First draft of Delegated Event Signing

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+NIP: 26
+=======
+
+Delegated Event Signing
+-----
+
+`draft` `mandatory` `author:markharding` `author:minds`
+
+This NIP defines how events should be verified and signed to support generating events on behalf of someone else. It should be possible to sign Nostr events from other keypairs.
+
+Another application of this proposal is to abstract away the use of the 'root' keypairs when interacting with clients. For example, a user could generate new keypairs for each client they wish to use and authorize those keypairs to generate events on behalf of their root pubkey, where the root keypair is stored in cold storage. 
+
+#### Introducing the 's' tag
+
+This NIP introduces a new tag: `s` which is formatted as follows:
+
+```json
+[
+  "s",
+  <signed pairing payload (base64 encoded)>,
+  <signed pairing signature (64-bytes shnnorr signature of the sha256 hash of the signed pairing payload>
+]
+```
+
+##### Signed Pairing Payload
+
+The Signed Pairing Payload should be a `base64` encoded JSON object as follows: 
+
+```json
+{
+  "signerkey": <32-bytes hex-encoded public key of who is authorized to sign>,
+  "created_at": <unix timestamp of issued time>,
+  "expires_at": <optional, if present unix timestamp of invalidation time>
+}
+```
+
+For example, the Signed Pairing Payload `eyJzaWduZXJrZXkiOiI2MjkwM2IxZmY0MTU1OWRhZjllZTk4ZWYxYWU2N2NjNTJmMzAxYmI1Y2UyNmQxNGJhYmEzMDUyZjY0OWMzZjQ5IiwiY3JlYXRlZF9hdCI6MTY1OTQ0NjMxNX0=` consists of the payload:
+
+```json
+{
+  "signerkey": "62903b1ff41559daf9ee98ef1ae67cc52f301bb5ce26d14baba3052f649c3f49",
+  "created_at": 1659446315
+}
+```
+
+##### Signed Pairing Signature
+
+The Signed Pairing Signature is a schnorr signature of the sha256 hash of the Signed Pairing Payload. For example `2ed3e4b8470ce37b7e1946441a323d1d71c8a846fe49787ec406e14a44632cc96e48cabccc4a526eedd51aca33bf2a5cf7fb85462d23ad6d4de29c8b91abc41b` is a signature of the payload mentioned above.
+
+#### Modifying event verification
+
+When the `s` tag is provided, events **must** be signed and verified by the respective private key of the `signerkey`. Clients/relays **should** confirm that no revocation have been created with a greater `created_at` value. 
+
+Clients **must** verify the token is valid. The token **may** include the `expires_at` field if it wishes the delegated signing to be temporary (ie. sign events for 2 hours, 7 days, etc). 
+
+> TODO: How to revoke. This could be a future NIP.
+
+
+#### Example
+
+Below is an example of an event published by `62903b1ff41559daf9ee98ef1ae67cc52f301bb5ce26d14baba3052f649c3f49`, on behalf of `86f0689bd48dcd19c67a19d994f938ee34f251d8c39976290955ff585f2db42e`.
+
+```json
+{
+   "id":"23bf557814cd294d77a52e43d16903862c231857319f950ba40474fde1b9c393",
+   "pubkey":"86f0689bd48dcd19c67a19d994f938ee34f251d8c39976290955ff585f2db42e",
+   "created_at":1652969505,
+   "kind":1,
+   "tags":[
+      [
+         "s",
+         "eyJzaWduZXJrZXkiOiI2MjkwM2IxZmY0MTU1OWRhZjllZTk4ZWYxYWU2N2NjNTJmMzAxYmI1Y2UyNmQxNGJhYmEzMDUyZjY0OWMzZjQ5IiwiY3JlYXRlZF9hdCI6MTY1OTQ0NjMxNX0=",
+         "2ed3e4b8470ce37b7e1946441a323d1d71c8a846fe49787ec406e14a44632cc96e48cabccc4a526eedd51aca33bf2a5cf7fb85462d23ad6d4de29c8b91abc41b"
+      ]
+   ],
+   "content":"Hello Nostr. This is Minds calling.",
+   "sig":"30f0ebb907bd23416dd32a14b905abeb1db3fde97c74a151c51e5503e91ed25d58fbdcd95910ea4c11dc49d3fca8c616c0e7489e0f8da37b0938a7ce393285e7"
+}
+```
+