Deal with versions/suites being contradictory

If we have TLS1.3 enabled but no TLS1.3 suites, we
shouldn't actually offer TLS1.3.

README.md

@@ -19,6 +19,7 @@ Rustls is currently in development and hence unstable.  [Here's what I'm working
   - Introduce client-side support for 0-RTT data in TLS1.3.
   - Fix a bug in rustls::Stream for non-blocking transports.
   - Move TLS1.3 support from draft 28 to final RFC8446 version.
+  - Don't offer (eg) TLS1.3 if no TLS1.3 suites are configured.
 * 0.13.1 (2018-08-17):
   - Fix a bug in rustls::Stream for non-blocking transports
     (backport).

src/client/hs.rs

@@ -247,8 +247,8 @@ fn emit_client_hello_for_retry(sess: &mut ClientSessionImpl,
         (SessionID::empty(), Vec::new(), ProtocolVersion::Unknown(0))
     };
 
-    let support_tls12 = sess.config.versions.contains(&ProtocolVersion::TLSv1_2);
-    let support_tls13 = sess.config.versions.contains(&ProtocolVersion::TLSv1_3);
+    let support_tls12 = sess.config.supports_version(ProtocolVersion::TLSv1_2);
+    let support_tls13 = sess.config.supports_version(ProtocolVersion::TLSv1_3);
 
     let mut supported_versions = Vec::new();
     if support_tls13 {
@@ -642,7 +642,7 @@ impl State for ExpectServerHello {
         trace!("We got ServerHello {:#?}", server_hello);
 
         use ProtocolVersion::{TLSv1_2, TLSv1_3};
-        let tls13_supported = sess.config.versions.contains(&TLSv1_3);
+        let tls13_supported = sess.config.supports_version(TLSv1_3);
 
         let server_version = if server_hello.legacy_version == TLSv1_2 {
             server_hello.get_supported_versions()
@@ -655,7 +655,7 @@ impl State for ExpectServerHello {
             TLSv1_3 if tls13_supported => {
                 sess.common.negotiated_version = Some(TLSv1_3);
             }
-            TLSv1_2 if sess.config.versions.contains(&TLSv1_2) => {
+            TLSv1_2 if sess.config.supports_version(TLSv1_2) => {
                 if sess.early_data.is_enabled() && sess.common.early_traffic {
                     // The client must fail with a dedicated error code if the server
                     // responds with TLS 1.2 when offering 0-RTT.

src/client/mod.rs

@@ -157,6 +157,14 @@ impl ClientConfig {
         }
     }
 
+    #[doc(hidden)]
+    /// We support a given TLS version if it's quoted in the configured
+    /// versions *and* at least one ciphersuite for this version is
+    /// also configured.
+    pub fn supports_version(&self, v: ProtocolVersion) -> bool {
+        self.versions.contains(&v) && self.ciphersuites.iter().any(|cs| cs.usable_for_version(v))
+    }
+
     #[doc(hidden)]
     pub fn get_verifier(&self) -> &verify::ServerCertVerifier {
         self.verifier.as_ref()

src/server/hs.rs

@@ -957,8 +957,8 @@ impl State for ExpectClientHello {
 
     fn handle(mut self: Box<Self>, sess: &mut ServerSessionImpl, m: Message) -> NextStateOrError {
         let client_hello = extract_handshake!(m, HandshakePayload::ClientHello).unwrap();
-        let tls13_enabled = sess.config.versions.contains(&ProtocolVersion::TLSv1_3);
-        let tls12_enabled = sess.config.versions.contains(&ProtocolVersion::TLSv1_2);
+        let tls13_enabled = sess.config.supports_version(ProtocolVersion::TLSv1_3);
+        let tls12_enabled = sess.config.supports_version(ProtocolVersion::TLSv1_2);
         trace!("we got a clienthello {:?}", client_hello);
 
         if !client_hello.compression_methods.contains(&Compression::Null) {

src/server/mod.rs

@@ -164,6 +164,14 @@ impl ServerConfig {
         }
     }
 
+    #[doc(hidden)]
+    /// We support a given TLS version if it's quoted in the configured
+    /// versions *and* at least one ciphersuite for this version is
+    /// also configured.
+    pub fn supports_version(&self, v: ProtocolVersion) -> bool {
+        self.versions.contains(&v) && self.ciphersuites.iter().any(|cs| cs.usable_for_version(v))
+    }
+
     #[doc(hidden)]
     pub fn get_verifier(&self) -> &verify::ClientCertVerifier {
         self.verifier.as_ref()

tests/api.rs

@@ -1143,9 +1143,12 @@ fn stream_write_swallows_underlying_io_error_after_plaintext_processed() {
 fn make_disjoint_suite_configs() -> (ClientConfig, ServerConfig) {
     let kt = KeyType::RSA;
     let mut server_config = make_server_config(kt);
-    server_config.ciphersuites = vec![];
+    server_config.ciphersuites = vec![find_suite(CipherSuite::TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256)];
 
-    (make_client_config(kt), server_config)
+    let mut client_config = make_client_config(kt);
+    client_config.ciphersuites = vec![find_suite(CipherSuite::TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384)];
+
+    (client_config, server_config)
 }
 
 #[test]