This is the iOS part of Bug 1426304. Since we will soon start deriving non-sync-related keys from kB, sync clients should avoid storing kB directly, and instead store the minimal set of derived keys necessary to talk to sync: * 64 bytes for the sync key bundle: kSync = HKDF(kB, undefined, "identity.mozilla.com/picl/v1/oldsync", 64) * 16 bytes for the tokenserver's X-Client-State header: kXCS = SHA256(kB)[:16]

In IRC discussion, it came up that this change might cause us issues if we ever want to add the `chrome.storage.sync` API on mobile. Desktop will be storing those keys as a separate derived secret [1] but here on iOS, we'd have to ask the user to sign in again in order to get the new derived secrets. Are we likely to ever want to add `chrome.storage.sync` on iOS, and if so, should we modify this to store the necessary derived secrets just in case? [1] https://bugzilla.mozilla.org/show_bug.cgi?id=1426306

Merged

After writing this: > this change might cause us issues if we ever want to add the `chrome.storage.sync` API on mobile I realized that we don't have any webextension support at all on iOS, so the "if we ever want" part of this seems extremely far away. James, :rnewman mentioned you as a good person to ping about webextensions on iOS, do you have any thoughts on whether we might one day want to add such an API?

There seems be very little appetite internally for even a limited subset of the web extensions API. Iff we did decide to support it, without significantly more developers, it would take a long time to get to us needing chrome.storage.sync. "if we ever want" is further away than your "extremely far way".