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

# 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/hash_details
```

This endpoint takes no parameters, and simply returns the current pepper as a JSON object:

```
{
  "pepper": "matrixrocks",
  "algorithm": "sha256",
}
```

Clients should request this endpoint every time before making a
`/(bulk_)lookup`, to handle identity servers which may rotate their pepper
values frequently.

In addition, the pepper and hashing algorithm the client used must be a request
body field for the new `/lookup` and `/bulk_lookup` endpoints, ensuring that
the client is using the right parameters. 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 the client will know to query the hash details again
instead of assuming that no contacts are registered on that identity server.

Thus, a call to `/bulk_lookup` would look like the following:

```
{
  "threepids": [
    [
      "email",
      "vNjEQuRCOmBp/KTuIpZ7RUJgPAbVAyqa0Uzh770tQaw"
    ],
    [
      "msisdn",
      "0VnvYk7YZpe08fP/CGqs3f39QtRjqAA2lPd14eLZXiw"
    ],
    [
      "email",
      "BJaLI0RrLFDMbsk0eEp5BMsYDYzvOzDneQP/9NTemYA"
    ]
  ],
  "pepper": "matrixrocks",
  "algorithm": "sha256"
}
```

If the pepper does not match the server's, the client should receive a `400
M_INVALID_PARAM` with the error `Provided pepper does not match
'$server_pepper'`. If the algorithm does not match the server's, the client
should receive a `400 M_INVALID_PARAM` with the error `Provided algorithm does
not match '$server_algorithm'`. Clients should ensure they don't enter an
infinite loop if they receive these errors more than once even after changing
to the correct pepper and hash.

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.

## Fallback considerations

`v1` versions of these endpoints may be disabled at the discretion of the
implementation, and should return a HTTP 403 with a `M_FORBIDDEN` `errcode` if
so.

If an identity server is too old and a HTTP 404 is received when accessing the
`v2` endpoint, they should fallback to the `v1` endpoint instead. However,
clients should be aware that plain-text 3pids are required, and should ask for
user consent accordingly.


## 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.

While a bit out of scope for this MSC, there has been debate over preventing
3pids as being kept as plain-text on disk. The argument against this was that
if the hashing algorithm (in this case SHA-256) was broken, we couldn't update
the hashing algorithm without having the plaintext 3PIDs. Well @toml helpfully
added that we could just take the old hashes and rehash them in the more secure
hashing algorithm, thus transforming the hash from SHA-256 to
SHA-256+SomeBetterAlg. This may spur on 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.