NIP-73

External Content IDs#

draft optional

There are certain established global content identifiers such as Book ISBNs, Podcast GUIDs, and Movie ISANs that are useful to reference in nostr events so that clients can query all the events associated with these ids.

i tags are used for referencing these external content ids, with k tags representing the external content id kind so that clients can query all the events for a specific kind.

Supported IDs#

Type	i tag	k tag
URLs	"<URL, normalized, no fragment>"	"web"
Books	"isbn:<id, without hyphens>"	"isbn"
Geohashes	"geo:<geohash, lowercase>"	"geo"
OSM Way	"osm:way:<POI id>"	"osm"
OSM Node	"osm:node:<Node id>"	"osm"
OSM Relation	"osm:relation:<Relation id>"	"osm"
Countries	"iso3166:<code, uppercase>"	"iso3166"
Movies	"isan:<id, without version part>"	"isan"
Papers	"doi:<id, lowercase>"	"doi"
Hashtags	"#<topic, lowercase>"	"#"
Podcast Feeds	"podcast:guid:<guid>"	"podcast:guid"
Podcast Episodes	"podcast:item:guid:<guid>"	"podcast:item:guid"
Podcast Publishers	"podcast:publisher:guid:<guid>"	"podcast:publisher:guid"
Blockchain Transaction	"<blockchain>:[<chainId>:]tx:<txid, hex, lowercase>"	"<blockchain>:tx"
Blockchain Address	"<blockchain>:[<chainId>:]address:<address>"	"<blockchain>:address"

---

Examples#

Webpages#

For the webpage "https://myblog.example.com/post/2012-03-27/hello-world" the "i" and "k" tags are:

[
  ["i", "https://myblog.example.com/post/2012-03-27/hello-world"],
  ["k", "web"]
]

Geohashes:#

• Geohash: ["i", "geo:ezs42e44yx96"] - https://www.movable-type.co.uk/scripts/geohash.html

Geohashes are a geocoding system that encodes geographic locations into short strings of letters and digits. They MUST be lowercase.

OpenStreetMap Object References:#

• Node: ["i", "osm:node:240949599"] – https://www.openstreetmap.org/node/240949599
• Way: ["i", "osm:way:5013364"] – https://www.openstreetmap.org/way/5013364
• Relation: ["i", "osm:relation:62422"] – https://www.openstreetmap.org/relation/62422

OpenStreetMap object IDs are globally unique identifiers. They use the lowercase format osm:<type>:<id> where <type> is one of node, way, or relation, and <id> is a numeric identifier.

Countries:#

ISO 3166 codes can reference countries (ISO 3166-1 alpha-2) or subdivisions like states/provinces (ISO 3166-2).

• Country (Venezuela): ["i", "iso3166:VE"]
• Subdivision (California, USA): ["i", "iso3166:US-CA"]

ISO 3166 codes MUST be uppercase. More info: https://en.wikipedia.org/wiki/ISO_3166

Books:#

• Book ISBN: ["i", "isbn:9780765382030"] - https://isbnsearch.org/isbn/9780765382030

Book ISBNs MUST be referenced without hyphens as many book search APIs return the ISBNs without hyphens. Removing hyphens from ISBNs is trivial, whereas adding the hyphens back in is non-trivial requiring a library.

Podcasts:#

• Podcast RSS Feed GUID: ["i", "podcast:guid:c90e609a-df1e-596a-bd5e-57bcc8aad6cc"] - https://podcastindex.org/podcast/c90e609a-df1e-596a-bd5e-57bcc8aad6cc
• Podcast RSS Item GUID: ["i", "podcast:item:guid:d98d189b-dc7b-45b1-8720-d4b98690f31f"]
• Podcast RSS Publisher GUID: ["i", "podcast:publisher:guid:18bcbf10-6701-4ffb-b255-bc057390d738"]

Movies:#

• Movie ISAN: ["i", "isan:0000-0000-401A-0000-7"] - https://web.isan.org/public/en/isan/0000-0000-401A-0000-7

Movie ISANs SHOULD be referenced without the version part as the versions / edits of movies are not relevant. More info on ISAN parts here - https://support.isan.org/hc/en-us/articles/360002783131-Records-relations-and-hierarchies-in-the-ISAN-Registry

Blockchain#

<blockchain> can be any layer 1 chain (bitcoin, ethereum, solana, ...). If necessary (e.g. for ethereum), you can specify a <chainId>.

Bitcoin#

bitcoin:address:<bech32, lowercase | base58, case sensitive>
bitcoin:tx:<txid, hex, lowercase>

E.g. https://blockstream.info/tx/a1075db55d416d3ca199f55b6084e2115b9345e16c5cf302fc80e9d5fbf5d48d

[
  ["i", "bitcoin:tx:a1075db55d416d3ca199f55b6084e2115b9345e16c5cf302fc80e9d5fbf5d48d"],
  ["k", "bitcoin:tx"]
]

E.g. https://blockstream.info/address/1HQ3Go3ggs8pFnXuHVHRytPCq5fGG8Hbhx

[
  ["i", "bitcoin:address:1HQ3Go3ggs8pFnXuHVHRytPCq5fGG8Hbhx"],
  ["k", "bitcoin:address"]
]

Ethereum (and other EVM chains)#

ethereum:<chainId, integer>:tx:<txHash, hex, lowercase>
ethereum:<chainId, integer>:address:<hex, lowercase>

E.g. https://etherscan.io/address/0xd8dA6BF26964aF9D7eEd9e03E53415D37aA96045

[
  ["i", "ethereum:1:address:0xd8da6bf26964af9d7eed9e03e53415d37aa96045"],
  ["k", "ethereum:address"]
]

E.g. https://gnosisscan.io/tx/0x98f7812be496f97f80e2e98d66358d1fc733cf34176a8356d171ea7fbbe97ccd

[
  ["i", "ethereum:100:tx:0x98f7812be496f97f80e2e98d66358d1fc733cf34176a8356d171ea7fbbe97ccd"],
  ["k", "ethereum:tx"]
]

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Optional URL Hints#

Each i tag MAY have a url hint as the second argument to redirect people to a website if the client isn't opinionated about how to interpret the id:

["i", "podcast:item:guid:d98d189b-dc7b-45b1-8720-d4b98690f31f", https://fountain.fm/episode/z1y9TMQRuqXl2awyrQxg]

["i", "isan:0000-0000-401A-0000-7", https://www.imdb.com/title/tt0120737]