add NIP-04.

closes https://github.com/fiatjaf/nostr/pull/10.

I couldn't merge that branch because it had unrelated stuff, so I used this opportunity to rewrite some things in the NIP.

nips/02.md

@@ -2,7 +2,7 @@ NIP-02
 ======
 
 Petname sharing through a special event `3`: "contact list"
--------------------------------------------------------
+-----------------------------------------------------------
 
 `draft` `optional`
 

nips/04.md

@@ -0,0 +1,43 @@
+NIP-04
+======
+
+Encrypted Direct Message
+------------------------
+
+`draft` `optional`
+
+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>"]`.
+
+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
+}
+```