cache the ephermal key for up to 1 min

fixes #136

handshake/ephermal_cache_test.go

@@ -0,0 +1,30 @@
+package handshake
+
+import (
+	"time"
+
+	"github.com/lucas-clemente/quic-go/crypto"
+	"github.com/lucas-clemente/quic-go/protocol"
+	. "github.com/onsi/ginkgo"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("Ephermal KEX", func() {
+	It("has a consistent KEX", func() {
+		kex1 := getEphermalKEX()
+		Expect(kex1).ToNot(BeNil())
+		kex2 := getEphermalKEX()
+		Expect(kex2).ToNot(BeNil())
+		Expect(kex1).To(Equal(kex2))
+	})
+
+	It("changes KEX", func() {
+		kexLifetime = time.Millisecond
+		defer func() {
+			kexLifetime = protocol.EphermalKeyLifetime
+		}()
+		kex := getEphermalKEX()
+		Expect(kex).ToNot(BeNil())
+		Eventually(func() crypto.KeyExchange { return getEphermalKEX() }).ShouldNot(Equal(kex))
+	})
+})