Nostr protocol users may have other online identities such as usernames, profile pages, keypairs etc. they control and they may want to include this data in their profile metadata so clients can parse, validate and display this information.
## `i` tag on a metadata event
A new optional `i` tag is introduced for `kind 0` metadata event contents in addition to name, about, picture fields as included in [NIP-01](https://github.com/nostr-protocol/nips/blob/master/01.md):
An `i` tag will have two parameters and an optional third parameter, which are defined as the following:
1.`type:identity`: This is the platform name (for example `github`) or the linked cryptographic identity type (for example `openpgp4fpr`), and the identity on that platform (for example `semisol`) or public key fingerprint (for example `1A04E0F1A78D982BD8885B7EB325A9C5F70849D0`) joined together with `:`.
2.`proof`: String that points to the proof of owning this identity or is the proof itself (crytographic signature).
3. The full public key in the case of a cryptographic identity.
Identity provider names SHOULD only include `a-z`, `0-9` and the characters `._-/` and MUST NOT include `:`.
Identity names SHOULD be normalized if possible by replacing uppercase letters with lowercase letters, and if there are multiple aliases for an entity the primary one should be used.
Proof: A GitHub Gist ID. This Gist should be created by `<identity>` with a single file that has the text `Verifying that I control the following Nostr public key: <npub encoded public key>`.
Proof: A Tweet ID. The tweet should be posted by `<identity>` and have the text `Verifying my account on nostr My Public Key: "<npub encoded public key>"`.
Proof: A Mastodon post ID. This post should be published by `<username>@<instance>` and have the text `Verifying that I control the following Nostr public key: "<npub encoded public key>"`.
Proof: A string in the format `<ref>/<id>` which points to a message published in the public channel or group with name `<ref>` and message ID `<id>`. This message should be sent by user ID `<identity>` and have the text `Verifying that I control the following Nostr public key: "<npub encoded public key>"`.
echo 'Verifying that I control the following Nostr public key: "<npubencodedpublickey>"' | openssl dgst -sha256 -sign privatekey.pem | openssl base64 -A