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-705]([xxx](https://github.com/motorina0/nips/blob/republish_events/705.md))).
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:
- 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)).
> **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 (similar to [BIP-44 address gap logic](https://github.com/bitcoin/bips/blob/master/bip-0044.mediawiki#user-content-Address_gap_limit)).
> **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`.