matrix-spec-proposals/proposals/2134-identity-hash-lookup.md

MSC2134: Identity Hash Lookups

Issue #2130 has been recently created in response to a security issue brought up by an independant party. To summarise the issue, lookups (of matrix userids) are performed using non-hashed 3pids which means that the 3pid is identifiable to anyone who can see the payload (e.g. willh AT matrix.org can be identified by a human).

The problem with this, is that a malicious identity service could then store the plaintext 3pid and make an assumption that the requesting entity knows the holder of the 3pid, even if the identity service does not know of the 3pid beforehand.

If the 3pid is hashed, the identity service could not determinethe owner of the 3pid unless the identity service has already been made aware of the 3pid by the owner themselves (using the /bind mechanism).

Note that this proposal does not stop a identity service from mapping hashed 3pids to many users, in an attempt to form a social graph. However the identity of the 3pid will remain a mystery until /bind is used.

It should be clear that there is a need to hide any address from the identity service that has not been explicitly bound to it, and this proposal aims to solve that for the lookup API.

Proposal

This proposal suggests making changes to the Identity Service API's lookup endpoints. Due to the nature of this proposal, the new endpoints should be on a v2 path:

• /_matrix/identity/api/v2/lookup
• /_matrix/identity/api/v2/bulk_lookup

The parameters will remain the same, but address should no longer be in a plain-text format. address will now take a SHA-256 format hash value, and the resulting digest should be encoded in base64 format. For example:

address = "willh@matrix.org"
digest = hashlib.sha256(address.encode()).digest()
result_address = base64.encodebytes(digest).decode()
print(result_address)
CpvOgBf0hFzdqZD4ASvWW0DAefErRRX5y8IegMBO98w=

Example request

SHA-256 has been chosen as it is currently used elsewhere in the Matrix protocol, and the only requirement for the hashing algorithm is that it cannot be used to guess the real value of the address

No parameter changes will be made to /bind, but identity services should keep a hashed value for each address it knows about in order to process lookups quicker and it is the recommendation that this is done at the time of bind.

v1 versions of these endpoints may be disabled at the discretion of the implementation, and should return a M_FORBIDDEN errcode if so.

Tradeoffs

• This approach means that the client now needs to calculate a hash by itself, but the belief is that most librarys provide a mechanism for doing so.
• There is a small cost incurred by doing hashes before requests, but this is outweighed by the privacy implications of sending plaintext addresses.

Potential issues

This proposal does not force a identity service to stop handling plaintext requests, because a large amount of the matrix ecosystem relies upon this behavior. However, a conscious effort should be made by all users to use the privacy respecting endpoints outlined above. Identity services may disallow use of the v1 endpoint.

Security considerations

None

Conclusion

This proposal outlines a quick and effective method to stop bulk collection of users contact lists and their social graphs without any disasterous side effects. All functionality which depends on the lookup service should continue to function unhindered by the use of hashes.