3.8 KiB
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 user ids) are performed using non-hashed 3pids which means that the 3pid is identifiable to anyone who can see the payload (e.g. willh@matrix.org can be identified).
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 determine the 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 languages 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.
Base64 has been chosen to encode the value due to it's ubiquitous support in many languages, however it does mean that special characters in the address will have to be encoded when used as a parameter value.
Security considerations
None
Conclusion
This proposal outlines a quick and effective method to stop bulk collection of user's 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.