Implement Crypto.subtle

[jdm]

https://www.w3.org/TR/WebCryptoAPI/#dfn-SubtleCrypto

This is used in the hub.link flow for joining a hubs room on a VR device.

[jdm]

Hubs relies on the deriveKey, generateKey, importKey, exportKey, encrypt, and decrypt methods.

[jdm]

Hubs relies specifically on:

• ECDH (deriveKey, generateKey)
• AES-CBC (deriveKey, encrypt, decrypt)
• jwk (exportKey, importKey)
• P-256 (importKey, generateKey)

[jdm]

I've started tinkering in https://github.com/jdm/servo/tree/subtle.

[jdm]

ring may be a useful basis for this work. Gecko relies on NSS (eg. https://searchfox.org/mozilla-central/rev/82c04b9cad5b98bdf682bd477f2b1e3071b004ad/dom/crypto/WebCryptoTask.cpp#2727 for the ECDH implementation for deriveKey).

https://docs.rs/ring/0.16.15/ring/agreement/index.html has APIs involving ECDH, the P256 curve. However, it doesn't seem to have AES-CBC support. The RustCrypto group has https://docs.rs/aes/0.4.0/aes/ which looks like it might suit. Given that, we may just want to rely on the elliptic_curve and p256 crates instead of ring.