func newServerHandshake(nodeID *ntor.NodeID, serverIdentity *ntor.Keypair, sessionKey *ntor.Keypair) *serverHandshake {
hs := new(serverHandshake)
hs.keypair = sessionKey
hs.nodeID = nodeID
hs.serverIdentity = serverIdentity
hs.padLen = csrand.IntRange(serverMinPadLength, serverMaxPadLength)
hs.mac = hmac.New(sha256.New, append(hs.serverIdentity.Public().Bytes()[:], hs.nodeID.Bytes()[:]...))
return hs
}
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
}
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)
}
func newTicketClientHandshake(mac hash.Hash, ticket *ssTicket) *ssTicketClientHandshake {
hs := &ssTicketClientHandshake{mac: mac, ticket: ticket}
hs.padLen = csrand.IntRange(ticketMinPadLength, ticketMaxPadLength)
return hs
}
// Decode decodes a stream of data and returns the length if any. ErrAgain is
// a temporary failure, all other errors MUST be treated as fatal and the
// session aborted.
func (decoder *Decoder) Decode(data []byte, frames *bytes.Buffer) (int, error) {
// A length of 0 indicates that we do not know how big the next frame is
// going to be.
if decoder.nextLength == 0 {
// Attempt to pull out the next frame length.
if lengthLength > frames.Len() {
return 0, ErrAgain
}
// Remove the length field from the buffer.
var obfsLen [lengthLength]byte
_, err := io.ReadFull(frames, obfsLen[:])
if err != nil {
return 0, err
}
// Derive the nonce the peer used.
if err = decoder.nonce.bytes(&decoder.nextNonce); err != nil {
return 0, err
}
// Deobfuscate the length field.
length := binary.BigEndian.Uint16(obfsLen[:])
lengthMask := decoder.drbg.NextBlock()
length ^= binary.BigEndian.Uint16(lengthMask)
if maxFrameLength < length || minFrameLength > length {
// Per "Plaintext Recovery Attacks Against SSH" by
// Martin R. Albrecht, Kenneth G. Paterson and Gaven J. Watson,
// there are a class of attacks againt protocols that use similar
// sorts of framing schemes.
//
// While obfs4 should not allow plaintext recovery (CBC mode is
// not used), attempt to mitigate out of bound frame length errors
// by pretending that the length was a random valid range as per
// the countermeasure suggested by Denis Bider in section 6 of the
// paper.
decoder.nextLengthInvalid = true
length = uint16(csrand.IntRange(minFrameLength, maxFrameLength))
}
decoder.nextLength = length
}
if int(decoder.nextLength) > frames.Len() {
return 0, ErrAgain
}
// Unseal the frame.
var box [maxFrameLength]byte
n, err := io.ReadFull(frames, box[:decoder.nextLength])
if err != nil {
return 0, err
}
out, ok := secretbox.Open(data[:0], box[:n], &decoder.nextNonce, &decoder.key)
if !ok || decoder.nextLengthInvalid {
// When a random length is used (on length error) the tag should always
// mismatch, but be paranoid.
return 0, ErrTagMismatch
}
// Clean up and prepare for the next frame.
decoder.nextLength = 0
decoder.nonce.counter++
return len(out), 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
}