nips/04.md

NIP-04

Encrypted Direct Message

final optional author:arcbtc

A special event with kind 4, meaning "encrypted direct message". It is supposed to have the following attributes:

content MUST be equal to the base64-encoded, aes-256-cbc encrypted string of anything a user wants to write, encrypted using a shared cipher generated by combining the recipient's public-key with the sender's private-key; this appended by the base64-encoded initialization vector as if it was a querystring parameter named "iv". The format is the following: "content": "<encrypted_text>?iv=<initialization_vector>".

tags MUST contain an entry identifying the receiver of the message (such that relays may naturally forward this event to them), in the form ["p", "<pubkey, as a hex string>"].

tags MAY contain an entry identifying the previous message in a conversation or a message we are explicitly replying to (such that contextual, more organized conversations may happen), in the form ["e", "<event_id>"].

Note: By default in the libsecp256k1 ECDH implementation, the secret is the SHA256 hash of the shared point (both X and Y coordinates). In Nostr, only the X coordinate of the shared point is used as the secret and it is NOT hashed. If using libsecp256k1, a custom function that copies the X coordinate must be passed as the hashfp argument in secp256k1_ecdh. See here.

Code sample for generating such an event in JavaScript:

import crypto from 'crypto'
import * as secp from 'noble-secp256k1'

let sharedPoint = secp.getSharedSecret(ourPrivateKey, '02' + theirPublicKey)
let sharedX = sharedPoint.substr(2, 64)

let iv = crypto.randomFillSync(new Uint8Array(16))
var cipher = crypto.createCipheriv(
  'aes-256-cbc',
  Buffer.from(sharedX, 'hex'),
  iv
)
let encryptedMessage = cipher.update(text, 'utf8', 'base64')
encryptedMessage += cipher.final('base64')
let ivBase64 = Buffer.from(iv.buffer).toString('base64')

let event = {
  pubkey: ourPubKey,
  created_at: Math.floor(Date.now() / 1000),
  kind: 4,
  tags: [['p', theirPublicKey]],
  content: encryptedMessage + '?iv=' + ivBase64
}