send a reserved version number in the EncryptedExtensions message

internal/handshake/tls_extension_handler_server.go

@@ -47,13 +47,14 @@ func (h *extensionHandlerServer) Send(hType mint.HandshakeType, el *mint.Extensi
 		// TODO(#855): generate a real token
 		transportParameter{statelessResetTokenParameterID, bytes.Repeat([]byte{42}, 16)},
 	)
-	supportedVersions := make([]uint32, len(h.supportedVersions))
-	for i, v := range h.supportedVersions {
-		supportedVersions[i] = uint32(v)
+	supportedVersions := protocol.GetGreasedVersions(h.supportedVersions)
+	versions := make([]uint32, len(supportedVersions))
+	for i, v := range supportedVersions {
+		versions[i] = uint32(v)
 	}
 	data, err := syntax.Marshal(encryptedExtensionsTransportParameters{
 		NegotiatedVersion: uint32(h.version),
-		SupportedVersions: supportedVersions,
+		SupportedVersions: versions,
 		Parameters:        transportParams,
 	})
 	if err != nil {

internal/handshake/tls_extension_handler_server_test.go

@@ -45,7 +45,8 @@ var _ = Describe("TLS Extension Handler, for the server", func() {
 
 		It("adds TransportParameters to the EncryptedExtensions message", func() {
 			handler.version = 666
-			handler.supportedVersions = []protocol.VersionNumber{13, 37, 42}
+			versions := []protocol.VersionNumber{13, 37, 42}
+			handler.supportedVersions = versions
 			err := handler.Send(mint.HandshakeTypeEncryptedExtensions, &el)
 			Expect(err).ToNot(HaveOccurred())
 			Expect(el).To(HaveLen(1))
@@ -55,8 +56,12 @@ var _ = Describe("TLS Extension Handler, for the server", func() {
 			eetp := &encryptedExtensionsTransportParameters{}
 			_, err = syntax.Unmarshal(ext.data, eetp)
 			Expect(err).ToNot(HaveOccurred())
-			Expect(eetp.SupportedVersions).To(Equal([]uint32{13, 37, 42}))
 			Expect(eetp.NegotiatedVersion).To(BeEquivalentTo(666))
+			// the SupportedVersions will contain one reserved version number
+			Expect(eetp.SupportedVersions).To(HaveLen(len(versions) + 1))
+			for _, version := range versions {
+				Expect(eetp.SupportedVersions).To(ContainElement(uint32(version)))
+			}
 		})
 	})