Stronger checks for invalid sync codes

[fmarier]

Here are a few small improvements that came up during a review of the Sync code:

• We should enforce a minimum size (32?) here: https://github.com/brave/brave-core/blob/d1c22d333d92f8fd76a6251e8c3c2fdfbdbae63c/components/brave_sync/crypto/crypto.cc#L22
• Validate that the seed is in the right format here: https://github.com/brave/brave-core/blob/d1c22d333d92f8fd76a6251e8c3c2fdfbdbae63c/components/sync/driver/brave_sync_service_impl.cc#L66
• We should treat this error as fatal: https://github.com/brave/brave-core/blob/d1c22d333d92f8fd76a6251e8c3c2fdfbdbae63c/components/brave_sync/crypto/crypto.cc#L135
• We should check that the sync code is of the expected length: https://github.com/brave/brave-core/blob/e2357907646d20fba395b0f736c4cec8fec50c31/browser/android/brave_sync_worker.cc#L97

None of these were found to be exploitable, but we'd like this code to be extra-paranoid about key-generation errors.

[rebron]

cc: @jsecretan

[AlexeyBarabash]

Not sure about

We should enforce a minimum size (32?) here: https://github.com/brave/brave-core/blob/d1c22d333d92f8fd76a6251e8c3c2fdfbdbae63c/components/brave_sync/crypto/crypto.cc#L22

maybe it will require server change
it is already 32

[fmarier]

That's right, it already defaults to 32. The change I'm asking for is just to enforce that in the function.

It could be as adding:

CHECK(size >= 32);

at the beginning of that function to crash the browser should this function ever get misused in the future.

[AlexeyBarabash]

@fmarier thanks
I am working on draft PR, how about brave/brave-core@3dc3784#diff-2ef0c64527a52e5910aa131da0822a63d0ab0bae6250bb768bf672d11b9b9287R22 ?

When having CHECK - this makes impossible to make the unit test , because we don't have EXPECT_CRASH option.

[AlexeyBarabash]

PR is opened for review

[stephendonner]

Verified PASSED using

Brave	1.32.103 Chromium: 96.0.4664.45 (Official Build) (x86_64)
Revision	76e4c1bb2ab4671b8beba3444e61c0f17584b2fc-refs/branch-heads/4664@{#947}
OS	macOS Version 11.6.1 (Build 20G224)

Steps:

1. new profile
2. launched Brave
3. loaded brave://settings/braveSync/setup and chose Start a new Sync Chain
4. copied code words into clipboard
5. shut down Brave
6. loaded brave://settings/braveSync/setup and confirmed my device was still on the Sync chain
7. clicked on Leave Sync Chain
8. clicked OK
9. confirmed my device left the Sync chain
10. loaded brave://settings/braveSync/setup and clicked on I have a Sync code
11. entered the code words from step 4 and clicked Confirm
12. confirmed via brave://settings/braveSync/setup my device is now again on the original chain created in step 3

example	example	example	example	example	example	example

Verification PASSED on Win 11 x64 using the following build:

Brave | 1.32.103 Chromium: 96.0.4664.45 (Official Build) (64-bit)
-- | --
Revision | 76e4c1bb2ab4671b8beba3444e61c0f17584b2fc-refs/branch-heads/4664@{#947}
OS | Windows 11 Version 21H2 (Build 22000.318)

• ensured that you can create a new sync chain via brave://settings/braveSync without any issues
• ensured that you can add other devices into the same sync chain via QR Code & code words without any issues
• ensured that the browser stays connected to the sync chain via brave://settings/braveSync after several browser restarts
• ensured that that leaving the sync chain works without any issues and that the second device is still connected
• ensured that re-joining the sync chain works without any issues

Initial Sync Chain	2nd Device on Sync Chain	Leaving Sync Chain	2nd Device after Leaving	Re-joining	Re-joined Sync Chain	2nd Device after re-joining

[kjozwiak]

Verification PASSED on Samsun S10+ running Android 11 using the following build:

Brave | 1.32.103 Chromium: 96.0.4664.45

• ensured that you can create a new sync chain via Settings -> Sync
• ensured that you can add other devices into the same sync chain via both QR Code & code word
• ensured that the browser stays connected to the sync chain after several browser restarts
• ensured that that leaving the sync chain works without any issues and that the second device is still connected
• ensured that re-joining the sync chain several times works without any issues

Initial Sync Chain	2nd Device on Sync Chain	Leaving Sync Chain	2nd Device after Leaving	Re-joining	Re-joined Sync Chain	2nd Device after re-joining

---

Verification passed on Brave v1.32.106 on Samsung Galaxy Tab S5e (Android 10.0)

Verified test plan from brave/brave-core#10960

• ensured that you can create a new sync chain via Settings -> Sync
• ensured that you can add other devices into the same sync chain via both QR Code & code word
• ensured that the browser stays connected to the sync chain after several browser restarts
• ensured that that leaving the sync chain works without any issues and that the second device is still connected
• ensured that re-joining the sync chain several times works without any issues