1426304 - [meta] Stop clients from storing kB directly; instead have them store a minimal set of derived values

Status: RESOLVED FIXED

(Firefox :: Sync, enhancement, P3)

Description

[Ryan Kelly [:rfkelly]]

With lockbox and notes coming up as new consumers of FxA's key-management services, it's no longer appropriate for the browser to be storing the raw `kA` and `kB` values on disk.

There's no guarantee that the security model of sync matches that of other consumers of FxA key material.  For example, Lockbox would like to use its FxA scoped key as a kind of "master password" to encrypt data locally, and having Sync store the top-level secret key kB in the same profile directory would defeat the whole point of that scheme.

Related mailing-list discussion here:  https://groups.google.com/a/mozilla.com/d/msg/fxa-staff/29A7-OFg_fk/1MY11AWWBAAJ

Concretely, we need to identify all the place where clients currently store the raw value of `kA` and/or `kB`, and update them to instead store the minimal set of derived key material necessary to function correctly.  This is a metabug for tracking that work across clients.

For sync, clients will need to store the following derived keys:

* 64 bytes for the sync key bundle [1]:

  kSync = HKDF(kB, undefined, "identity.mozilla.com/picl/v1/oldsync", 64)

* 16 bytes for the tokenserver's X-Client-State header [2]:

  kXCS = SHA256(kB)[:16]

* Optionally, 64 bytes for the webext chrome.storage.sync master key [3]:

  kExtSync = HKDF(kB, undefined, "identity.mozilla.com/picl/v1/chrome.storage.sync", 64)

I can't find any other places in the Firefox codebase that currently use kB, but please comment below if I've missed any.


[1] https://dxr.mozilla.org/mozilla-central/source/services/sync/modules/browserid_identity.js#137
[2] https://dxr.mozilla.org/mozilla-central/source/services/sync/modules/browserid_identity.js#415
[3] https://dxr.mozilla.org/mozilla-central/source/toolkit/components/extensions/ExtensionStorageSync.jsm#279

Comment 1

[Ryan Kelly [:rfkelly]]

Well this bug tree was resolved surprisingly quickly, thanks all!