Add RustCrypto cryptographic backend about rustls

[djc]

I don't think the Rustls project will develop a first-party RustCrypto-based provider, but others have already started on that as part of the RustCrypto project.

For the time being I would recommend just using the first-party ring provider if that works on your platform.

[wiktor-k]

Ok!

Thanks a lot for the comments and references. Even though I believe pure Rust backend would be better in several ways I understand the engineering time to maintain multiple backends here would be significant.

👋

[wiktor-k]

That's reassuring to hear. Thanks for the explanation! 🙏

[cpu]

Describe the solution you'd like
I believe using pure Rust libraries, such as the ones from the RustCrypto project would make cross-compilation easier. I'd use a RustCrypto backend even if it is implementing a small set of only modern cipher suites.

I think the solution you're describing exists today: https://github.com/RustCrypto/rustls-rustcrypto

It's a crate in the Rust Crypto org and I think that's the best place for it to live. The Rust Crypto developers are better positioned to support that crate than we would be in the Rustls org.

[cpu]

We're certainly supportive of the goal of a pure rust cryptography backend. Much of the motivation for the cryptography provider interface work was to enable everyone to choose a backend to use based on their own prioritization weights.

I understand the engineering time to maintain multiple backends here would be significant.

It's worth noting we don't meaningfully maintain the underlying cryptography provider options that are built-in either, just the surface points for where they meet Rustls. aws-lc-rs has a team at Amazon that maintains it and *ring* (also an OpenSSL derivative like aws-lc-rs) is maintained by Brian Smith. We'll also work closely with the Rust Crypto team to help maintain an end-to-end solution that works for folks, but in all cases the Rustls team is collaborating with third parties.