func makePad(padLen int) ([]byte, error) {
pad := make([]byte, padLen)
if err := csrand.Bytes(pad); err != nil {
return nil, err
}
return pad, nil
}
// NewSeed returns a Seed initialized with the runtime CSPRNG.
func NewSeed() (seed *Seed, err error) {
seed = new(Seed)
if err = csrand.Bytes(seed.Bytes()[:]); err != nil {
return nil, err
}
return
}
func (conn *obfs3Conn) handshake() error {
// The party who opens the connection is the 'initiator'; the one who
// accepts it is the 'responder'. Each begins by generating a
// UniformDH keypair, and a random number PADLEN in [0, MAX_PADDING/2].
// Both parties then send:
//
// PUB_KEY | WR(PADLEN)
privateKey, err := uniformdh.GenerateKey(csrand.Reader)
if err != nil {
return err
}
padLen := csrand.IntRange(0, maxPadding/2)
blob := make([]byte, uniformdh.Size+padLen)
publicKey, err := privateKey.PublicKey.Bytes()
if err != nil {
return err
}
copy(blob[0:], publicKey)
if err := csrand.Bytes(blob[uniformdh.Size:]); err != nil {
return err
}
if _, err := conn.Conn.Write(blob); err != nil {
return err
}
// Read the public key from the peer.
rawPeerPublicKey := make([]byte, uniformdh.Size)
if _, err := io.ReadFull(conn.Conn, rawPeerPublicKey); err != nil {
return err
}
var peerPublicKey uniformdh.PublicKey
if err := peerPublicKey.SetBytes(rawPeerPublicKey); err != nil {
return err
}
// After retrieving the public key of the other end, each party
// completes the DH key exchange and generates a shared-secret for the
// session (named SHARED_SECRET).
sharedSecret, err := uniformdh.Handshake(privateKey, &peerPublicKey)
if err != nil {
return err
}
if err := conn.kdf(sharedSecret); err != nil {
return err
}
return nil
}
// New creates a new ReplayFilter instance.
func New(ttl time.Duration) (filter *ReplayFilter, err error) {
// Initialize the SipHash-2-4 instance with a random key.
var key [16]byte
if err = csrand.Bytes(key[:]); err != nil {
return
}
filter = new(ReplayFilter)
filter.filter = make(map[uint64]*entry)
filter.fifo = list.New()
filter.key[0] = binary.BigEndian.Uint64(key[0:8])
filter.key[1] = binary.BigEndian.Uint64(key[8:16])
filter.ttl = ttl
return
}
func (conn *obfs3Conn) Write(b []byte) (n int, err error) {
// If this is the first time we write data post handshake, send the
// padding/magic value.
if conn.txMagic != nil {
padLen := csrand.IntRange(0, maxPadding/2)
blob := make([]byte, padLen+len(conn.txMagic))
if err = csrand.Bytes(blob[:padLen]); err != nil {
conn.Close()
return
}
copy(blob[padLen:], conn.txMagic)
if _, err = conn.Conn.Write(blob); err != nil {
conn.Close()
return
}
conn.txMagic = nil
}
return conn.tx.Write(b)
}
// NewKeypair generates a new Curve25519 keypair, and optionally also generates
// an Elligator representative of the public key.
func NewKeypair(elligator bool) (*Keypair, error) {
keypair := new(Keypair)
keypair.private = new(PrivateKey)
keypair.public = new(PublicKey)
if elligator {
keypair.representative = new(Representative)
}
for {
// Generate a Curve25519 private key. Like everyone who does this,
// run the CSPRNG output through SHA256 for extra tinfoil hattery.
priv := keypair.private.Bytes()[:]
if err := csrand.Bytes(priv); err != nil {
return nil, err
}
digest := sha256.Sum256(priv)
digest[0] &= 248
digest[31] &= 127
digest[31] |= 64
copy(priv, digest[:])
if elligator {
// Apply the Elligator transform. This fails ~50% of the time.
if !extra25519.ScalarBaseMult(keypair.public.Bytes(),
keypair.representative.Bytes(),
keypair.private.Bytes()) {
continue
}
} else {
// Generate the corresponding Curve25519 public key.
curve25519.ScalarBaseMult(keypair.public.Bytes(),
keypair.private.Bytes())
}
return keypair, nil
}
}
func newJSONServerState(stateDir string, js *jsonServerState) (err error) {
// Generate everything a server needs, using the cryptographic PRNG.
var st obfs4ServerState
rawID := make([]byte, ntor.NodeIDLength)
if err = csrand.Bytes(rawID); err != nil {
return
}
if st.nodeID, err = ntor.NewNodeID(rawID); err != nil {
return
}
if st.identityKey, err = ntor.NewKeypair(false); err != nil {
return
}
if st.drbgSeed, err = drbg.NewSeed(); err != nil {
return
}
st.iatMode = iatNone
// Encode it into JSON format and write the state file.
js.NodeID = st.nodeID.Hex()
js.PrivateKey = st.identityKey.Private().Hex()
js.PublicKey = st.identityKey.Public().Hex()
js.DrbgSeed = st.drbgSeed.Hex()
js.IATMode = st.iatMode
var encoded []byte
if encoded, err = json.Marshal(js); err != nil {
return
}
if err = ioutil.WriteFile(path.Join(stateDir, stateFile), encoded, 0600); err != nil {
return err
}
return nil
}
func (conn *obfs2Conn) handshake() error {
// Each begins by generating a seed and a padding key as follows.
// The initiator generates:
//
// INIT_SEED = SR(SEED_LENGTH)
// INIT_PAD_KEY = MAC("Initiator obfuscation padding", INIT_SEED)[:KEYLEN]
//
// And the responder generates:
//
// RESP_SEED = SR(SEED_LENGTH)
// RESP_PAD_KEY = MAC("Responder obfuscation padding", INIT_SEED)[:KEYLEN]
//
// Each then generates a random number PADLEN in range from 0 through
// MAX_PADDING (inclusive).
var seed [seedLen]byte
if err := csrand.Bytes(seed[:]); err != nil {
return err
}
var padMagic []byte
if conn.isInitiator {
padMagic = []byte(initiatorPadString)
} else {
padMagic = []byte(responderPadString)
}
padKey, padIV := hsKdf(padMagic, seed[:], conn.isInitiator)
padLen := uint32(csrand.IntRange(0, maxPadding))
hsBlob := make([]byte, hsLen+padLen)
binary.BigEndian.PutUint32(hsBlob[0:4], magicValue)
binary.BigEndian.PutUint32(hsBlob[4:8], padLen)
if padLen > 0 {
if err := csrand.Bytes(hsBlob[8:]); err != nil {
return err
}
}
// The initiator then sends:
//
// INIT_SEED | E(INIT_PAD_KEY, UINT32(MAGIC_VALUE) | UINT32(PADLEN) | WR(PADLEN))
//
// and the responder sends:
//
// RESP_SEED | E(RESP_PAD_KEY, UINT32(MAGIC_VALUE) | UINT32(PADLEN) | WR(PADLEN))
txBlock, err := aes.NewCipher(padKey)
if err != nil {
return err
}
txStream := cipher.NewCTR(txBlock, padIV)
conn.tx = &cipher.StreamWriter{S: txStream, W: conn.Conn}
if _, err := conn.Conn.Write(seed[:]); err != nil {
return err
}
if _, err := conn.Write(hsBlob); err != nil {
return err
}
// Upon receiving the SEED from the other party, each party derives
// the other party's padding key value as above, and decrypts the next
// 8 bytes of the key establishment message.
var peerSeed [seedLen]byte
if _, err := io.ReadFull(conn.Conn, peerSeed[:]); err != nil {
return err
}
var peerPadMagic []byte
if conn.isInitiator {
peerPadMagic = []byte(responderPadString)
} else {
peerPadMagic = []byte(initiatorPadString)
}
peerKey, peerIV := hsKdf(peerPadMagic, peerSeed[:], !conn.isInitiator)
rxBlock, err := aes.NewCipher(peerKey)
if err != nil {
return err
}
rxStream := cipher.NewCTR(rxBlock, peerIV)
conn.rx = &cipher.StreamReader{S: rxStream, R: conn.Conn}
hsHdr := make([]byte, hsLen)
if _, err := io.ReadFull(conn, hsHdr[:]); err != nil {
return err
}
// If the MAGIC_VALUE does not match, or the PADLEN value is greater than
// MAX_PADDING, the party receiving it should close the connection
// immediately.
if peerMagic := binary.BigEndian.Uint32(hsHdr[0:4]); peerMagic != magicValue {
return fmt.Errorf("invalid magic value: %x", peerMagic)
}
padLen = binary.BigEndian.Uint32(hsHdr[4:8])
if padLen > maxPadding {
return fmt.Errorf("padlen too long: %d", padLen)
}
// Otherwise, it should read the remaining PADLEN bytes of padding data
// and discard them.
tmp := make([]byte, padLen)
if _, err := io.ReadFull(conn.Conn, tmp); err != nil { // Note: Skips AES.
return err
}
// Derive the actual keys.
if err := conn.kdf(seed[:], peerSeed[:]); err != nil {
return err
}
return nil
}
