Contribute to the Rust HPKE crate

[ipetr0v]

There is an existing implementation of the RFC 9180 Hybrid Public Key Encryption in Rust: https://crates.io/crates/hpke

But this implementation lacks the bidirectional encryption, which is explained in the RFC but in the Security Considerations section - so it's technically not a part of the protocol.

We need bidirectional encryption for our remote attestation use-cases - so we need to think about contributing to this crate, and making it work with bidirectional encryption.

[ipetr0v]

cc @tiziano88 @conradgrobler @sidtelang

[ipetr0v]

Also the default-features include x25519-dalek, which we would want to remove from our codebase: https://github.com/rozbb/rust-hpke/blob/b53f23ca707efe78051dec5330dd18e6007e2384/Cargo.toml#L17-L18

We probably need to split p256 into a separate feature, so we won't have to rely on the default-features.

[ipetr0v]

Another contribution that we could make is to allow serialization for the Context objects.

We cannot share private keys with the enclave application and only want to share session keys/contexts. But it would require sending them iver gRPC.