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diff --git a/docs/human-id-rules.rst b/docs/human-id-rules.rst new file mode 100644 index 0000000000..36987ddd0d --- /dev/null +++ b/docs/human-id-rules.rst @@ -0,0 +1,71 @@ +This document outlines the format for human-readable IDs within matrix. + +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, +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 the the end-user. This opens up a security vulnerability with +phishing/spoofing of IDs, commonly known as a homograph attack. + +Web browers 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, which will +then warn the client about the potentially misleading ID. However, Matrix does +not use punycode, and so does not show raw punycode on a failed check. Instead, +home servers must outright reject these misleading IDs. + +Types of human-readable IDs +--------------------------- +There are two main human-readable IDs in question: + + - Room aliases + - User IDs + +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. + +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 +------ +- Similar to web browsers. +- blacklisted chars (e.g. non-printable characters) +- mix of language sets from 'preferred' language not allowed. +- Language sets from CLDR dataset. +- Treated in segments (localpart, domain) + +Rejecting +--------- +- Home servers MUST reject room aliases which do not pass the check, both on + GETs and PUTs. +- Home servers MUST reject user ID localparts which do not pass the check, both + on creation and on events. +- Any home server whose domain does not pass this check, MUST use their punycode + domain name instead of the IDN, to prevent other home servers rejecting you. +- Error code is M_FAILED_HOMOGRAPH_CHECK. +- Error message MAY go into further information about which characters were + rejected and why. + +Other considerations +-------------------- +- Basic security: Informational key on the event attached by HS to say "unsafe + ID". Problem: clients can just ignore it, and since it will appear only very + rarely, easy to forget when implementing clients. +- Moderate security: Requires client handshake. Forces clients to implement + a check, else they cannot communicate with the misleading ID. However, this is + extra overhead in both client implementations and round-trips. +- High security: Outright rejection of the ID at the point of creation / + receiving event. Point of creation rejection is preferable to avoid the ID + entering the system in the first place. However, malicious HSes can just allow + the ID. Hence, other home servers must reject them if they see them in events. + Client never sees the problem ID, provided the HS is correctly implemented. +- High security decided; client doesn't need to worry about it, no additional + protocol complexity aside from rejection of an event. \ No newline at end of file |