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100 lines
3.6 KiB
Markdown
100 lines
3.6 KiB
Markdown
NIP-92
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==========================
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Rendezvous Beacons
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------------------
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`draft` `optional` `author:shafemtol`
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A rendezvous beacon is used to anonymously deliver a secret event to a recipient
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without prior communication between sender and recipient. Apart from its
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timestamp, a rendezvous beacon is indistinguishable from any other rendezvous
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beacon of the same kind on the same relay.
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A stored rendezvous beacon has kind `9992`, while an ephemeral rendezvous beacon
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has kind `29992`.
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`pubkey` MUST be a unique public key with no publicly known relation to any
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other event.
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`content` MUST be the 8-character base64 encoded string of a 6-byte combined
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hash/nonce, generated as described below.
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For stored beacons, `tags` SHOULD contain a `NIP-40` `expiration` tag with a
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value of `created_at` plus exactly `10000000` (ten million) seconds and no other
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tags. It is not recommended to use stored rendezvous beacons on relays that do
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not support `NIP-40`.
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For ephemeral beacons, `tags` SHOULD be an empty list.
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`created_at` SHOULD be approximately the time the event is submitted to the
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relay.
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Public User Metadata
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--------------------
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In order for the sender to know how to reach the recipient, the recipient needs
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to publish some parameters in their `set_metadata` event, with the following
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attribute:
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`"bip92": [<subprefix>,<recipient_pubkey>,<relays>...]`
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`<subprefix>` MUST be either an empty string or a single hexadecimal character.
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In order to match the recipient's subscription filter, the `id` of the beacon
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MUST have a prefix that matches `"0000" + <subprefix>`.
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`<recipient_pubkey>` is the pubkey to use together with the key for the secret
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beacon to derive a shared secret as described below.
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`<relays>` are the remaining items in the list, specifying the relays where the
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recipient expects to receive a beacon.
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Beacon Generation
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-----------------
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- Generate a random keypair `a, A`.
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- Derive the shared secret `S` as described below.
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- Let `k = bytes(tagged_hash("NIP0092/beacon", bytes(x(S))))`.
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- Let `i` be the 0-indexed position in `<relays>` where the secret event is to
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be found.
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- Let `d` be a random odd positive integer below `2³²`.
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- For `j` in `0, 1, ...`:
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- Let `p` be the 2-byte encoding of `i` followed by the 4-byte encoding of
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`(j * d) mod 2³²`, each with the most significant byte first.
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- Let `c` be the byte-wise `xor` of `p` and `k[0:6]`.
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- Construct the beacon event, with `A` as the `pubkey` and `c` as the
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`content`.
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- If the resulting `id` matches the required prefix, break out of the loop.
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- Sign the beacon event using `a` and deliver it to one of the relays listed in
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`<relays>`.
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Shared Secret Derivation
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------------------------
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Given the keypair `a, A` of the beacon (`A` being its `pubkey`) and the keypair
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`b, B` of the recipient (`B` being their `recipient_pubkey`), the shared
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secret is simply the point `S` resulting from ECDH operation, that is, `S =
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a⋅B`, or equivalently, `S = b⋅A`. The shared secret is used in subsequent
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derivations as `tagged_hash(tag, bytes(x(S)))`, with the functions as defined in
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[BIP-340](https://bips.xyz/340).
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Secret Event Delivery
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---------------------
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The secret event to be delivered to the recipient is given the rendezvous
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keypair `r, R` (`R` being the `pubkey`), calculated as follows:
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- `t = bytes(tagged_hash("NIP0092/tweak", bytes(x(S))))`
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- `r = (a + t) mod n`
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- `R = A + t⋅G`
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The sender is able to derive `r` and uses this to construct and sign the secret
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event.
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From the beacon, the recipient derives the shared secret and is thus able to
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learn `R`. The recipient uses `R` to find the secret event on the relay
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indicated in the beacon.
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