in the Matrix protocol. Additionally a hardcoded salt (“matrix” or something)
in the Matrix protocol. Additionally a
must be prepended to the data before hashing in order to serve as a weak
[pepper](https://en.wikipedia.org/wiki/Pepper_(cryptography)) must be prepended
defense against existing rainbow tables. As time goes on, this algorithm may be
to the data before hashing in order to serve as a weak defense against existing
changed provided a spec bump is performed. Then, clients making a request to
rainbow tables. This pepper will be specified by the identity server in order
`/lookup` must use the hashing algorithm defined in whichever version of the CS
to prevent a single rainbow table being generated for all identity servers. As
spec they and the IS have agreed to speaking.
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
No parameter changes will be made to /bind, but identity services should keep a
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
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.
quicker. It is the recommendation that this is done during the act of binding.
@ -87,11 +137,19 @@ 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
Additionally, only storing 3pid hashes at rest instead of the plain-text
versions is impractical if the hashing algorithm ever needs to be changed.
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.
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
We could let an identity server specify its own salt for the hashes, however it
that needs updating every time a new bind is made. Further considered solutions
would require an extra network call before uploading 3pid hashes in order for
are explored in https://signal.org/blog/contact-discovery/ Signal's eventual
the client to ask the server which salt it requires.
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.