tls: Add support for X25519Kyber768Draft00 post-quantum "curve" when compiled with cfgo.

[bwesterb]

It's not ideal to have to compile with a different toolchain, and I'd understand if you'd rather wait for upstream Go.

With this patch, we can get PQ caddy:

git clone https://github.com/bwesterb/caddy
git clone https://github.com/cloudflare/go
cd go/src
./make.bash
cd ../../caddy/cmd/caddy
git checkout pq
../../../go/bin/go build

This enables PQ for browser -> Caddy by default. For PQ Caddy -> upstream we need this patch, and then we can use:

localhost {
    reverse_proxy https://pq.cloudflareresearch.com {
        header_up Host {upstream_hostport}
        transport http {
            tls_curves X25519Kyber768Draft00 x25519 secp256r1
        }
    }
}

For Caddy -> upstream we're not enabling it by default, as not all upstreams might support it, and it would incur an extra roundtrip (HelloRetryRequest). One can check for PQ caddy -> upstream in this case by visiting https://localhost/cdn-cgi/trace:

$ curl https://localhost/cdn-cgi/trace | grep kex
kex=X25519Kyber768Draft00

[mholt]

Thank you. This is a simple and elegant change. It does seem like a new feature but also requires explicit build tag so it doesn't affect existing build flows. Normally we are blocking new features for 2.8 so we can focus on testing but this is a separate opt in feature. So I dunno, we can at least merge this in, and decide later if we want to include it before 2.9.

[bwesterb]

Thanks, sgtm.

[francislavoie]

Could we add a code comment in this file to explain why it exists and how it can be used, in short terms? It can link to this PR as context. I think it might be confusing as-is to understand why it exists without context.

[bwesterb]

Could we add a code comment in this file to explain why it exists and how it can be used, in short terms?

@francislavoie Added a comment. Let me know if you want more elaborate instructions.

[francislavoie]

Thanks! That's helpful 👍

[bwesterb]

By the way, what about adding the line to init()

defaultCurves = append(
        []tls.CurveID{tls.X25519Kyber768Draft00},
        defaultCurves...,
)

You wouldn't compile with our fork of Go unless you want PQ anyway.

[mholt]

Probably a good idea! I'd approve of that.

[bwesterb]

Done.

[mholt]

Thanks for the contribution!

[bwesterb]

Thanks for the contribution!

Thanks for taking the PR! 🙏

[high3eam]

I temporarily replaced my standard go with cf-go and use Caddy v2.7.5 and xcaddy v0.3.5

$ git clone https://github.com/cloudflare/go
$ cd go/src
$ ./make.bash

in my .bashrc

GOROOT=$HOME/go
GOPATH=$HOME/gopath
PATH=$PATH:$GOROOT/bin

go version command outputs:

$ source ~/.bashrc
$ go version
go version go1.21.3-devel-cf linux/amd64

Building Caddy:

$ ./xcaddy build v2.7.5 --with github.com/caddy-dns/cloudflare@master --output caddy

And without changing my Caddyfile config, it just works 🙌 (latest Edge browser v117.0.2045.60)