Complete secreview followups for rust-ece crate

[data-sync-user]

Some time ago, we had :mt give us a security-focussed review of the rust-ece crate (which is used by the push component). He didn't find any problems but he did find some suggested areas for improvement. Ryan had a work-in-progress PR to address them (https://github.com/mozilla/rust-ece/pull/27)) but it has languished on the backlog for a while. Let's pick it up and finish it off so that rust-ece can be in a nice stable state.

┆Issue is synchronized with this Jira Task
┆Story Points: 3
┆Epic: Sync Maintainability Focus
┆Sprint: SYNC - 2021-04-16

[data-sync-user]

➤ Ryan Kelly commented:

I've completed the main one of these that I intended to do, over in mozilla/rust-ece#52 ( mozilla/rust-ece#52 ) and the corresponding rust-ece v2.0.0 release. We have an appservices PR to consume the new release as well: #3941 ( #3941 )

I've spun off two follow-up issues for other specific crypto-related cleanups:

• Clean up record-chunking code, and make it apply only to aes128gcm rust-ece#55 ( Clean up record-chunking code, and make it apply only to aes128gcm rust-ece#55 )
• Pad to multiples of a fixed size, rather than padding randomly rust-ece#54 ( Pad to multiples of a fixed size, rather than padding randomly rust-ece#54 )

But I don't think either of these are particularly urgent. If I run out of things to do later this week I'll take a look at them, otherwise let's put this crate back down for a while and we can look at them again some other time.

[data-sync-user]

➤ Ryan Kelly commented:

I ended up following through on the above two issues, have a cut a new release and updated appservices to use it here: #4000 ( #4000 )

I consider the work in this ticket complete.