udp: Add AES-based connection ID generation

udp/connection.go

@@ -0,0 +1,50 @@
+// Copyright 2015 The Chihaya Authors. All rights reserved.
+// Use of this source code is governed by the BSD 2-Clause license,
+// which can be found in the LICENSE file.
+
+package udp
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+)
+
+var connectionKey, connectionIV []byte
+
+func InitConnectionIDEncryption() error {
+	connectionKey = make([]byte, 16)
+	_, err := rand.Read(connectionKey)
+	if err != nil {
+		return err
+	}
+
+	connectionIV = make([]byte, 16)
+	_, err = rand.Read(connectionIV)
+	return err
+}
+
+func GenerateConnectionID(ip []byte) []byte {
+	block, err := aes.NewCipher(connectionKey)
+	if err != nil {
+		panic(err)
+	}
+
+	if len(ip) > 16 {
+		panic("IP larger than 16 bytes")
+	}
+
+	for len(ip) < 8 {
+		ip = append(ip, ip...) // Not enough bits in output.
+	}
+
+	ct := make([]byte, 16)
+	stream := cipher.NewCFBDecrypter(block, connectionIV)
+	stream.XORKeyStream(ct, ip)
+
+	for i := len(ip) - 1; i >= 8; i-- {
+		ct[i-8] ^= ct[i]
+	}
+
+	return ct[:8]
+}