[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