lnpbp-0015

LNPBP: 0015
Vertical: Lightning network protocol
Title: LNP handshake and encryption in network communications based on Noise_XK (BOLT-8 extract)
Author: BOLT-8 and Noise_XK protocol authors
Comments-URI: n/a
Status: Proposal
Type: Standards Track
Created: 2020-12-05
License: CC0-1.0
​Abstract​
​Background​
​Motivation​
​Design​
​Authenticated Key Agreement Handshake​
​Handshake Versioning​
​Noise Protocol Instantiation​
​Specification​
​Handshake State​
​Handshake State Initialization​
​Handshake Exchange​
​Act One​
​Act Two​
​Act Three​
​Compatibility​
​Rationale​
​Reference implementation​
​Acknowledgements​
​References​
​Copyright​
​Test vectors​
Abstract
Background
Motivation
Design
Prior to sending any 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.
Before any actual data transfer, both nodes participate in an authenticated key agreement handshake, which is based on the Noise Protocol Framework [2].
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) 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 75391.
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.
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 private key, and rk, which is a secp256k1 public key within the finite field, as defined by the curve parameters
The returned value is the SHA256 of the DER-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:
1.h = SHA-256(protocolName)
where protocolName = "Noise_XK_secp256k1_ChaChaPoly_SHA256" encoded as an ASCII string
2.ck = h
3.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 DER-compressed format:
h = SHA-256(h || rs.pub.serializeCompressed())
The responding node mixes in their local static public key serialized in Bitcoin's DER-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:
1.e = generateKey()
2.h = SHA-256(h || e.pub.serializeCompressed())
The newly generated ephemeral key is accumulated into the running handshake digest.
3.es = ECDH(e.priv, rs)
The initiator performs an ECDH between its newly generated ephemeral key and the remote node's static public key.
4.ck, temp_k1 = HKDF(ck, es)
A new temporary encryption key is generated, which is used to generate the authenticating MAC.
5.c = encryptWithAD(temp_k1, 0, h, zero)
where zero is a zero-length plaintext
6.h = SHA-256(h || c)
Finally, the generated ciphertext is accumulated into the authenticating handshake digest.
7.Send m = 0 || e.pub.serializeCompressed() || c to the responder over the network buffer.
Receiver Actions:
1.Read exactly 50 bytes from the network buffer.
2.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.
3.If v is an unrecognized handshake version, then the responder MUST abort the connection attempt.
4.h = SHA-256(h || re.serializeCompressed())
The responder accumulates the initiator's ephemeral key into the authenticating handshake digest.
5.es = ECDH(s.priv, re)
The responder performs an ECDH between its static private key and the initiator's ephemeral public key.
6.ck, temp_k1 = HKDF(ck, es)
A new temporary encryption key is generated, which will shortly be used to check the authenticating MAC.
7.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.
8.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.
Sender Actions:
1.e = generateKey()
2.h = SHA-256(h || e.pub.serializeCompressed())
The newly generated ephemeral key is accumulated into the running handshake digest.
3.ee = ECDH(e.priv, re)
where re is the ephemeral key of the initiator, which was received during Act One
4.ck, temp_k2 = HKDF(ck, ee)
A new temporary encryption key is generated, which is used to generate the authenticating MAC.
5.c = encryptWithAD(temp_k2, 0, h, zero)
where zero is a zero-length plaintext
6.h = SHA-256(h || c)
Finally, the generated ciphertext is accumulated into the authenticating handshake digest.
7.Send m = 0 || e.pub.serializeCompressed() || c to the initiator over the network buffer.
Receiver Actions:
1.Read exactly 50 bytes from the network buffer.
2.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.
3.If v is an unrecognized handshake version, then the responder MUST abort the connection attempt.
4.h = SHA-256(h || re.serializeCompressed())
5.ee = ECDH(e.priv, re)
where re is the responder's ephemeral public key
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.
6.ck, temp_k2 = HKDF(ck, ee)
A new temporary encryption key is generated, which is used to generate the authenticating MAC.
7.p = decryptWithAD(temp_k2, 0, h, c)
If the MAC check in this operation fails, then the initiator MUST terminate the connection without any further messages.
8.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 Three
   -> s, se
Act Three is the final phase in the authenticated key agreement described in this section. This act is sent from the initiator to the responder as a concluding step. Act Three is executed if and only if Act Two was successful. During Act Three, the initiator transports its static public key to the responder encrypted with strong forward secrecy, using the accumulated HKDF derived secret key at this point of the handshake.
The handshake is exactly 66 bytes: 1 byte for the handshake version, 33 bytes for the static public key encrypted with the ChaCha20 stream cipher, 16 bytes for the encrypted public key's tag generated via the AEAD construction, and 16 bytes for a final authenticating tag.
Sender Actions:
1.c = encryptWithAD(temp_k2, 1, h, s.pub.serializeCompressed())
where s is the static public key of the initiator
2.h = SHA-256(h || c)
3.se = ECDH(s.priv, re)
where re is the ephemeral public key of the responder
4.ck, temp_k3 = HKDF(ck, se)
The final intermediate shared secret is mixed into the running chaining key.
5.t = encryptWithAD(temp_k3, 0, h, zero)
where zero is a zero-length plaintext
6.sk, rk = HKDF(ck, zero)
where zero is a zero-length plaintext, sk is the key to be used by the initiator to encrypt messages to the responder, and rk is the key to be used by the initiator to decrypt messages sent by the responder
The final encryption keys, to be used for sending and receiving messages for the duration of the session, are generated.
7.rn = 0, sn = 0
The sending and receiving nonces are initialized to 0.
8.Send m = 0 || c || t over the network buffer.
Receiver Actions:
1.Read exactly 66 bytes from the network buffer.
2.Parse the read message (m) into v, c, and t:
where v is the first byte of m, c is the next 49 bytes of m, and t is the last 16 bytes of m
3.If v is an unrecognized handshake version, then the responder MUST abort the connection attempt.
4.rs = decryptWithAD(temp_k2, 1, h, c)
At this point, the responder has recovered the static public key of the initiator.
5.h = SHA-256(h || c)
6.se = ECDH(e.priv, rs)
where e is the responder's original ephemeral key
7.ck, temp_k3 = HKDF(ck, se)
8.p = decryptWithAD(temp_k3, 0, h, t)
If the MAC check in this operation fails, then the responder MUST terminate the connection without any further messages.
9.rk, sk = HKDF(ck, zero)
where zero is a zero-length plaintext, rk is the key to be used by the responder to decrypt the messages sent by the initiator, and sk is the key to be used by the responder to encrypt messages to the initiator
The final encryption keys, to be used for sending and receiving messages for the duration of the session, are generated.
10.rn = 0, sn = 0
The sending and receiving nonces are initialized to 0.
Compatibility
Rationale
Reference implementation
Acknowledgements
References
1.  BOLT-8: Encrypted and Authenticated Transport. Version 1. https://github.com/lightningnetwork/lightning-rfc/blob/v1.0/08-transport.md​
2.  Trevor Perrin. The Noise Protocol Framework. Revision 34, 2018-07-11. http://noiseprotocol.org/noise.pdf​
Copyright
This document is licensed under the Creative Commons CC0 1.0 Universal license.
Test vectors