From 2bc1e11ce514d0e42dc240ed0bba397b1e7f8f23 Mon Sep 17 00:00:00 2001
From: Bas Westerbaan <bas@westerbaan.name>
Date: Tue, 27 Aug 2024 00:10:13 +0200
Subject: [PATCH] tls: use Go default kex for the moment that include PQC

By default Go 1.23 enables X25519Kyber768, a post-quantum key agreement
method that is enabled by default on Chrome. Go 1.23 does not expose
the CurveID, so we cannot add it by specifying it in CurvePreferences.
The reason is that X25519Kyber768 is a preliminary key agreement that
will be supplanted by X25519MLKEM768. For the moment there is value
in enabling it.

A consequence of this is that by default Caddy will enable support
for P-384 and P-521.

This PR also removes the special code to add support for X25519Kyber768
via the Cloudflare Go branch.

Cf #6540
---
 cmd/caddy/main.go              |  2 ++
 modules/caddytls/cf.go         | 24 ------------------------
 modules/caddytls/connpolicy.go | 10 +++++++++-
 modules/caddytls/values.go     |  5 +++++
 4 files changed, 16 insertions(+), 25 deletions(-)
 delete mode 100644 modules/caddytls/cf.go

diff --git a/cmd/caddy/main.go b/cmd/caddy/main.go
index 48fa149aa081..5da89b408537 100644
--- a/cmd/caddy/main.go
+++ b/cmd/caddy/main.go
@@ -1,3 +1,5 @@
+//go:debug tlskyber=1
+
 // Copyright 2015 Matthew Holt and The Caddy Authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
diff --git a/modules/caddytls/cf.go b/modules/caddytls/cf.go
deleted file mode 100644
index e61a59c09e1a..000000000000
--- a/modules/caddytls/cf.go
+++ /dev/null
@@ -1,24 +0,0 @@
-//go:build cfgo
-
-package caddytls
-
-// This file adds support for X25519Kyber768Draft00, a post-quantum
-// key agreement that is currently being rolled out by Chrome [1]
-// and Cloudflare [2,3]. For more context, see the PR [4].
-//
-// [1] https://blog.chromium.org/2023/08/protecting-chrome-traffic-with-hybrid.html
-// [2] https://blog.cloudflare.com/post-quantum-for-all/
-// [3] https://blog.cloudflare.com/post-quantum-to-origins/
-// [4] https://github.com/caddyserver/caddy/pull/5852
-
-import (
-	"crypto/tls"
-)
-
-func init() {
-	SupportedCurves["X25519Kyber768Draft00"] = tls.X25519Kyber768Draft00
-	defaultCurves = append(
-		[]tls.CurveID{tls.X25519Kyber768Draft00},
-		defaultCurves...,
-	)
-}
diff --git a/modules/caddytls/connpolicy.go b/modules/caddytls/connpolicy.go
index 4ec0e673a774..e2890c848ab6 100644
--- a/modules/caddytls/connpolicy.go
+++ b/modules/caddytls/connpolicy.go
@@ -841,7 +841,15 @@ func setDefaultTLSParams(cfg *tls.Config) {
 	cfg.CipherSuites = append([]uint16{tls.TLS_FALLBACK_SCSV}, cfg.CipherSuites...)
 
 	if len(cfg.CurvePreferences) == 0 {
-		cfg.CurvePreferences = defaultCurves
+		// We would want to write
+		//
+		//	cfg.CurvePreferences = defaultCurves
+		//
+		// but that would disable the post-quantum key agreement X25519Kyber768
+		// supported in Go 1.23, for which the CurveID is not exported.
+		// Instead, we'll set CurvePreferences to nil, which will enable PQC.
+		// See https://github.com/caddyserver/caddy/issues/6540
+		cfg.CurvePreferences = nil
 	}
 
 	if cfg.MinVersion == 0 {
diff --git a/modules/caddytls/values.go b/modules/caddytls/values.go
index 4e8c1adc244f..20fe45ff8fd1 100644
--- a/modules/caddytls/values.go
+++ b/modules/caddytls/values.go
@@ -108,6 +108,11 @@ var supportedCertKeyTypes = map[string]certmagic.KeyType{
 // implementation exists (e.g. P256). The latter ones can be
 // found here:
 // https://github.com/golang/go/tree/master/src/crypto/elliptic
+//
+// Temporily we ignore these default, to take advantage of X25519Kyber768
+// in Go's defaults (X25519Kyber768, X25519, P-256, P-384, P-521), which
+// isn't exported. See https://github.com/caddyserver/caddy/issues/6540
+// nolint:unused
 var defaultCurves = []tls.CurveID{
 	tls.X25519,
 	tls.CurveP256,