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": "?iv="`. **`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", ""]`. **`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", ""]`. **Note**: By default in the [libsecp256k1](https://github.com/bitcoin-core/secp256k1) 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](https://github.com/bitcoin-core/secp256k1/blob/master/src/modules/ecdh/main_impl.h#L29). Code sample for generating such an event in JavaScript: ```js 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 } ```