package brontide import ( "crypto/cipher" "crypto/sha256" "encoding/binary" "errors" "fmt" "io" "math" "golang.org/x/crypto/hkdf" "github.com/aead/chacha20" "github.com/btcsuite/fastsha256" "github.com/roasbeef/btcd/btcec" ) const ( // protocolName is the precise instantiation of the Noise protocol // handshake at the center of Brontide. This value will be used as part // of the prologue. If the initiator and responder aren't using the // exact same string for this value, along with prologue of the Bitcoin // network, then the initial handshake will fail. protocolName = "Noise_XK_secp256k1_ChaChaPoly_SHA256" // macSize is the length in bytes of the tags generated by poly1305. macSize = 16 // lengthHeaderSize is the number of bytes used to prefix encode the // length of a message payload. lengthHeaderSize = 2 // keyRotationInterval is the number of messages sent on a single // cipher stream before the keys are rotated forwards. keyRotationInterval = 1000 ) var ( ErrMaxMessageLengthExceeded = errors.New("the generated payload exceeds " + "the max allowed message length of (2^16)-1") ) // TODO(roasbeef): free buffer pool? // ecdh performs an ECDH operation between pub and priv. The returned value is // the sha256 of the compressed shared point. func ecdh(pub *btcec.PublicKey, priv *btcec.PrivateKey) []byte { s := &btcec.PublicKey{} x, y := btcec.S256().ScalarMult(pub.X, pub.Y, priv.D.Bytes()) s.X = x s.Y = y h := fastsha256.Sum256(s.SerializeCompressed()) return h[:] } // cipherState encapsulates the state for the AEAD which will be used to // encrypt+authenticate any payloads sent during the handshake, and messages // sent once the handshake has completed. type cipherState struct { // nonce is the nonce passed into the chacha20-poly1305 instance for // encryption+decryption. The nonce is incremented after each successful // encryption/decryption. // // TODO(roasbeef): this should actually be 96 bit nonce uint64 // secretKey is the shared symmetric key which will be used to // instantiate the cipher. // // TODO(roasbeef): m-lock?? secretKey [32]byte // salt is an additional secret which is used during key rotation to // generate new keys. salt [32]byte // cipher is an instance of the ChaCha20-Poly1305 AEAD construction // created using the secretKey above. cipher cipher.AEAD } // Encrypt returns a ciphertext which is the encryption of the plainText // observing the passed associatedData within the AEAD construction. func (c *cipherState) Encrypt(associatedData, cipherText, plainText []byte) []byte { defer func() { c.nonce++ if c.nonce == keyRotationInterval { c.rotateKey() } }() var nonce [12]byte binary.LittleEndian.PutUint64(nonce[4:], c.nonce) return c.cipher.Seal(cipherText, nonce[:], plainText, associatedData) } // Decrypt attempts to decrypt the passed ciphertext observing the specified // associatedData within the AEAD construction. In the case that the final MAC // check fails, then a non-nil error will be returned. func (c *cipherState) Decrypt(associatedData, plainText, cipherText []byte) ([]byte, error) { defer func() { c.nonce++ if c.nonce == keyRotationInterval { c.rotateKey() } }() var nonce [12]byte binary.LittleEndian.PutUint64(nonce[4:], c.nonce) return c.cipher.Open(plainText, nonce[:], cipherText, associatedData) } // InitializeKey initializes the secret key and AEAD cipher scheme based off of // the passed key. func (c *cipherState) InitializeKey(key [32]byte) { c.secretKey = key c.nonce = 0 c.cipher = chacha20.NewChaCha20Poly1305(&c.secretKey) } // InitializeKeyWithSalt is identical to InitializeKey however it also sets the // cipherState's salt field which is used for key rotation. func (c *cipherState) InitializeKeyWithSalt(salt, key [32]byte) { c.salt = salt c.InitializeKey(key) } // rotateKey rotates the current encryption/decryption key for this cipherState // instance. Key rotation is performed by ratcheting the current key forward // using an HKDF invocation with the cipherState's salt as the salt, and the // current key as the input. func (c *cipherState) rotateKey() { var ( info []byte nextKey [32]byte ) oldKey := c.secretKey h := hkdf.New(sha256.New, oldKey[:], c.salt[:], info) // hkdf(ck, k, zero) // | // | \ // | \ // ck k' h.Read(c.salt[:]) h.Read(nextKey[:]) c.InitializeKey(nextKey) } // symmetricState encapsulates a cipherState object and houses the ephemeral // handshake digest state. This struct is used during the handshake to derive // new shared secrets based off of the result of ECDH operations. Ultimately, // the final key yielded by this struct is the result of an incremental // Triple-DH operation. type symmetricState struct { cipherState // chainingKey is used as the salt to the HKDF function to derive a new // chaining key as well as a new tempKey which is used for // encryption/decryption. chainingKey [32]byte // tempKey is the latter 32 bytes resulted from the latest HKDF // iteration. This key is used to encrypt/decrypt any handshake // messages or payloads sent until the next DH operation is executed. tempKey [32]byte // handshakeDigest is the cumulative hash digest of all handshake // messages sent from start to finish. This value is never transmitted // to the other side, but will be used as the AD when // encrypting/decrypting messages using our AEAD construction. handshakeDigest [32]byte } // mixKey is implements a basic HKDF-based key ratchet. This method is called // with the result of each DH output generated during the handshake process. // The first 32 bytes extract from the HKDF reader is the next chaining key, // then latter 32 bytes become the temp secret key using within any future AEAD // operations until another DH operation is performed. func (s *symmetricState) mixKey(input []byte) { var info []byte secret := input salt := s.chainingKey h := hkdf.New(sha256.New, secret, salt[:], info) // hkdf(ck, input, zero) // | // | \ // | \ // ck k h.Read(s.chainingKey[:]) h.Read(s.tempKey[:]) // cipher.k = temp_key s.InitializeKey(s.tempKey) } // mixHash hashes the passed input data into the cumulative handshake digest. // The running result of this value (h) is used as the associated data in all // decryption/encryption operations. func (s *symmetricState) mixHash(data []byte) { h := sha256.New() h.Write(s.handshakeDigest[:]) h.Write(data) copy(s.handshakeDigest[:], h.Sum(nil)) } // EncryptAndHash returns the authenticated encryption of the passed plaintext. // When encrypting the handshake digest (h) is used as the associated data to // the AEAD cipher. func (s *symmetricState) EncryptAndHash(plaintext []byte) []byte { ciphertext := s.Encrypt(s.handshakeDigest[:], nil, plaintext) s.mixHash(ciphertext) return ciphertext } // DecryptAndHash returns the authenticated decryption of the passed // ciphertext. When encrypting the handshake digest (h) is used as the // associated data to the AEAD cipher. func (s *symmetricState) DecryptAndHash(ciphertext []byte) ([]byte, error) { plaintext, err := s.Decrypt(s.handshakeDigest[:], nil, ciphertext) if err != nil { return nil, err } s.mixHash(ciphertext) return plaintext, nil } // InitializeSymmetric initializes the symmetric state by setting the handshake // digest (h) and the chaining key (ck) to protocol name. func (s *symmetricState) InitializeSymmetric(protocolName []byte) { var empty [32]byte s.handshakeDigest = sha256.Sum256(protocolName) s.chainingKey = s.handshakeDigest s.InitializeKey(empty) } // handshakeState encapsulates the symmetricState and keeps track of all the // public keys (static and ephemeral) for both sides during the handshake // transcript. If the handshake completes successfully, then two instances of a // cipherState are emitted: one to encrypt messages from initiator to // responder, and the other for the opposite direction. type handshakeState struct { symmetricState initiator bool localStatic *btcec.PrivateKey localEphemeral *btcec.PrivateKey remoteStatic *btcec.PublicKey remoteEphemeral *btcec.PublicKey } // newHandshakeState returns a new instance of the handshake state initialized // with the prologue and protocol name. If this is the responder's handshake // state, then the remotePub can be nil. func newHandshakeState(initiator bool, prologue []byte, localPub *btcec.PrivateKey, remotePub *btcec.PublicKey) handshakeState { h := handshakeState{ initiator: initiator, localStatic: localPub, remoteStatic: remotePub, } // Set the current chaining key and handshake digest to the hash of the // protocol name, and additionally mix in the prologue. If either sides // disagree about the prologue or protocol name, then the handshake // will fail. h.InitializeSymmetric([]byte(protocolName)) h.mixHash(prologue) // In Noise_XK, then initiator should know the responder's static // public key, therefore we include the responder's static key in the // handshake digest. If the initiator gets this value wrong, then the // handshake will fail. if initiator { h.mixHash(remotePub.SerializeCompressed()) } else { h.mixHash(localPub.PubKey().SerializeCompressed()) } return h } // BrontideMachine is a state-machine which implements Brontide: an // Authenticated-key Exchange in Three Acts. Brontide is derived from the Noise // framework, specifically implementing the Noise_XK handshake. Once the // initial 3-act handshake has completed all messages are encrypted with a // chacha20 AEAD cipher. On the wire, all messages are prefixed with an // authenticated+encrypted length field. Additionally, the encrypted+auth'd // length prefix is used as the AD when encrypting+decryption messages. This // construction provides confidentiality of packet length, avoids introducing // a padding-oracle, and binds the encrypted packet length to the packet // itself. // // The acts proceeds the following order (initiator on the left): // GenActOne() -> // RecvActOne() // <- GenActTwo() // RecvActTwo() // GenActThree() -> // RecvActThree() // // This exchange corresponds to the following Noise handshake: // <- s // ... // -> e, es // <- e, ee // -> s, se type BrontideMachine struct { sendCipher cipherState recvCipher cipherState handshakeState } // NewBrontideMachine creates a new instance of the brontide state-machine. If // the responder (listener) is creating the object, then the remotePub should // be nil. The handshake state within brontide is initialized using the ascii // string "bitcoin" as the prologue. func NewBrontideMachine(initiator bool, localPub *btcec.PrivateKey, remotePub *btcec.PublicKey) *BrontideMachine { handshake := newHandshakeState(initiator, []byte("lightning"), localPub, remotePub) return &BrontideMachine{handshakeState: handshake} } const ( // HandshakeVersion is the expected version of the brontide handshake. // Any messages that carry a different version will cause the handshake // to abort immediately. HandshakeVersion = byte(0) // ActOneSize is the size of the packet sent from initiator to // responder in ActOne. The packet consists of a handshake version, an // ephemeral key in compressed format, and a 16-byte poly1305 tag. // // 1 + 33 + 16 ActOneSize = 50 // ActTwoSize is the size the packet sent from responder to initiator // in ActTwo. The packet consists of a handshake version, an ephemeral // key in compressed format and a 16-byte poly1305 tag. // // 1 + 33 + 16 ActTwoSize = 50 // ActThreeSize is the size of the packet sent from initiator to // responder in ActThree. The packet consists of a handshake version, // the initiators static key encrypted with strong forward secrecy and // a 16-byte poly1035 // tag. // // 1 + 33 + 16 + 16 ActThreeSize = 66 ) // GenActOne generates the initial packet (act one) to be sent from initiator // to responder. During act one the initiator generates a fresh ephemeral key, // hashes it into the handshake digest, and performs an ECDH between this key // and the responder's static key. Future payloads are encrypted with a key // derived from this result. // // -> e, es func (b *BrontideMachine) GenActOne() ([ActOneSize]byte, error) { var ( err error actOne [ActOneSize]byte ) // e b.localEphemeral, err = btcec.NewPrivateKey(btcec.S256()) if err != nil { return actOne, err } ephemeral := b.localEphemeral.PubKey().SerializeCompressed() b.mixHash(ephemeral) // es s := ecdh(b.remoteStatic, b.localEphemeral) b.mixKey(s[:]) authPayload := b.EncryptAndHash([]byte{}) actOne[0] = HandshakeVersion copy(actOne[1:34], ephemeral) copy(actOne[34:], authPayload) return actOne, nil } // RecvActOne processes the act one packet sent by the initiator. The responder // executes the mirrored actions to that of the initiator extending the // handshake digest and deriving a new shared secret based on a ECDH with the // initiator's ephemeral key and responder's static key. func (b *BrontideMachine) RecvActOne(actOne [ActOneSize]byte) error { var ( err error e [33]byte p [16]byte ) // If the handshake version is unknown, then the handshake fails // immediately. if actOne[0] != HandshakeVersion { return fmt.Errorf("Invalid handshake version: %v, only %v is "+ "valid", actOne[0], HandshakeVersion) } copy(e[:], actOne[1:34]) copy(p[:], actOne[34:]) // e b.remoteEphemeral, err = btcec.ParsePubKey(e[:], btcec.S256()) if err != nil { return err } b.mixHash(b.remoteEphemeral.SerializeCompressed()) // es s := ecdh(b.remoteEphemeral, b.localStatic) b.mixKey(s) // If the initiator doesn't know our static key, then this operation // will fail. if _, err := b.DecryptAndHash(p[:]); err != nil { return err } return nil } // GenActTwo generates the second packet (act two) to be sent from the // responder to the initiator. The packet for act two is identify to that of // act one, but then results in a different ECDH operation between the // initiator's and responder's ephemeral keys. // // <- e, ee func (b *BrontideMachine) GenActTwo() ([ActTwoSize]byte, error) { var ( err error actTwo [ActTwoSize]byte ) // e b.localEphemeral, err = btcec.NewPrivateKey(btcec.S256()) if err != nil { return actTwo, err } ephemeral := b.localEphemeral.PubKey().SerializeCompressed() b.mixHash(b.localEphemeral.PubKey().SerializeCompressed()) // ee s := ecdh(b.remoteEphemeral, b.localEphemeral) b.mixKey(s) authPayload := b.EncryptAndHash([]byte{}) actTwo[0] = HandshakeVersion copy(actTwo[1:34], ephemeral) copy(actTwo[34:], authPayload) return actTwo, nil } // RecvActTwo processes the second packet (act two) sent from the responder to // the initiator. A successful processing of this packet authenticates the // initiator to the responder. func (b *BrontideMachine) RecvActTwo(actTwo [ActTwoSize]byte) error { var ( err error e [33]byte p [16]byte ) // If the handshake version is unknown, then the handshake fails // immediately. if actTwo[0] != HandshakeVersion { return fmt.Errorf("Invalid handshake version: %v, only %v is "+ "valid", actTwo[0], HandshakeVersion) } copy(e[:], actTwo[1:34]) copy(p[:], actTwo[34:]) // e b.remoteEphemeral, err = btcec.ParsePubKey(e[:], btcec.S256()) if err != nil { return err } b.mixHash(b.remoteEphemeral.SerializeCompressed()) // ee s := ecdh(b.remoteEphemeral, b.localEphemeral) b.mixKey(s) if _, err := b.DecryptAndHash(p[:]); err != nil { return err } return nil } // GenActThree creates the final (act three) packet of the handshake. Act three // is to be sent from the initiator to the responder. The purpose of act three // is to transmit the initiator's public key under strong forward secrecy to // the responder. This act also includes the final ECDH operation which yields // the final session. // // -> s, se func (b *BrontideMachine) GenActThree() ([ActThreeSize]byte, error) { var actThree [ActThreeSize]byte ourPubkey := b.localStatic.PubKey().SerializeCompressed() ciphertext := b.EncryptAndHash(ourPubkey) s := ecdh(b.remoteEphemeral, b.localStatic) b.mixKey(s) authPayload := b.EncryptAndHash([]byte{}) actThree[0] = HandshakeVersion copy(actThree[1:50], ciphertext) copy(actThree[50:], authPayload) // With the final ECDH operation complete, derive the session sending // and receiving keys. b.split() return actThree, nil } // RecvActThree processes the final act (act three) sent from the initiator to // the responder. After processing this act, the responder learns of the // initiator's static public key. Decryption of the static key serves to // authenticate the initiator to the responder. func (b *BrontideMachine) RecvActThree(actThree [ActThreeSize]byte) error { var ( err error s [33 + 16]byte p [16]byte ) // If the handshake version is unknown, then the handshake fails // immediately. if actThree[0] != HandshakeVersion { return fmt.Errorf("Invalid handshake version: %v, only %v is "+ "valid", actThree[0], HandshakeVersion) } copy(s[:], actThree[1:33+16+1]) copy(p[:], actThree[33+16+1:]) // s remotePub, err := b.DecryptAndHash(s[:]) if err != nil { return err } b.remoteStatic, err = btcec.ParsePubKey(remotePub, btcec.S256()) if err != nil { return err } // se se := ecdh(b.remoteStatic, b.localEphemeral) b.mixKey(se) if _, err := b.DecryptAndHash(p[:]); err != nil { return err } // With the final ECDH operation complete, derive the session sending // and receiving keys. b.split() return nil } // split is the final wrap-up act to be executed at the end of a successful // three act handshake. This function creates to internal cipherState // instances: one which is used to encrypt messages from the initiator to the // responder, and another which is used to encrypt message for the opposite // direction. func (b *BrontideMachine) split() { var ( empty []byte sendKey [32]byte recvKey [32]byte ) h := hkdf.New(sha256.New, empty, b.chainingKey[:], empty) // If we're the initiator the first 32 bytes are used to encrypt our // messages and the second 32-bytes to decrypt their messages. For the // responder the opposite is true. if b.initiator { h.Read(sendKey[:]) b.sendCipher = cipherState{} b.sendCipher.InitializeKeyWithSalt(b.chainingKey, sendKey) h.Read(recvKey[:]) b.recvCipher = cipherState{} b.recvCipher.InitializeKeyWithSalt(b.chainingKey, recvKey) } else { h.Read(recvKey[:]) b.recvCipher = cipherState{} b.recvCipher.InitializeKeyWithSalt(b.chainingKey, recvKey) h.Read(sendKey[:]) b.sendCipher = cipherState{} b.sendCipher.InitializeKeyWithSalt(b.chainingKey, sendKey) } } // WriteMessage writes the next message p to the passed io.Writer. The // ciphertext of the message is pre-pended with an encrypt+auth'd length which // must be used as the AD to the AEAD construction when being decrypted by the // other side. func (b *BrontideMachine) WriteMessage(w io.Writer, p []byte) error { // The total length of each message payload including the MAC size // payload exceed the largest number encodable within a 16-bit unsigned // integer. if len(p) > math.MaxUint16 { return ErrMaxMessageLengthExceeded } // The full length of the packet is only the packet length, and does // NOT include the MAC. fullLength := uint16(len(p)) var pktLen [2]byte binary.BigEndian.PutUint16(pktLen[:], fullLength) // First, write out the encrypted+MAC'd length prefix for the packet. cipherLen := b.sendCipher.Encrypt(nil, nil, pktLen[:]) if _, err := w.Write(cipherLen); err != nil { return err } // Finally, write out the encrypted packet itself. We only write out a // single packet, as any fragmentation should have taken place at a // higher level. cipherText := b.sendCipher.Encrypt(nil, nil, p) if _, err := w.Write(cipherText); err != nil { return err } return nil } // ReadMessage attempts to read the next message from the passed io.Reader. In // the case of an authentication error, a non-nil error is returned. func (b *BrontideMachine) ReadMessage(r io.Reader) ([]byte, error) { var cipherLen [lengthHeaderSize + macSize]byte if _, err := io.ReadFull(r, cipherLen[:]); err != nil { return nil, err } // Attempt to decrypt+auth the packet length present in the stream. pktLenBytes, err := b.recvCipher.Decrypt(nil, nil, cipherLen[:]) if err != nil { return nil, err } // Next, using the length read from the packet header, read the // encrypted packet itself. pktLen := uint32(binary.BigEndian.Uint16(pktLenBytes)) + macSize cipherText := make([]byte, pktLen) if _, err := io.ReadFull(r, cipherText[:]); err != nil { return nil, err } return b.recvCipher.Decrypt(nil, nil, cipherText) }