add nips/13.md

nips/13.md

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+NIP-13
+======
+
+Nostr Alias
+-----------
+
+`draft` `optional` `author:vinliao`
+
+The event with kind `13` links two pubkeys together, creating an alt pubkey from a main pubkey. Two pubkeys (main and alt) can be cryptographically linked by creating these two events:
+
+`main event` (created by main):
+```
+{
+  "id": <main event id>
+  "pubkey": <main's pubkey>,
+  "created_at": <unix timestamp in seconds>,
+  "kind": 13,
+  "tags": [
+    ["p", <alt's pubkey>],
+  ]
+  "content": <empty string>,
+  "sig": <64-bytes signature of the sha256 hash of the serialized event data, which is the same as the "id" field>,
+}
+```
+
+`alt event` (created by alt):
+```
+{
+  "id": <alt event id>
+  "pubkey": <alt's pubkey>,
+  "created_at": <unix timestamp in seconds>,
+  "kind": 13,
+  "tags": [
+    ["e", <main event id>],
+    ["p", <main's pubkey>],
+  ]
+  "content": <empty string>,
+  "sig": <64-bytes signature of the sha256 hash of the serialized event data, which is the same as the "id" field>,
+}
+```
+
+The tag of `main event` MUST contain alt's pubkey. The tag of `alt event` MUST contain main's pubkey and the event id of `main event`. 
+
+A main or alt can prove its main-alt relationship by providing `main event` and `alt event`; the main-alt relationship is invalid until `alt event` is presented.
+
+`main event` and `alt event` can be posted publicly like a NIP-01 event or sent privately with NIP-04. Clients MAY treat alt as the same entity as main by displaying the same "petname."