Respect rustls default crypto provider after upgrading rustls to 0.23

[thynson]

Description of changes:

This is a follow up of #2200, making s2n-quic-rustls respect the CryptoProvider::get_default().

Call-outs:

Code that relying on aws_lc_rs directly was replaced with CryptoProvider::get_default().unwrap().

Testing:

Existing unit tests and examples pass.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[toidiu]

@thynson can you please share more about why you don't/aren't able to use the aws-lc-rs provider?

We would like to avoid supporting multiple providers since it makes integration less clean (as can be seen from the default_crypto_provider() code you submitted).

[thynson]

One of my teammate is trying to port and deploy our app to an embedded system, but failed to cross compile aws_lc_rs for various reason. For this reason we are staying on an old version internal fork of s2n-quic to avoid diverging the code base for now.
While my goal is to deploy our app on the cloud, actually I would like to switch to aws_lc_rs if it's possible for maximum throughput.

[thynson]

Actually we're blocked on #2069 too.

[toidiu]

@thynson Thanks for opening this PR and raising the issue. We considered what it would take to support both CryptoProviders and decided that it is better to only support a single CryptoProvider in the variant of aws-lc-rs.

Why choose aws-lc-rs:

In order to provider good performance/sercurity/interop we extensively test our library in our CI. Since each new feature requires more testing, we must carefully choose which features to support. In this instance we decided that it is better to not expose rustls's underlying CryptoProvider feature and instead set one by default. We have decided to only support aws-lc-rs because it is the default provider that rustls ships with and also because it offers FIPs compliance.

Still a build issue?

aws/aws-lc-rs#297
Since aws-lc-rs has removed its CMake dependency, I wonder if building aws-lc-rs is still an issue for you. We also encourage you to open an issue with aws-lc-rs to help resolve build issues.