MSC2134: Identity Hash Lookups

proposals/2134-identity-hash-lookup.md

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+# MSC2134: Identity Hash Lookups
+
+[Issue #2130](https://github.com/matrix-org/matrix-doc/issues/2130) has been
+recently created in response to a security issue brought up by an independent
+party. To summarise the issue, lookups (of matrix user ids) are performed using
+non-hashed 3pids (third-party IDs) which means that the identity server can
+identify and record every 3pid that the user wants to check, whether that
+address is already known by the identity server or not.
+
+If the 3pid is hashed, the identity service could not determine the address
+unless it has already seen that address in plain-text during a previous call of
+the /bind mechanism.
+
+Note that in terms of privacy, this proposal does not stop an identity service
+from mapping hashed 3pids to users, resulting in a social graph. However, the
+identity of the 3pid will at least remain a mystery until /bind is used.
+
+This proposal thus calls for the Identity Service’s /lookup API to use hashed
+3pids instead of their plain-text counterparts.
+
+## 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 (we also drop the `/api` in order to preserve consistency across
+other endpoints):
+
+- `/_matrix/identity/v2/lookup`
+- `/_matrix/identity/v2/bulk_lookup`
+
+`address` should no longer be in a plain-text format, but will now take a hash
+value, and the resulting digest should be encoded in unpadded base64. For
+example:
+
+```python
+address = "user@example.org"
+pepper = "matrix"
+digest = hashlib.sha256((pepper + address).encode()).digest()
+result_address = unpaddedbase64.encode_base64(digest)
+print(result_address)
+CpvOgBf0hFzdqZD4ASvWW0DAefErRRX5y8IegMBO98w
+```
+
+SHA-256 has been chosen as it is [currently used
+elsewhere](https://matrix.org/docs/spec/server_server/r0.1.2#adding-hashes-and-signatures-to-outgoing-events)
+in the Matrix protocol. Additionally a
+[pepper](https://en.wikipedia.org/wiki/Pepper_(cryptography)) must be prepended
+to the data before hashing in order to serve as a weak defense against existing
+rainbow tables. This pepper will be specified by the identity server in order
+to prevent a single rainbow table being generated for all identity servers. As
+time goes on, this algorithm may be changed provided a spec bump is performed.
+Then, clients making a request to `/lookup` must use the hashing algorithm
+defined in whichever version of the CS spec they and the IS have agreed to
+speaking.
+
+Identity servers can specify their own peppers, which can be handy if a rainbow table is released for their current one. Identity servers could also set a timer for rotating this value to further impede rainbow table publishing. As such, it must be possible for clients to be able to query what pepper an identity server requires before sending it hashes. Thus a new endpoint must be added:
+
+```
+GET /_matrix/identity/v2/lookup_pepper
+```
+
+This endpoint takes no parameters, and simply returns the current pepper as a JSON object:
+
+```
+{
+  "pepper": "matrixrocks"
+}
+```
+
+In addition, the pepper the client used must be appended as a parameter to the
+new `/lookup` and `/bulk_lookup` endpoints, ensuring that the client is using
+the right one. If it does not match what the server has on file (which may be
+the case is it rotated right after the client's request for it), then client
+will know to query the pepper again instead of just getting a response saying
+no contacts are registered on that identity server.
+
+Thus, a call to `/bulk_lookup` would look like the following:
+
+```
+{
+  "threepids": [
+    [
+      "email",
+      "user@example.org"
+    ],
+    [
+      "msisdn",
+      "123456789"
+    ],
+    [
+      "email",
+      "user2@example.org"
+    ]
+  ],
+  "pepper": "matrixrocks"
+}
+```
+
+If the pepper does not match the server's, the client should receive a `400
+M_INVALID_PARAM` with the error `Provided pepper value does not match
+'$server_pepper'`. Clients should ensure they don't enter an infinite loop if
+they receive this error more than once even after changing to the correct
+pepper.
+
+No parameter changes will be made to /bind, but identity servers should keep a
+hashed value for each address it knows about in order to process lookups
+quicker. It is the recommendation that this is done during the act of binding.
+
+`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 plain-text addresses.
+
+## Potential issues
+
+This proposal does not force an identity service to stop handling plain-text
+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.
+
+Unpadded base64 has been chosen to encode the value due to its 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.
+
+## Other considered solutions
+
+Ideally identity servers would never receive plain-text addresses, however it
+is necessary for the identity server to send an email/sms message during a
+bind, as it cannot trust a homeserver to do so as the homeserver may be lying.
+Additionally, only storing 3pid hashes at rest instead of the plain-text
+versions is impractical if the hashing algorithm ever needs to be changed.
+
+Bloom filters are an alternative method of providing private contact discovery,
+however does not scale well due to clients needing to download a large filter
+that needs updating every time a new bind is made. Further considered solutions
+are explored in https://signal.org/blog/contact-discovery/ Signal's eventual
+solution of using SGX is considered impractical for a Matrix-style setup.
+
+Bit out of scope for this MSC, but there was an argument for not keeping all
+IDs as hashed on disk in the identity server, that being if a hashing algorithm
+was broken, we couldn't update the hashing algorithm without having the
+plaintext 3PIDs. Well @toml helpfully said that we could just take the old
+hashes and rehash them in the more secure hashing algorithm, thus transforming
+the algo from ex. SHA256 to SHA256+SomeBetterAlg. This may spur an MSC in the
+future that supports this, unless it is just an implementation detail.
+
+## Conclusion
+
+This proposal outlines an effective method to stop bulk collection of user's
+contact lists and their social graphs without any disastrous side effects. All
+functionality which depends on the lookup service should continue to function
+unhindered by the use of hashes.
+