MSC4009: Expanding the Matrix ID grammar to enable E.164 IDs

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

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+# MSC4009: E.164 Matrix 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 the localpart of a Matrix ID: `@+15558675309:example.com`.
+
+It could additionally lower the impedance mismatch when bridging if the identifiers
+use the full E.164 phone numbers as identifiers.
+
+## 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.
+
+## 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
+
+None noted.
+
+## Unstable prefix
+
+N/A
+
+## Dependencies
+
+N/A
+
+[^1]: E.g. Signal, WhatsApp, etc.