NIP-704
======

Key derivation for Encrypted Direct Messages
-----------------------------------

`draft` `optional` `author:motorina0`

This NIP defines a way for two clients to derive `one-use-only` keys for sending and recieving `kind:4` events.

## Motivation
The content of `Direct Messages` [NIP-04](https://github.com/nostr-protocol/nips/blob/master/04.md) is encrypted, but everyone can see who is chatting with whom. Privacy wise this is far from ideal.

This NIP describes a way to obfuscate DM communications from the "general public", it does not deal with the relay tracking of clients (for that see [NIP XXX](xxx)).

## Suggestion
For the maximum of privacy the two participants of a `Direct Message` exchange SHOULD use a different public key for **each** `kind:4` event.
This means that each participant has to:
 - build a `direct message parent key` from which it will derive keys to send and keys to receive (listen for) `kind:4` events
 - share this `direct message parent key` with its DM peer

Each client has a `master` key (denoted with `m`). This key can be the profile `nsec...`, but it is not mandatory.

## Derive the `direct message parent key`
A client must generate multiple `direct message parent keys`, one for each peer that it is communicating with. The [BIP32](https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki) structure to be used is:
```
m / purpose' / conin_type' / part1' / part2' / ... / part8'
```

- this NIP defines the `purpose` `25709'` (`dm` -> `0x646d` -> `25709`) for deriving `Direct Messages` related keys
- nostr `coin_type'` is `1237'` (see [NIP-06](https://github.com/nostr-protocol/nips/blob/master/06.md))
- `part1' / part2' / ... / part8'` is the public key hex string (of the peer) split in 8 chunks:
  - the reason for using the peer's (`Bob`) public key is to always arive at the same value even if prio state is lost
  - the reason for splitting the public key is that each level of the path can have a max value of 2<sup>32</sup>-1 

 
<details>
 <summary><b>Example</b></summary> 
If Alice wants to build he <code>dm parent key</code> for Bob then she has to:
 <ul>
  <li>get the public key of `Bob` (in hex). Eg: <code>3bf0c63fcb93463407af97a5e5ee64fa883d107ef9e558472c4eb9aaaefa459d</code></li>
  <li>split the public key hex string in 8 chunks:</li>
       - <code>3bf0c63f</code>, <code>cb934634</code>, <code>07af97a5</code>, <code>e5ee64fa</code>, <code>883d107e</code>, <code>f9e55847</code>, <code>2c4eb9aa</code>, <code>aefa459d</code>
  <li>derive the <code>dm parent key</code>: <code>m/25709'/1237'/3bf0c63f'/cb934634'/.../aefa459d'</code></li>
 </ul>
</details>

We notate the above derived `direct message public key` with  `dmpk`. Then we can define paths of the form `dmpk/<action>/index`.

| Action Name           | Value  | Path                | Derive keys for                   |
|-----------------------|--------|---------------------|-----------------------------------|
| **send**              | `0`    | `dmpk/0/<index>`    | sending `direct messages`         |
| **receive**           | `1`    | `dmpk/1/<index>`    | receiving `direct messages`       |
| `draft`: republish    | `2`    | `dmpk/2/<index>`    | sending `republish events`        |
| `draft`: market-order | `4500` | `dmpk/4500/<index>` | sending NIP45 `market orders`     |

The client (creator of the `dmpk`) must:
 - use a new send key (`dmpk/0/<index>`) for each event it signs. It starts from `0` and increments after an event is signed.
 - create filters for the public keys it expects to receive messages to (`dmpk/1/<index>`). It is recommended to listen for the next `10` keys and increment the index once a key is used (see [BIP-44 address gap logic](https://github.com/bitcoin/bips/blob/master/bip-0044.mediawiki#user-content-Address_gap_limit)).

## Exchange the `direct message parent key`
If `Alice` wants to signal `Bob` that she is ready to use this NIP (for more privacy) she must:
 - build a JSON data of the form:
```json
 {
  "key": <the direct message parent key (dmpk)>,
  "send_index": <integer (optional), the index of the last key used to sign an event>,
  "receive_index": <integer (optional), the index of the last key an event was received to>,
 }
 ```
  > **Note** `send_index` and `receive_index` are optional, but they help the client a lot in knowing what the state is. Alternatively (when an account is retored) the client would have to scan the public keys until unused ones are found.
 
 - publish a `Parameterized Replaceable Event` ([NIP-33](https://github.com/nostr-protocol/nips/blob/master/33.md)) having:

```json
 {
  ...
  "kind": 35709,
  "content": <NIP-04 encrypted content of the JSON data>,
  "tags:" [
    "d": <sha256 of the shared secret as defined in NIP-04>
  ]
 }
```
 
 > **Note** the reason for using `sha256(shared_secret)` for the `d` tag is so that outside observers do not even know that `Alice` and `Bob` have started to communicate. Any other value for the `d` tag would reveal that the message is intended for `Bob.`
 
 After both `Alice` and `Bob` have published the `kind: 35709` event, they can start to publish and listen to events using the `one-use-keys`.