Speak protocol 3.0 and fall back to 2.0 only when that is the reflector's maximum.

A reflector that has moved to 3.0 will not accept a station that always announces 2.0, and a 2.x reflector still needs one explicit plaintext session.

Co-authored-by: Cursor <cursoragent@cursor.com>

station/tls.go

@@ -0,0 +1,60 @@
+package station
+
+import (
+	"crypto/tls"
+	"crypto/x509"
+	"errors"
+	"net"
+	"strings"
+)
+
+var errHostMismatch = errors.New("host mismatch")
+
+func tlsConfig(host string, caPEM []byte, client tls.Certificate, present bool) (*tls.Config, error) {
+	pool := x509.NewCertPool()
+	if !pool.AppendCertsFromPEM(caPEM) {
+		return nil, errors.New("ca bundle")
+	}
+	cfg := &tls.Config{
+		MinVersion:         tls.VersionTLS12,
+		RootCAs:            pool,
+		InsecureSkipVerify: true,
+		VerifyConnection: func(cs tls.ConnectionState) error {
+			return verifyPeer(host, pool, cs)
+		},
+	}
+	if present {
+		cfg.Certificates = []tls.Certificate{client}
+	}
+	return cfg, nil
+}
+
+func verifyPeer(host string, pool *x509.CertPool, cs tls.ConnectionState) error {
+	if len(cs.PeerCertificates) == 0 {
+		return errors.New("no peer certificate")
+	}
+	leaf := cs.PeerCertificates[0]
+	opts := x509.VerifyOptions{Roots: pool, KeyUsages: []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth}}
+	if len(cs.PeerCertificates) > 1 {
+		opts.Intermediates = x509.NewCertPool()
+		for _, cert := range cs.PeerCertificates[1:] {
+			opts.Intermediates.AddCert(cert)
+		}
+	}
+	if _, err := leaf.Verify(opts); err != nil {
+		return err
+	}
+	host = strings.TrimSuffix(host, ".")
+	if ip := net.ParseIP(host); ip != nil {
+		for _, cand := range leaf.IPAddresses {
+			if cand.Equal(ip) {
+				return nil
+			}
+		}
+		return errHostMismatch
+	}
+	if err := leaf.VerifyHostname(host); err != nil {
+		return errHostMismatch
+	}
+	return nil
+}