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drafts/human-id-rules.rst
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This document outlines the format for human-readable IDs within matrix.
Summary
-------
- Human-readable IDs are Room Aliases and User IDs.
- They MUST be Unicode as UTF-8.
- If spoof checks fail, the user ID in question MUST be rewritten to be punycode
with an additional ``@`` prefix.
Room aliases cannot be rewritten.
- Spoof Checks:
- MUST NOT contain one of the 107 blacklisted characters on this list:
http://kb.mozillazine.org/Network.IDN.blacklist_chars
- MUST NOT contain characters from >1 language, defined by
http://cldr.unicode.org/
- User IDs MUST NOT contain a ``:`` or start with a ``@`` or ``.``
- Room aliases MUST NOT contain a ``:``
- User IDs SHOULD be case-insensitive.
Overview
--------
UTF-8 is quickly becoming the standard character encoding set on the web. As
such, Matrix requires that all strings MUST be encoded as UTF-8. However,
Abstract
========
This document outlines the format for human-readable IDs within Matrix.
Background
----------
UTF-8 is the dominant character encoding for Unicode on the web. However,
using Unicode as the character set for human-readable IDs is troublesome. There
are many different characters which appear identical to each other, but would
identify different users. In addition, there are non-printable characters which
cannot be rendered by the end-user. This opens up a security vulnerability with
produce different IDs. In addition, there are non-printable characters which
cannot be rendered by the end-user. This creates an opportunity for
phishing/spoofing of IDs, commonly known as a homograph attack.
Web browsers encountered this problem when International Domain Names were
introduced. A variety of checks were put in place in order to protect users. If
an address failed the check, the raw punycode would be displayed to
disambiguate the address. Similar checks are performed by home servers in
Matrix in order to protect users. In the event of a failed check, the raw
punycode is displayed as the user ID along with a special escape sequence to
indicate the change.
disambiguate the address.
Types of human-readable IDs
~~~~~~~~~~~~~~~~~~~~~~~~~~~
There are two main human-readable IDs in question:
The human-readable IDs in Matrix are Room Aliases and User IDs.
Room aliases look like ``#localpart:domain``. These aliases point to opaque
non human-readable room IDs. These pointers can change to point at a different
room ID at any time. User IDs look like ``@localpart:domain``. These represent
actual end-users (there is no indirection).
- Room aliases
- User IDs
Proposal
========
Room aliases look like ``#localpart:domain``. These aliases point to opaque
non human-readable room IDs. These pointers can change, so there is already an
issue present with the same ID pointing to a different destination at a later
date. Checks SHOULD be applied to room aliases, but they cannot be renamed in
punycode as that would break the alias. As a result, the checks in this document
apply to user IDs, although HSes may wish to enforce them on room alias
creation.
User IDs look like ``@localpart:domain``. These represent actual end-users, and
unlike room aliases, there is no layer of indirection. This presents a much
greater concern with homograph attacks. Checks MUST be applied to user IDs.
Spoof Checks
------------
First, each ID is split into segments (localpart/domain) around the ``:``. For
this reason, ``:`` is a reserved character and cannot be a localpart or domain
character.
User IDs which start with an ``@`` are used as an escape sequence for failed
user IDs. As a result, the localpart MUST NOT start with an ``@`` in order to
avoid namespace clashes.
The checks are similar to web browsers for IDNs. The first check is that the
segment MUST NOT contain a blacklisted character on this list:
http://kb.mozillazine.org/Network.IDN.blacklist_chars - NB: Even though
this is Mozilla, Chrome follows the same list as per
http://www.chromium.org/developers/design-documents/idn-in-google-chrome
The second check is that it MUST NOT contain characters from more than 1
language. This is defined by this dataset http://cldr.unicode.org/ and is
applied after stripping " 0-9, +, -, [, ], _, and the space character"
( http://www.chromium.org/developers/design-documents/idn-in-google-chrome )
Consequences of a failed check
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If a user ID fails the check, the user ID on the event is renamed. This is
possible because user IDs contain routing information. This doesn't require
extra work for clients, and users will see an odd user ID rather than a spoofed
name. Renaming is done in order to protect users of a given HS, so if a
User IDs and Room Aliases MUST be Unicode as UTF-8. Checks are performed on
these IDs by homeservers to protect users from phishing/spoofing attacks.
These checks are:
User ID Localparts:
- MUST NOT contain a ``:`` or start with a ``@`` or ``.``
- MUST NOT contain one of the 107 blacklisted characters on this list:
http://kb.mozillazine.org/Network.IDN.blacklist_chars
- After stripping " 0-9, +, -, [, ], _, and the space character it MUST NOT
contain characters from >1 language, defined by http://cldr.unicode.org/
Room Alias Localparts:
- MUST NOT contain a ``:``
- MUST NOT contain one of the 107 blacklisted characters on this list:
http://kb.mozillazine.org/Network.IDN.blacklist_chars
- After stripping " 0-9, +, -, [, ], _, and the space character it MUST NOT
contain characters from >1 language, defined by http://cldr.unicode.org/
In the event of a failed user ID check, well behaved homeservers MUST:
- Rewrite user IDs in the offending events to be punycode with an additional ``@``
prefix **before** delivering them to clients. There are no guarantees for
consistency between homeserver ID checking implementations. As a result, user
IDs MUST be sent in their *original* form over federation. This can be done in
a stateless manner as the punycode form has no information loss.
In the event of a failed room alias check, well behaved homeservers MUST:
- Send an HTTP status code 400 with an ``errcode`` of ``M_FAILED_HUMAN_ID_CHECK``
to the client if the client is attempting to *create* this alias.
- Send an HTTP status code 400 with an ``errcode`` of ``M_FAILED_HUMAN_ID_CHECK``
to the client if the client is attempting to *join* a room via this alias.
Examples::
@ebаy:domain.com (Cyrillic 'a', everything else English)
@@xn--eby-7cd:domain.com (Punycode with additional '@')
Homeservers SHOULD NOT allow two user IDs that differ only by case. This
SHOULD be applied based on the capitalisation rules in the CLDR dataset:
http://cldr.unicode.org/
This check SHOULD be applied when the user ID is created, in order to prevent
registration with the same name and different capitalisations, e.g.
``@foo:bar`` vs ``@Foo:bar`` vs ``@FOO:bar``. Home servers MAY canonicalise
the user ID to be completely lower-case if desired.
Rationale
=========
Each ID is split into segments (localpart/domain) around the ``:``. For
this reason, ``:`` is a reserved character and cannot be a localpart character.
The 107 blacklisted characters are used to prevent non-printable characters and
spaces from being used. The decision to ban characters from more than 1 language
matches the behaviour of Google Chrome for IDN handling. This is to protect
against common homograph attacks such as ebаy.com (Cyrillic "a", rest is
English). This would always result in a failed check. Even with this though
there are limitations. For example, сахар is entirely Cyrillic, whereas caxap is
entirely Latin.
User ID localparts cannot start with ``@`` so that a namespace of localparts
beginning with ``@`` can be created. This namespace is used for user IDs which
fail the ID checks. A failed ID could look like ``@@xn--c1yn36f:domain.com``.
If a user ID fails the check, the user ID on the event is renamed. This doesn't
require extra work for clients, and users will see an odd user ID rather than a
spoofed name. Renaming is done in order to protect users of a given HS, so if a
malicious HS doesn't rename their IDs, it doesn't affect any other HS.
- The HS MAY reject the creation of the room alias or user ID. This is the
preferred choice but it is entirely benevolent: other HSes may not apply this
rule so checks on incoming events MUST still be applied. The error code returned
for the rejection is ``M_FAILED_HUMAN_ID_CHECK``, which is generic enough for
both failing due to homograph attacks, and failing due to including ``:`` s.
Error message MAY go into further information about which characters were
rejected and why.
- The HS MUST rename the localpart which failed the check. It SHOULD be
represented as punycode. The HS MUST prefix the punycode with the escape
sequence ``@`` on user ID localparts, e.g. ``@@somepunycode:domain``. Room
aliases do not need to be escaped, and indeed they cannot be, as the originating
HS will not understand the rewritten alias. If a HS renames a user ID, it MUST
be able to apply the reverse mapping in case the user wishes to communicate with
the ID which failed the check.
Room aliases cannot be rewritten as punycode and sent to the HS the alias is
referring to as the HS will not necessarily understand the rewritten alias.
Other rejected solutions for failed checks
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@ -115,29 +113,14 @@ Other rejected solutions for failed checks
implemented. However, it is difficult to ensure that ALL HSes will come to the
same conclusion (given the CLDR dataset does come out with new versions).
Namespacing
-----------
Bots
~~~~
User IDs representing real users SHOULD NOT start with a ``.``. User IDs which
act on behalf of a real user (e.g. an IRC/XMPP bot) SHOULD start with a ``.``.
This namespaces real/generated user IDs. Further namespacing SHOULD be applied
based on the service being used, getting progressively more specific, similar to
event types: e.g. ``@.irc.freenode.matrix.<username>:domain``. Ultimately, the
HS in question has control over their user ID namespace, so this is just a
recommendation.
Additional recommendations
--------------------------
Outstanding Problems
====================
Capitalisation
~~~~~~~~~~~~~~
The home server SHOULD NOT allow two user IDs that differ only by case. This SHOULD be applied based on the
capitalisation rules in the CLDR dataset: http://cldr.unicode.org/
--------------
This check SHOULD be applied when the user ID is created, in order to prevent
registration with the same name and different capitalisations, e.g.
``@foo:bar`` vs ``@Foo:bar`` vs ``@FOO:bar``. Home servers MAY canonicalise
the user ID to be completely lower-case if desired.
The capitalisation rules outlined above are nice but do not fully resolve issues
where ``@alice:example.com`` tries to speak with ``@bob:domain.com`` using
``@Bob:domain.com``. It is up to ``domain.com`` to map ``Bob`` to ``bob`` in
a sensible way.

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