Add tls_prefer_rsa as promised a while back

Author: jedisct1

dnscrypt-proxy/config.go

@@ -92,6 +92,7 @@ type Config struct {
 	LogMaxBackups            int                         `toml:"log_files_max_backups"`
 	TLSDisableSessionTickets bool                        `toml:"tls_disable_session_tickets"`
 	TLSCipherSuite           []uint16                    `toml:"tls_cipher_suite"`
+	TLSPreferRSA             bool                        `toml:"tls_prefer_rsa"`
 	TLSKeyLogFile            string                      `toml:"tls_key_log_file"`
 	NetprobeAddress          string                      `toml:"netprobe_address"`
 	NetprobeTimeout          int                         `toml:"netprobe_timeout"`
@@ -156,6 +157,7 @@ func newConfig() Config {
 		LogMaxBackups:            1,
 		TLSDisableSessionTickets: false,
 		TLSCipherSuite:           nil,
+		TLSPreferRSA:             false,
 		TLSKeyLogFile:            "",
 		NetprobeTimeout:          60,
 		OfflineMode:              false,

dnscrypt-proxy/config_loader.go

@@ -65,6 +65,7 @@ func configureLogging(proxy *Proxy, flags *ConfigFlags, config *Config) {
 // configureXTransport - Configures the XTransport
 func configureXTransport(proxy *Proxy, config *Config) error {
 	proxy.xTransport.tlsDisableSessionTickets = config.TLSDisableSessionTickets
+	proxy.xTransport.tlsPreferRSA = config.TLSPreferRSA
 	proxy.xTransport.http3 = config.HTTP3
 	proxy.xTransport.http3Probe = config.HTTP3Probe
 

dnscrypt-proxy/example-dnscrypt-proxy.toml

@@ -287,6 +287,15 @@ cert_refresh_delay = 240
 # tls_disable_session_tickets = false
 
 
+## Prefer RSA certificates over ECDSA for TLS connections.
+## When this is enabled, some servers may become impossible to use,
+## or may stop to work later as they upgrade their configuratione.
+## Changing this setting is generally not recommended, but it may
+## reduce CPU usage on small routers with slow CPUs.
+
+# tls_prefer_rsa = false
+
+
 ## Log TLS key material to a file, for debugging purposes only.
 ## This file will contain the TLS master key, which can be used to decrypt
 ## all TLS traffic to/from DoH servers.

dnscrypt-proxy/xtransport.go

@@ -28,8 +28,13 @@ import (
 	"github.com/quic-go/quic-go/http3"
 	"golang.org/x/net/http2"
 	netproxy "golang.org/x/net/proxy"
+	"golang.org/x/sys/cpu"
 )
 
+var hasAESGCMHardwareSupport = cpu.X86.HasAES && cpu.X86.HasPCLMULQDQ ||
+	cpu.ARM64.HasAES && cpu.ARM64.HasPMULL ||
+	cpu.S390X.HasAES && cpu.S390X.HasAESGCM
+
 const (
 	DefaultBootstrapResolver    = "9.9.9.9:53"
 	DefaultKeepAlive            = 5 * time.Second
@@ -77,6 +82,7 @@ type XTransport struct {
 	http3                    bool
 	http3Probe               bool
 	tlsDisableSessionTickets bool
+	tlsPreferRSA             bool
 	proxyDialer              *netproxy.Dialer
 	httpProxyFunction        func(*http.Request) (*url.URL, error)
 	tlsClientCreds           DOHClientCreds
@@ -99,6 +105,7 @@ func NewXTransport() *XTransport {
 		useIPv6:                  false,
 		http3Probe:               false,
 		tlsDisableSessionTickets: false,
+		tlsPreferRSA:             false,
 		keyLogWriter:             nil,
 	}
 	return &xTransport
@@ -328,6 +335,28 @@ func (xTransport *XTransport) rebuildTransport() {
 	if xTransport.tlsDisableSessionTickets {
 		tlsClientConfig.SessionTicketsDisabled = true
 	}
+	if xTransport.tlsPreferRSA {
+		tlsClientConfig.MaxVersion = tls.VersionTLS12
+		if hasAESGCMHardwareSupport {
+			tlsClientConfig.CipherSuites = []uint16{
+				tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
+				tls.TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
+				tls.TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256,
+				tls.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
+				tls.TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
+				tls.TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256,
+			}
+		} else {
+			tlsClientConfig.CipherSuites = []uint16{
+				tls.TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256,
+				tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
+				tls.TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
+				tls.TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256,
+				tls.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
+				tls.TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
+			}
+		}
+	}
 	transport.TLSClientConfig = &tlsClientConfig
 	if http2Transport, _ := http2.ConfigureTransports(transport); http2Transport != nil {
 		http2Transport.ReadIdleTimeout = timeout