BOLT 8specificationlightningkey-managementscriptp2p

BOLT 8: Encrypted and Authenticated Transport

All communications between Lightning nodes is encrypted in order to provide confidentiality for all transcripts between nodes and is authenticated in order to avoid malicious interference. Each node has a known long-term identifier that is a public key on Bitcoin's `secp256k1` curve. This long-term public key is used within the protocol to establish an encrypted and authenticated connection with p

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Specification

BOLT #8: Encrypted and Authenticated Transport

All communications between Lightning nodes is encrypted in order to provide confidentiality for all transcripts between nodes and is authenticated in order to avoid malicious interference. Each node has a known long-term identifier that is a public key on Bitcoin's secp256k1 curve. This long-term public key is used within the protocol to establish an encrypted and authenticated connection with peers, and also to authenticate any information advertised on behalf of a node.

Cryptographic Messaging Overview
Authenticated Key Exchange Handshake Specification
Lightning Message Specification
- Encrypting and Sending Messages
- Receiving and Decrypting Messages
Lightning Message Key Rotation
Security Considerations
Appendix A: Transport Test Vectors
Acknowledgments
References
Authors

Cryptographic Messaging Overview

Prior to sending any Lightning messages, nodes MUST first initiate the cryptographic session state that is used to encrypt and authenticate all messages sent between nodes. The initialization of this cryptographic session state is completely distinct from any inner protocol message header or conventions.

The transcript between two nodes is separated into two distinct segments:

Before any actual data transfer, both nodes participate in an authenticated key agreement handshake, which is based on the Noise Protocol Framework2.
If the initial handshake is successful, then nodes enter the Lightning message exchange phase. In the Lightning message exchange phase, all messages are Authenticated Encryption with Associated Data (AEAD) ciphertexts.

Authenticated Key Agreement Handshake

The handshake chosen for the authenticated key exchange is Noise_XK. As a pre-message, the initiator must know the identity public key of the responder. This provides a degree of identity hiding for the responder, as its static public key is never transmitted during the handshake. Instead, authentication is achieved implicitly via a series of Elliptic-Curve Diffie-Hellman (ECDH) operations followed by a MAC check.

The authenticated key agreement (Noise_XK) is performed in three distinct steps (acts). During each act of the handshake the following occurs: some (possibly encrypted) keying material is sent to the other party; an ECDH is performed, based on exactly which act is being executed, with the result mixed into the current set of encryption keys (ck the chaining key and k the encryption key); and an AEAD payload with a zero-length cipher text is sent. As this payload has no length, only a MAC is sent across. The mixing of ECDH outputs into a hash digest forms an incremental TripleDH handshake.

Using the language of the Noise Protocol, e and s (both public keys with e being the ephemeral key and s being the static key which in our case is usually the nodeid) indicate possibly encrypted keying material, and es, ee, and se each indicate an ECDH operation between two keys. The handshake is laid out as follows:

    Noise_XK(s, rs):
       <- s
       ...
       -> e, es
       <- e, ee
       -> s, se

All of the handshake data sent across the wire, including the keying material, is incrementally hashed into a session-wide "handshake digest", h. Note that the handshake state h is never transmitted during the handshake; instead, digest is used as the Associated Data within the zero-length AEAD messages.

Authenticating each message sent ensures that a man-in-the-middle (MITM) hasn't modified or replaced any of the data sent as part of a handshake, as the MAC check would fail on the other side if so.

A successful check of the MAC by the receiver indicates implicitly that all authentication has been successful up to that point. If a MAC check ever fails during the handshake process, then the connection is to be immediately terminated.

Handshake Versioning

Each message sent during the initial handshake starts with a single leading byte, which indicates the version used for the current handshake. A version of 0 indicates that no change is necessary, while a non-zero version indicate that the client has deviated from the protocol originally specified within this document.

Clients MUST reject handshake attempts initiated with an unknown version.

Noise Protocol Instantiation

Concrete instantiations of the Noise Protocol require the definition of three abstract cryptographic objects: the hash function, the elliptic curve, and the AEAD cipher scheme. For Lightning, SHA-256 is chosen as the hash function, secp256k1 as the elliptic curve, and ChaChaPoly-1305 as the AEAD construction.

The composition of ChaCha20 and Poly1305 that are used MUST conform to RFC 84391.

The official protocol name for the Lightning variant of Noise is Noise_XK_secp256k1_ChaChaPoly_SHA256. The ASCII string representation of this value is hashed into a digest used to initialize the starting handshake state. If the protocol names of two endpoints differ, then the handshake process fails immediately.

Authenticated Key Exchange Handshake Specification

The handshake proceeds in three acts, taking 1.5 round trips. Each handshake is a fixed sized payload without any header or additional meta-data attached. The exact size of each act is as follows:

Act One: 50 bytes
Act Two: 50 bytes
Act Three: 66 bytes

Handshake State

Throughout the handshake process, each side maintains these variables:

ck: the chaining key. This value is the accumulated hash of all previous ECDH outputs. At the end of the handshake, ck is used to derive the encryption keys for Lightning messages.
h: the handshake hash. This value is the accumulated hash of all handshake data that has been sent and received so far during the handshake process.
temp_k1, temp_k2, temp_k3: the intermediate keys. These are used to encrypt and decrypt the zero-length AEAD payloads at the end of each handshake message.
e: a party's ephemeral keypair. For each session, a node MUST generate a new ephemeral key with strong cryptographic randomness.
s: a party's static keypair (ls for local, rs for remote)

The following functions will also be referenced:

ECDH(k, rk): performs an Elliptic-Curve Diffie-Hellman operation using k, which is a valid secp256k1 private key, and rk, which is a valid public key
- The returned value is the SHA256 of the compressed format of the generated point.
HKDF(salt,ikm): a function defined in RFC 58693, evaluated with a zero-length info field
- All invocations of HKDF implicitly return 64 bytes of cryptographic randomness using the extract-and-expand component of the HKDF.
encryptWithAD(k, n, ad, plaintext): outputs encrypt(k, n, ad, plaintext)
- Where encrypt is an evaluation of ChaCha20-Poly1305 (IETF variant) with the passed arguments, with nonce n encoded as 32 zero bits, followed by a little-endian 64-bit value. Note: this follows the Noise Protocol convention, rather than our normal endian.
decryptWithAD(k, n, ad, ciphertext): outputs decrypt(k, n, ad, ciphertext)
- Where decrypt is an evaluation of ChaCha20-Poly1305 (IETF variant) with the passed arguments, with nonce n encoded as 32 zero bits, followed by a little-endian 64-bit value.
generateKey(): generates and returns a fresh secp256k1 keypair
- Where the object returned by generateKey has two attributes:
  - .pub, which returns an abstract object representing the public key
  - .priv, which represents the private key used to generate the public key
- Where the object also has a single method:
  - .serializeCompressed()
a || b denotes the concatenation of two byte strings a and b

Handshake State Initialization

Before the start of Act One, both sides initialize their per-sessions state as follows:

h = SHA-256(protocolName)
- where protocolName = "Noise_XK_secp256k1_ChaChaPoly_SHA256" encoded as an ASCII string
ck = h
h = SHA-256(h || prologue)
- where prologue is the ASCII string: lightning

As a concluding step, both sides mix the responder's public key into the handshake digest:

The initiating node mixes in the responding node's static public key serialized in Bitcoin's compressed format:
- h = SHA-256(h || rs.pub.serializeCompressed())
The responding node mixes in their local static public key serialized in Bitcoin's compressed format:
- h = SHA-256(h || ls.pub.serializeCompressed())

Handshake Exchange

Act One

    -> e, es

Act One is sent from initiator to responder. During Act One, the initiator attempts to satisfy an implicit challenge by the responder. To complete this challenge, the initiator must know the static public key of the responder.

The handshake message is exactly 50 bytes: 1 byte for the handshake version, 33 bytes for the compressed ephemeral public key of the initiator, and 16 bytes for the poly1305 tag.

Sender Actions:

e = generateKey()
h = SHA-256(h || e.pub.serializeCompressed())
- The newly generated ephemeral key is accumulated into the running handshake digest.
es = ECDH(e.priv, rs)
- The initiator performs an ECDH between its newly generated ephemeral key and the remote node's static public key.
ck, temp_k1 = HKDF(ck, es)
- A new temporary encryption key is generated, which is used to generate the authenticating MAC.
c = encryptWithAD(temp_k1, 0, h, zero)
- where zero is a zero-length plaintext
h = SHA-256(h || c)
- Finally, the generated ciphertext is accumulated into the authenticating handshake digest.
Send m = 0 || e.pub.serializeCompressed() || c to the responder over the network buffer.

Receiver Actions:

Read exactly 50 bytes from the network buffer.
Parse the read message (m) into v, re, and c:
- where v is the first byte of m, re is the next 33 bytes of m, and c is the last 16 bytes of m
- The raw bytes of the remote party's ephemeral public key (re) are to be deserialized into a point on the curve using affine coordinates as encoded by the key's serialized composed format.
If v is an unrecognized handshake version, then the responder MUST abort the connection attempt.
h = SHA-256(h || re.serializeCompressed())
- The responder accumulates the initiator's ephemeral key into the authenticating handshake digest.
es = ECDH(s.priv, re)
- The responder performs an ECDH between its static private key and the initiator's ephemeral public key.
ck, temp_k1 = HKDF(ck, es)
- A new temporary encryption key is generated, which will shortly be used to check the authenticating MAC.
p = decryptWithAD(temp_k1, 0, h, c)
- If the MAC check in this operation fails, then the initiator does not know the responder's static public key. If this is the case, then the responder MUST terminate the connection without any further messages.
h = SHA-256(h || c)
- The received ciphertext is mixed into the handshake digest. This step serves to ensure the payload wasn't modified by a MITM.

Act Two

   <- e, ee

Act Two is sent from the responder to the initiator. Act Two will only take place if Act One was successful. Act One was successful if the responder was able to properly decrypt and check the MAC of the tag sent at the end of Act One.

The handshake is exactly 50 bytes: 1 byte for the handshake version, 33 bytes for the compressed ephemeral public key of the responder, and 16 bytes for the poly1305 tag.