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00.md

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+NIP-XX
+======
+
+Access Control Upgrade
+----------------------
+
+`draft` `optional`
+
+This NIP defines how to upgrade a public event with the property of limiting access to itself.
+
+## Upgrading a Public Event
+
+A public event (with no access control), such as of `kind:1`,
+must be published or updated with a `full_event` tag pointing to a
+`kind:40042` event address.
+
+It's data should be set to a warning that the event is private or to a preview version of the full event, in case the
+goal is to tease an user to pay for content, for example.
+
+For example, a `kind:1` event could have it's `.content` set to "This event is private." to be showed
+to users that don't have acces to the event or that are using clients that don't support this NIP. On the other hand,
+if the goal is to convince viewing users to pay for the full event, the `.content` could be set to a text excerpt like the following:
+"This is the first sentence of the full text. This text is trimmed to 5% of the character count of the full text... Subscribe
+to read the full text".
+
+Taking the `kind:1063` metadata event of an image as an example, it could had set its `url` tag to a blurred version of the image
+and/or use the `.content` (that according to NIP-94 means "caption") set to "This event is private".
+
+How the public event data will be set is entirely up to the client offering
+the feature that requires event access control. Such feature could be
+a "Publish only to Close Friends" or "Publish to Paying Subscribers", for example.
+
+Example:
+
+```json
+{
+  "kind": 1063,
+  "tags": [
+    ["full_event", "40042:<32-bytes lowercase hex of a pubkey>:abc", "<recommended relay URL, optional>"],
+    ["url", "<string with PREVIEW url>"],
+    ["dim", "800x600"]
+  ],
+  "content": "",
+  ...
+}
+```
+
+## Full Event
+
+A "Full Event" is of `kind:40042`. It has [NIP-42](42.md) `A` tags the author
+can update at will with the pubkeys allowed to access it.
+
+The `.content` and other tags aren't fixed. The structure depends on the
+kind of the public event it is protecting. For example, if it is
+protecting a `kind:30023` event, it should have the structure defined on [NIP-23](23.md).
+
+An interesting side effect of this being an event separate from the public one
+is that it doesn't need to be owned by the public event author. This way
+an always online bot or DVM can update the `A` tag list in response to
+a detected payment, for example.
+
+Example of a `kind:40042` event using a `kind:1063` event structure:
+
+```json
+{
+  "kind": 40042,
+  "pubkey": "<author_pubkey>",
+  "tags": [
+    ["d", "<random>"], // act as parameterized replaceable event
+    ["A", "<author_pubkey>"], // important to keep author access right
+    ["A", "<buyer_1_pubkey>"],
+    ["A", "<buyer_2_pubkey>"],
+    ["url", "https://some.place/file.webp"],
+    ["dim", "800x600"]
+  ],
+  "content": "Happy New Year, my dear supporters!!",
+  ...
+}
+```
+
+## Requesting the Event
+
+The client is expected to initially request the public event. For example, a microblogging client
+will request `kind:1` events as usual.
+
+If one of these events includes a `full_event` tag,
+supporting clients should request it with a filter that includes
+a `#A` filter key set to the client's user pubkey.
+Clients that don't understand this NIP, however, will show the `kind:1` `.content` as usual
+and ignore the `full_event` tag.
+
+If the user has rights to access the full event, the relay will send it
+after the NIP-42 authentication flow.
+
+Example of how to request the full event:
+
+`["REQ", "sub_id", { "kinds": [40042], "authors": ["abc"], #d: ["xyz"], "#A": ["<client's user pubkey>"], "limit": 1 }]`