LUD-16: Paying to static internet identifiers.
==============================================

`author: akumaigorodski` `author: andreneves` `author: fiatjaf` `discussion: https://t.me/lnurl/11610`

---

## Paying to [internet identifiers](https://datatracker.ietf.org/doc/html/rfc5322#section-3.4.1) (email-like addresses)

The idea here is that a `SERVICE` can offer human-readable addresses for users or specific internal endpoints that use the format `<username>@<domainname>`, e.g. satoshi@bitcoin.org. A user can then type these on a `WALLET`. The `<username>` is limited to `a-z0-9-_.` (and `+` if the `SERVICE` supports tags). Please note that this is way more strict than common email addresses as it allows fewer symbols and only lowercase characters.

Upon seeing such an address, `WALLET` makes a GET request to `https://<domain>/.well-known/lnurlp/<username>` endpoint if `domain` is clearnet or `http://<domain>/.well-known/lnurlp/<username>` if `domain` is onion. For example, if the address is `satoshi@bitcoin.org`, the request is to be made to `https://bitcoin.org/.well-known/lnurlp/satoshi`.

The response from `SERVICE` then MUST be the same as in [LUD-06](06.md), step 3, and the flow is the same.

### Default identifier

A `SERVICE` MAY offer a default internet identifier for a domain by using `_` as the `<username>`.

Upon seeing an address in the format `@<domainname>`, a `WALLET` MAY treat it as shorthand for `_@<domainname>` and make a GET request to the `https://<domain>/.well-known/lnurlp/_` endpoint if `domain` is clearnet or `http://<domain>/.well-known/lnurlp/_` if `domain` is onion. For example, if the address is `@bitcoin.org`, the request is to be made to `https://bitcoin.org/.well-known/lnurlp/_`.

This shorthand is optional. A `WALLET` that does not implement it MAY reject `@<domainname>` as an invalid internet identifier, and a `SERVICE` that does not offer a default identifier MAY return an error or 404 for `/.well-known/lnurlp/_`. Existing `<username>@<domainname>` identifiers MUST continue to resolve as specified above.

If a `SERVICE` returns metadata for a default identifier, it SHOULD use `_@<domainname>` as the canonical `text/identifier` value.

If providing such a scheme, `SERVICE` MUST add to the `metadata` JSON array either a `text/email` entry or a `text/identifier` entry, as in examples below:

```
[
    "text/identifier", // indication that this payment link is associated with an internet identifier string
    content // an internet identifier string in standard user@site.com format
]
```

or

```
[
    "text/email", // indication that this payment link is associated with an email address
    content // an email string in standard user@site.com format
]
```

The `text/email` entry MUST be used if the internet identifier corresponds to an actual email address.

### Tags

A `SERVICE` can offer multiple identifiers resolving to the same user by using the format `<username>+<tag>@<domainname>`.

Upon seeing such an address, `WALLET` makes a GET request to `https://<domain>/.well-known/lnurlp/<username>+<tag>` endpoint.

A `SERVICE` SHOULD strip the `+<tag>` part and continue as normal and MAY add a `text/tag` entry to the `metadata` JSON array:

```
[
    "text/tag", // optional
    content // the `<tag>` that has been passed in `<username>+<tag>`
]
```