client/etc.

### From client to relay: sending events and creating subscriptions

Clients can send 3 types of messages, which must be JSON arrays, according to the following patterns:

  * `["EVENT", <event JSON as defined above>]`, used to publish events.
  * `["REQ", <subscription_id>, <filters1>, <filters2>, ...]`, used to request events and subscribe to new updates.
  * `["CLOSE", <subscription_id>]`, used to stop previous subscriptions.

The `<subscription_id>` is an arbitrary, non-empty string of max length 64 chars generated by the client. It represents a subscription per connection. Relays MUST manage `<subscription_id>`s independently for each WebSocket connection. `<subscription_id>`s are not guaranteed to be globally unique.

`<filtersX>` is a JSON object containing filter attributes to determine which events will be sent in that subscription. The possible attributes are listed below, and they are filtered with logical AND. If an attribute contains a list (JSON array), the list is filtered with logical OR. 

```json
{
  "ids": <a list of event ids>,
  "authors": <a list of lowercase pubkeys>,
  "kinds": <a list of a kind numbers>,
  "#<single-letter (a-zA-Z)>": <a list of tag values, for #e — a list of event ids, for #p — a list of pubkeys, etc.>,
  "since": <an integer Unix timestamp in seconds, events must be newer than this to pass>,
  "until": <an integer Unix timestamp in seconds, events must be older than this to pass>,
  "limit": <maximum number of events relays SHOULD return in the INITIAL query>
}
```

Upon receiving a `REQ` message, the relay SHOULD query its internal database and return events that match the filter, then store that filter and send again all future events it receives to that same websocket until the websocket is closed. The `CLOSE` event is received with the same `<subscription_id>` or a new `REQ` is sent using the same `<subscription_id>`, in which case relay MUST overwrite the previous subscription.

A `REQ` message may contain multiple filters. In this case, events that match any of the filters are to be returned, i.e., multiple filters are to be interpreted as `||` conditions.

The `limit` property of a filter is only valid for the initial query and MUST be ignored afterwards. When `limit: n` is present it is assumed that the events returned in the initial query will be the last `n` events ordered by the `created_at`. It is acceptable to return fewer events than `limit` specifies, but it is expected that relays do not return (much) more events than requested, to avoid clients getting overwhelmed by data.

### From relay to client: sending events and notices

Relays can send 5 types of messages, which must also be JSON arrays, according to the following patterns:

  * `["EVENT", <subscription_id>, <event JSON as defined above>]`, used to send events requested by clients.
  * `["OK", <event_id>, <true|false>, <message>]`, used to indicate acceptance or denial of an `EVENT` message.
  * `["EOSE", <subscription_id>]`, used to indicate the _end of stored events_ and the beginning of events newly received in real-time.
  * `["CLOSED", <subscription_id>, <message>]`, used to indicate that a subscription was ended on the server side.
  * `["NOTICE", <message>]`, used to send human-readable error messages or other things to clients.

This NIP defines no rules for how `NOTICE` messages should be sent or treated.

- `EVENT` messages MUST be sent only with a subscription ID related to a subscription previously initiated by the client (using the `REQ` message above).
- `OK` messages MUST be sent in response to `EVENT` messages received from clients, they must have the 3rd parameter set to `true` when an event has been accepted by the relay, `false` otherwise. The 4th parameter MUST always be present, but MAY be an empty string when the 3rd is `true`, otherwise it MUST be a string formed by a machine-readable single-word prefix followed by a `:` and then a human-readable message. Some examples:
  * `["OK", "b1a649ebe8...", true, ""]`
  * `["OK", "b1a649ebe8...", true, "pow: difficulty 25>=24"]`
  * `["OK", "b1a649ebe8...", true, "duplicate: already have this event"]`
  * `["OK", "b1a649ebe8...", false, "blocked: you are banned from posting here"]`
  * `["OK", "b1a649ebe8...", false, "blocked: please register your pubkey at https://my-expensive-relay.example.com"]`
  * `["OK", "b1a649ebe8...", false, "rate-limited: slow down there chief"]`
  * `["OK", "b1a649ebe8...", false, "invalid: event creation date is too far off from the current time"]`
  * `["OK", "b1a649ebe8...", false, "pow: difficulty 26 is less than 30"]`
  * `["OK", "b1a649ebe8...", false, "error: could not connect to the database"]`
- `CLOSED` messages MUST be sent in response to a `REQ` when the relay refuses to fulfill it. It can also be sent when a relay decides to kill a subscription on its side before a client has disconnected or sent a `CLOSE`. This message uses the same pattern of `OK` messages with the machine-readable prefix and human-readable message. Some examples:
  * `["CLOSED", "sub1", "duplicate: sub1 already opened"]`
  * `["CLOSED", "sub1", "unsupported: filter contains unknown elements"]`
  * `["CLOSED", "sub1", "error: could not connect to the database"]`
  * `["CLOSED", "sub1", "error: shutting down idle subscription"]`
- The standardized machine-readable prefixes for `OK` and `CLOSED` are: `duplicate`, `pow`, `blocked`, `rate-limited`, `invalid`, and `error` for when none of that fits.