Replace aes-gcm with ring for AES encryption

[Kailai-Wang]

Context

companion of litentry/tee-worker#144

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[Kailai-Wang]

About the ring crate for wasm:

It failed to compile because ring 0.16.20implicitly depends on web-sys which has std dep - even for default-features = false

Btw the crate version 0.16.20 is rather old as they haven't updated the crate version for a while.

However, trying to switch to the up-to-date version won't work either, even though web-sys is removed:

ring = { git = "https://github.com/briansmith/ring", default-features = false, features = [ "alloc", "wasm32_unknown_unknown_js" ] }

as it's blocked by getrandom std (similar problem as above)

Solutions:

1. keep using aes-gcm, as it's for mock anyway
2. switch to patched version of ring, e.g. integritee-branch
3. switch to ring-oxus, see Switch to ring-xous integritee-network/pallets#115

[BillyWooo]

I would suggest we keep using aes-gcm for the moment to save the bandwidth for other more important tasks. And wait for INTEGRITEE's next movement. If necessary, we can propose our input to them for decision making about ring.

[Kailai-Wang]

OK let's keep aes-gcm. We need to add a comment sometime to explain why we didn't switch to ring. Integritee has more knowledge about it, I don't think they need any suggestion for now :/

[Kailai-Wang]

closing it now, nothing to do