Append Let's Encrypt ISRG Root X1 to cert pool

This is a workaround for older versions of android that do not trust
the Let's Encrypt root certificate.
https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/issues/40087

client/lib/rendezvous.go

@@ -4,6 +4,7 @@
 package snowflake_client
 
 import (
+	"crypto/tls"
 	"errors"
 	"fmt"
 
@@ -14,6 +15,7 @@ import (
 
 	"github.com/pion/webrtc/v3"
 	utls "github.com/refraction-networking/utls"
+	"gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/v2/common/certs"
 	"gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/v2/common/event"
 	"gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/v2/common/messages"
 	"gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/v2/common/nat"
@@ -50,7 +52,10 @@ type BrokerChannel struct {
 // and TLSHandshakeTimeout settings. But we want to disable the default
 // ProxyFromEnvironment setting.
 func createBrokerTransport() http.RoundTripper {
-	transport := http.DefaultTransport.(*http.Transport)
+	tlsConfig := &tls.Config{
+		RootCAs: certs.GetRootCAs(),
+	}
+	transport := &http.Transport{TLSClientConfig: tlsConfig}
 	transport.Proxy = nil
 	transport.ResponseHeaderTimeout = 15 * time.Second
 	return transport
@@ -70,7 +75,9 @@ func newBrokerChannelFromConfig(config ClientConfig) (*BrokerChannel, error) {
 		if err != nil {
 			return nil, fmt.Errorf("unable to create broker channel: %v", err)
 		}
-		utlsConfig := &utls.Config{}
+		utlsConfig := &utls.Config{
+			RootCAs: certs.GetRootCAs(),
+		}
 		brokerTransport = utlsutil.NewUTLSHTTPRoundTripper(utlsClientHelloID, utlsConfig, brokerTransport, config.UTLSRemoveSNI)
 	}
 

common/certs/certs.go

@@ -0,0 +1,54 @@
+package certs
+
+import (
+	"crypto/x509"
+	"log"
+)
+
+// https://crt.sh/?id=3958242236
+const LetsEncryptRootCert = `-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----`
+
+// GetRootCAs is a workaround for older versions of Android that do not trust
+// Let's Encrypt's ISRG Root X1. This manually adds the ISRG root to the device's
+// existing cert pool.
+func GetRootCAs() *x509.CertPool {
+	rootCerts, err := x509.SystemCertPool()
+	if err != nil {
+		rootCerts = x509.NewCertPool()
+	}
+	if ok := rootCerts.AppendCertsFromPEM([]byte(LetsEncryptRootCert)); !ok {
+		log.Println("Error appending Let's Encrypt root certificate to cert poool")
+		return nil
+	}
+	return rootCerts
+}