add "n" tag with sender encryption key to seal.

17.md

@@ -37,7 +37,10 @@ Following [NIP-59](59.md), the **unsigned** chat messages must be sealed (`kind:
       "pubkey": senderIdentityKey.public,
       "created_at": randomTimeUpTo2DaysInThePast,
       "kind": 13, // seal
-      "tags": [], // no tags
+      "tags": [
+        // when senderEncryptionKey != senderIdentityKey (see below)
+        ["n", senderEncryptionKey.public]
+      ],
       "content": nip44.encrypt(
         unsignedMessageRumor,
         nip44.compute_conversation_key(senderEncryptionKey.private, receiverEncryptionKey.public)
@@ -79,6 +82,8 @@ The existence of this event shows that the receiver wants to receive messages en
 
 When sending, `p`-tagging the receiver should still use their _identity key_; and when receiving messages the receiver should still assume the `"author"` field of the seal event contains the _identity key_.
 
+An `n` tag set to `senderEncryptionKey.public` MUST be added to the seal event when the _encryption key_ is different from the _identity key_.
+
 ## Publishing
 
 Kind `10050` indicates the user's preferred relays to receive DMs. The event MUST include a list of `relay` tags with relay URIs.