okhttp: enable TLS 1.3 on Android, retain TLS 1.2-only for desktop JVM

The ConnectionSpec used by OkHttpChannelBuilder had TLS 1.3 explicitly
disabled since Dec 2020 due to a Conscrypt/SunJSSE incompatibility.
However, this incompatibility does not affect Android. The previous
code applied the TLS 1.2-only restriction unconditionally to all platforms.

Regulatory impact: TLS 1.2 is classified as a legacy mechanism in ENISA
Agreed Cryptographic Mechanisms v2.0 (April 2025), with TLS 1.3 listed as
the recommended protocol. This limitation has been forcing all downstream
components using grpc-okhttp on Android to operate with a legacy protocol,
creating compliance friction with the EU Radio Equipment Directive (RED) and
EU Cyber Resilience Act (CRA) certification requirements.

Fixes: #7431 (Android only)
Fixes: #7765 (Android only)

Author: Myk Syr

okhttp/src/main/java/io/grpc/okhttp/OkHttpChannelBuilder.java

@@ -116,17 +116,26 @@ private enum NegotiationType {
               CipherSuite.TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
               CipherSuite.TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
               CipherSuite.TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256,
-              CipherSuite.TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256
-
-              // TLS 1.3 does not work so far. See issues:
-              // https://github.com/grpc/grpc-java/issues/7765
-              //
-              // TLS 1.3
-              //CipherSuite.TLS_AES_128_GCM_SHA256,
-              //CipherSuite.TLS_AES_256_GCM_SHA384,
-              //CipherSuite.TLS_CHACHA20_POLY1305_SHA256
-              )
-          .tlsVersions(/*TlsVersion.TLS_1_3,*/ TlsVersion.TLS_1_2)
+              CipherSuite.TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256,
+              CipherSuite.TLS_AES_128_GCM_SHA256,
+              CipherSuite.TLS_AES_256_GCM_SHA384,
+              CipherSuite.TLS_CHACHA20_POLY1305_SHA256)
+          .tlsVersions(TlsVersion.TLS_1_3, TlsVersion.TLS_1_2)
+          .supportsTlsExtensions(true)
+          .build();
+
+  // @VisibleForTesting
+  static final ConnectionSpec INTERNAL_LEGACY_CONNECTION_SPEC =
+      new ConnectionSpec.Builder(ConnectionSpec.MODERN_TLS)
+          .cipherSuites(
+              // The following items should be sync with Netty's Http2SecurityUtil.CIPHERS.
+              CipherSuite.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
+              CipherSuite.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
+              CipherSuite.TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
+              CipherSuite.TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
+              CipherSuite.TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256,
+              CipherSuite.TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256)
+          .tlsVersions(TlsVersion.TLS_1_2)
           .supportsTlsExtensions(true)
           .build();
 
@@ -184,7 +193,7 @@ public static OkHttpChannelBuilder forTarget(String target, ChannelCredentials c
   private SSLSocketFactory sslSocketFactory;
   private final boolean freezeSecurityConfiguration;
   private HostnameVerifier hostnameVerifier;
-  private ConnectionSpec connectionSpec = INTERNAL_DEFAULT_CONNECTION_SPEC;
+  private ConnectionSpec connectionSpec;
   private NegotiationType negotiationType = NegotiationType.TLS;
   private long keepAliveTimeNanos = KEEPALIVE_TIME_NANOS_DISABLED;
   private long keepAliveTimeoutNanos = DEFAULT_KEEPALIVE_TIMEOUT_NANOS;
@@ -199,6 +208,12 @@ public static OkHttpChannelBuilder forTarget(String target, ChannelCredentials c
    */
   private final boolean useGetForSafeMethods = false;
 
+  private static ConnectionSpec initialConnectionSpec() {
+    return (OkHttpProtocolNegotiator.get() instanceof OkHttpProtocolNegotiator.AndroidNegotiator)
+        ? INTERNAL_DEFAULT_CONNECTION_SPEC
+        : INTERNAL_LEGACY_CONNECTION_SPEC;
+  }
+
   private OkHttpChannelBuilder(String host, int port) {
     this(GrpcUtil.authorityFromHostAndPort(host, port));
   }
@@ -209,6 +224,7 @@ private OkHttpChannelBuilder(String target) {
         new OkHttpChannelDefaultPortProvider());
     this.freezeSecurityConfiguration = false;
     this.channelCredentials = null;
+    this.connectionSpec = initialConnectionSpec();
   }
 
   OkHttpChannelBuilder(
@@ -222,6 +238,7 @@ private OkHttpChannelBuilder(String target) {
     this.negotiationType = factory == null ? NegotiationType.PLAINTEXT : NegotiationType.TLS;
     this.freezeSecurityConfiguration = true;
     this.channelCredentials = channelCreds;
+    this.connectionSpec = initialConnectionSpec();
   }
 
   private final class OkHttpChannelTransportFactoryBuilder

okhttp/src/main/java/io/grpc/okhttp/SslSocketFactoryServerCredentials.java

@@ -41,7 +41,7 @@ static final class ServerCredentials extends io.grpc.ServerCredentials {
     private final ConnectionSpec connectionSpec;
 
     ServerCredentials(SSLSocketFactory factory) {
-      this(factory, OkHttpChannelBuilder.INTERNAL_DEFAULT_CONNECTION_SPEC);
+      this(factory, OkHttpChannelBuilder.INTERNAL_LEGACY_CONNECTION_SPEC);
     }
 
     ServerCredentials(SSLSocketFactory factory, ConnectionSpec connectionSpec) {