MSC4009: Expanding the Matrix ID grammar to enable E.164 IDs (#4009)

* E.164 Matrix IDs.

* Fix typo.

* small typo fixes

* Expand examples.

Another reasonable way around this is to use a prefix,
instead of character mapping.

The downsides are essentially the same.

* Updates from feedback.

* Fix typos.

Co-authored-by: Hubert Chathi <hubertc@matrix.org>

* Add note about re-using IDs.

---------

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>
Co-authored-by: Hubert Chathi <hubertc@matrix.org>

Author: 3 people

proposals/4009-e.164-matrix-ids.md

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+# MSC4009: Expanding the Matrix ID grammar to enable E.164 IDs
+
+[E.164](https://www.itu.int/rec/T-REC-E.164) is a set of international standards
+for telephone numbering. This defines a phone number as a `+` followed by a country
+code (e.g. `1` for the US, `44` for the UK) followed by a local phone number.
+For example, a E.164 phone number would look like: `+15558675309`.
+
+It is somewhat common[^1] for social networks or messaging applications to use phone
+numbers as identifiers instead of relying on users remembering separate usernames
+(and needing to tell other users how to find them).
+
+[Matrix user identifiers](https://spec.matrix.org/v1.6/appendices/#user-identifiers)
+are *almost* compatible with E.164:
+
+> The `localpart` of a user ID is an opaque identifier for that user. It MUST NOT
+> be empty, and MUST contain only the characters `a-z`, `0-9`, `.`, `_`, `=`, `-`,
+> and `/`.
+
+## Proposal
+
+Add `+` to the list of allowed characters in a Matrix user identifier. This would
+allow a full E.164 phone number as a user's username on a server, which is common
+practice for many applications. This should hopefully ease bridging of those
+services to Matrix or help them natively speak Matrix in the future. Users would
+not need to learn a new Matrix ID, but can continue using their phone number, as
+today.
+
+Although E.164 IDs are meant to be globally unique they are still namespaced under
+a domain under this proposal, e.g. `@+15558675309:example.com`, as the same user may
+already be registered on multiple service providers.
+
+## Potential issues
+
+Homeservers and clients must already be
+[more tolerant of Matrix user IDs](https://spec.matrix.org/v1.6/appendices/#historical-user-ids):
+
+> Older versions of this specification were more tolerant of the characters
+> permitted in user ID localparts. [...] clients and servers MUST accept user IDs
+> with localparts from the expanded character set:
+>
+> `extended_user_id_char = %x21-39 / %x3B-7E  ; all ASCII printing chars except :`
+
+Thus, it is expected to not cause any issues, although clients will need to identify
+that the `+` character is valid for a homeserver. This could be from the supported
+Matrix versions which the homeserver advertises.
+
+----
+
+A user having two accounts with the same identifier on different services is not
+ideal, but is not different than today where a user may log into both WhatsApp
+and Signal with their phone number. This MSC does *not* attempt to help with mapping
+of an E.164 identifier to an actual Matrix ID, that's best left to the current
+[identity service `/lookup` endpoint](https://spec.matrix.org/v1.6/identity-service-api/#post_matrixidentityv2lookup)
+or future discovery services.
+
+----
+
+If a service uses E.164 identifiers as Matrix IDs then it must be careful to avoid
+leaking history when reassigning IDs to new users. This is related to [#234](https://github.com/matrix-org/matrix-spec/issues/234),
+but only applies to the localpart of the Matrix ID, not the entire domain. The
+solution remains the same: using portable identifiers ([MSC1228](https://github.com/matrix-org/matrix-spec-proposals/pull/1228)
+or [MSC4014](https://github.com/matrix-org/matrix-spec-proposals/pull/4014)).
+
+## Alternatives
+
+The `+` could continue to be disallowed and left off the beginning of the Matrix
+IDs. Note that Synapse reserves digit-only usernames for guest users, although this
+seems to be an implementation detail and not mandated by the specification.
+
+Another option would be to [map from other character sets](https://spec.matrix.org/v1.6/appendices/#mapping-from-other-character-sets)
+or prefix the Matrix ID with something (e.g. `msisdn`).
+This would generate a Matrix ID like `@=2b15558675309:example.com` or
+`@msisdn_5558675309:example.com`, which would dramatically impact usability
+for the users on homeservers which use phone numbers as identifiers.
+
+----
+
+Although E.164 limits the `+` character to the initial character there seems to
+be no reason to limit that for Matrix identifiers.
+
+## Security considerations
+
+E.164 IDs are globally unique, but the proposed change only enforces per-homeserver
+uniqueness. If a homeserver does not diligently check that the phone number belongs
+to a user then this may allow additional spoofing attacks. The author does not consider
+this to be much worse than today's situation:
+
+1. Many current applications need to verify that a phone number truly is owned
+   by an account. Any current best practices should be followed by a service
+   taking advantage of this MSC.
+2. It is already quite easy today to spoof a user's ID by registering the same
+   user localpart on a different homeserver. The same issue occurs with email or
+   other distributed system (Mastodon, etc.).
+
+## Unstable prefix
+
+N/A
+
+## Dependencies
+
+N/A
+
+[^1]: E.g. Signal, WhatsApp, etc.