Remove psudorandom padding

cmd/ck-client/ck-client.go

@@ -100,7 +100,7 @@ func makeSession(sta *client.State) *mux.Session {
 
 	sta.UpdateIntervalKeys()
 
-	_, tthKey, _ := sta.GetIntervalKeys()
+	_, tthKey := sta.GetIntervalKeys()
 	sesh := mux.MakeSession(sta.SessionID, mux.UNLIMITED_VALVE, mux.MakeObfs(tthKey, sta.Cipher), mux.MakeDeobfs(tthKey, sta.Cipher), util.ReadTLS)
 
 	var wg sync.WaitGroup

internal/client/auth.go

@@ -29,34 +29,28 @@ func MakeRandomField(sta *State) []byte {
 	return ret
 }
 
+const SESSION_TICKET_LEN = 192
+const PUB_KEY_LEN = 32
+const AUTH_TAG_LEN = 16
+const STEGANO_LEN = SESSION_TICKET_LEN - PUB_KEY_LEN - AUTH_TAG_LEN
+
 func MakeSessionTicket(sta *State) []byte {
-	// sessionTicket: [marshalled ephemeral pub key 32 bytes][encrypted UID 16 bytes, proxy method 16 bytes, encryption method 1 byte][16 bytes authentication tag][padding 111 bytes]
+	// sessionTicket: [marshalled ephemeral pub key 32 bytes][encrypted UID 16 bytes, proxy method 16 bytes, encryption method 1 byte][reserved 111 bytes][16 bytes authentication tag]
 	// The first 12 bytes of the marshalled ephemeral public key is used as the nonce
 	// for encrypting the UID
 
-	ticket := make([]byte, 192)
+	ticket := make([]byte, SESSION_TICKET_LEN)
 
 	//TODO: error when the interval has expired
-	ephPub, intervalKey, seed := sta.GetIntervalKeys()
+	ephPub, intervalKey := sta.GetIntervalKeys()
-	copy(ticket[0:32], ecdh.Marshal(ephPub))
+	copy(ticket[0:PUB_KEY_LEN], ecdh.Marshal(ephPub))
 
-	plain := make([]byte, 33)
+	plain := make([]byte, STEGANO_LEN)
 	copy(plain, sta.UID)
-	copy(plain[16:32], []byte(sta.ProxyMethod))
+	copy(plain[16:32], sta.ProxyMethod)
 	plain[32] = sta.EncryptionMethod
 
 	cipher, _ := util.AESGCMEncrypt(ticket[0:12], intervalKey, plain)
-	copy(ticket[32:81], cipher)
+	copy(ticket[PUB_KEY_LEN:], cipher)
-	// The purpose of adding sessionID is that, the generated padding of sessionTicket needs to be unpredictable.
-	// As shown in auth.go, the padding is generated by a psudo random generator. The seed
-	// needs to be the same for each TicketTimeHint interval. However the value of epoch/TicketTimeHint
-	// is public knowledge, so is the psudo random algorithm used by math/rand. Therefore not only
-	// can the firewall tell that the padding is generated in this specific way, this padding is identical
-	// for all ckclients in the same TicketTimeHint interval. This will expose us.
-	//
-	// With the sessionID value generated at startup of ckclient and used as a part of the seed, the
-	// sessionTicket is still identical for each TicketTimeHint interval, but others won't be able to know
-	// how it was generated. It will also be different for each client.
-	copy(ticket[81:192], util.PsudoRandBytes(111, seed))
 	return ticket
 }

internal/client/state.go

@@ -4,7 +4,6 @@ import (
 	"crypto"
 	"crypto/rand"
 	"encoding/base64"
-	"encoding/binary"
 	"encoding/json"
 	"errors"
 	"io/ioutil"
@@ -32,7 +31,6 @@ type tthIntervalKeys struct {
 	ephPv       crypto.PrivateKey
 	ephPub      crypto.PublicKey
 	intervalKey []byte
-	seed        int64
 }
 
 // State stores global variables
@@ -82,14 +80,13 @@ func (sta *State) UpdateIntervalKeys() {
 	sta.intervalData.interval = currentInterval
 	ephPv, ephPub, _ := ecdh.GenerateKey(rand.Reader)
 	intervalKey := ecdh.GenerateSharedSecret(ephPv, sta.staticPub)
-	seed := int64(binary.BigEndian.Uint64(ephPv.(*[32]byte)[0:8]))
+	sta.intervalData.ephPv, sta.intervalData.ephPub, sta.intervalData.intervalKey = ephPv, ephPub, intervalKey
-	sta.intervalData.ephPv, sta.intervalData.ephPub, sta.intervalData.intervalKey, sta.intervalData.seed = ephPv, ephPub, intervalKey, seed
 }
 
-func (sta *State) GetIntervalKeys() (crypto.PublicKey, []byte, int64) {
+func (sta *State) GetIntervalKeys() (crypto.PublicKey, []byte) {
 	sta.intervalDataM.Lock()
 	defer sta.intervalDataM.Unlock()
-	return sta.intervalData.ephPub, sta.intervalData.intervalKey, sta.intervalData.seed
+	return sta.intervalData.ephPub, sta.intervalData.intervalKey
 }
 
 // semi-colon separated value. This is for Android plugin options

internal/server/auth.go

@@ -10,11 +10,16 @@ import (
 	"log"
 )
 
+const SESSION_TICKET_LEN = 192
+const PUB_KEY_LEN = 32
+const AUTH_TAG_LEN = 16
+const USED_STAGNO_LEN = 16 + 16 + 1
+
 func decryptSessionTicket(staticPv crypto.PrivateKey, ticket []byte) (UID []byte, proxyMethod string, encryptionMethod byte, tthKey []byte) {
-	// sessionTicket: [marshalled ephemeral pub key 32 bytes][encrypted UID 16 bytes, proxy method 16 bytes, encryption method 1 byte][16 bytes authentication tag][padding 111 bytes]
+	// sessionTicket: [marshalled ephemeral pub key 32 bytes][encrypted UID 16 bytes, proxy method 16 bytes, encryption method 1 byte][reserved 111 bytes][padding 111 bytes]
-	ephPub, _ := ecdh.Unmarshal(ticket[0:32])
+	ephPub, _ := ecdh.Unmarshal(ticket[0:PUB_KEY_LEN])
 	tthKey = ecdh.GenerateSharedSecret(staticPv, ephPub)
-	plain, err := util.AESGCMDecrypt(ticket[0:12], tthKey, ticket[32:81])
+	plain, err := util.AESGCMDecrypt(ticket[0:12], tthKey, ticket[PUB_KEY_LEN:])
 	if err != nil {
 		return
 	}
@@ -51,7 +56,7 @@ func TouchStone(ch *ClientHello, sta *State) (isCK bool, UID []byte, sessionID u
 
 	ticket := ch.extensions[[2]byte{0x00, 0x23}]
 
-	if len(ticket) < 81 {
+	if len(ticket) < PUB_KEY_LEN+USED_STAGNO_LEN+AUTH_TAG_LEN {
 		return
 	}