use cryptographic random to generate new connection IDs

fixes #348

client.go

@@ -3,7 +3,6 @@ package quic
 import (
 	"bytes"
 	"errors"
-	"math/rand"
 	"net"
 	"strings"
 	"sync/atomic"
@@ -52,9 +51,10 @@ func NewClient(host string, cryptoChangeCallback CryptoChangeCallback, versionNe
 		return nil, err
 	}
 
-	// TODO: generate cryptographically secure random ConnectionID
+	connectionID, err := utils.GenerateConnectionID()
-	rand.Seed(time.Now().UTC().UnixNano())
+	if err != nil {
-	connectionID := protocol.ConnectionID(rand.Int63())
+		return nil, err
+	}
 
 	hostname, _, err := net.SplitHostPort(host)
 	if err != nil {

utils/connection_id.go

@@ -0,0 +1,18 @@
+package utils
+
+import (
+	"crypto/rand"
+	"encoding/binary"
+
+	"github.com/lucas-clemente/quic-go/protocol"
+)
+
+// GenerateConnectionID generates a connection ID using cryptographic random
+func GenerateConnectionID() (protocol.ConnectionID, error) {
+	b := make([]byte, 8, 8)
+	_, err := rand.Read(b)
+	if err != nil {
+		return 0, err
+	}
+	return protocol.ConnectionID(binary.LittleEndian.Uint64(b)), nil
+}

utils/connection_id_test.go

@@ -0,0 +1,17 @@
+package utils
+
+import (
+	. "github.com/onsi/ginkgo"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("Connection ID generation", func() {
+	It("generates random connection IDs", func() {
+		c1, err := GenerateConnectionID()
+		Expect(err).ToNot(HaveOccurred())
+		Expect(c1).ToNot(BeZero())
+		c2, err := GenerateConnectionID()
+		Expect(err).ToNot(HaveOccurred())
+		Expect(c1).ToNot(Equal(c2))
+	})
+})