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 is encrypted, but everyone can see who is chatting with whom. This is far from ideal from a privacy perspective.

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).

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

BIP32 is used to derive the paths. A client can generate multiple direct message parent keys, one for each peer that it is communicating with. Nostr coin_type' is 1237' (see NIP-06). In this NIP we define the purpose 25709' (dm -> 0x646d -> 25709) for deriving Direct Messages related keys.

When client Alice wants to build its dm parent key for Bob it has to:

get the public key of Bob (in hex). Eg: 3bf0c63fcb93463407af97a5e5ee64fa883d107ef9e558472c4eb9aaaefa459d.
split the public key hex string in 8 chunks:
- 3bf0c63f, cb934634, 07af97a5, e5ee64fa, 883d107e, f9e55847, 2c4eb9aa, aefa459d
derive the dm parent key: m/25709'/1237'/p1'/p2'/.../p8'

Note the reason for using the peer's public key (Bob) in the dm parent key derivation is to always arive at the same value even if prio state is lost.

Note the reason for splitting is that each level of the path can have a max value of 2³²-1.

dm//index send, receive, marketplace

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:

 {
  "key": <the direct message parent key>,
  "send_index": <integer (optional), ...>,
  "receive_index": <integer (optional), ...>,
 }

publish a Parameterized Replaceable Event (NIP-33) having:

 {
  ...
  "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.

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