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The RLPx Transport Protocol

This specification defines the RLPx transport protocol, a TCP-based transport protocol used for communication among Ethereum nodes. The protocol carries encrypted messages belonging to one or more 'capabilities' which are negotiated during connection establishment. RLPx is named after the RLP serialization format. The name is not an acronym and has no particular meaning.

The current protocol version is 5. You can find a list of changes in past versions at the end of this document.

Notation

X || Y
    denotes concatenation of X and Y.
X ^ Y
    is byte-wise XOR of X and Y.
X[:N]
    denotes an N-byte prefix of X.
[X, Y, Z, ...]
    denotes recursive encoding as an RLP list.
keccak256(MESSAGE)
    is the Keccak256 hash function as used by Ethereum.
ecies.encrypt(PUBKEY, MESSAGE, AUTHDATA)
    is the asymmetric authenticated encryption function as used by RLPx.
    AUTHDATA is authenticated data which is not part of the resulting ciphertext,
    but written to HMAC-256 before generating the message tag.
ecdh.agree(PRIVKEY, PUBKEY)
    is elliptic curve Diffie-Hellman key agreement between PRIVKEY and PUBKEY.

ECIES Encryption

ECIES (Elliptic Curve Integrated Encryption Scheme) is an asymmetric encryption method used in the RLPx handshake. The cryptosystem used by RLPx is

  • The elliptic curve secp256k1 with generator G.
  • KDF(k, len): the NIST SP 800-56 Concatenation Key Derivation Function
  • MAC(k, m): HMAC using the SHA-256 hash function.
  • AES(k, iv, m): the AES-128 encryption function in CTR mode.

Alice wants to send an encrypted message that can be decrypted by Bobs static private key kB. Alice knows about Bobs static public key KB.

To encrypt the message m, Alice generates a random number r and corresponding elliptic curve public key R = r * G and computes the shared secret S = Px where (Px, Py) = r * KB. She derives key material for encryption and authentication as kE || kM = KDF(S, 32) as well as a random initialization vector iv. Alice sends the encrypted message R || iv || c || d where c = AES(kE, iv , m) and d = MAC(sha256(kM), iv || c) to Bob.

For Bob to decrypt the message R || iv || c || d, he derives the shared secret S = Px where (Px, Py) = kB * R as well as the encryption and authentication keys kE || kM = KDF(S, 32). Bob verifies the authenticity of the message by checking whether d == MAC(sha256(kM), iv || c) then obtains the plaintext as m = AES(kE, iv || c).

Node Identity

All cryptographic operations are based on the secp256k1 elliptic curve. Each node is expected to maintain a static secp256k1 private key which is saved and restored between sessions. It is recommended that the private key can only be reset manually, for example, by deleting a file or database entry.

Initial Handshake

An RLPx connection is established by creating a TCP connection and agreeing on ephemeral key material for further encrypted and authenticated communication. The process of creating those session keys is the 'handshake' and is carried out between the 'initiator' (the node which opened the TCP connection) and the 'recipient' (the node which accepted it).

  1. initiator connects to recipient and sends its auth message
  2. recipient accepts, decrypts and verifies auth (checks that recovery of signature == keccak256(ephemeral-pubk))
  3. recipient generates auth-ack message from remote-ephemeral-pubk and nonce
  4. recipient derives secrets and sends the first encrypted frame containing the Hello message
  5. initiator receives auth-ack and derives secrets
  6. initiator sends its first encrypted frame containing initiator Hello message
  7. recipient receives and authenticates first encrypted frame
  8. initiator receives and authenticates first encrypted frame
  9. cryptographic handshake is complete if MAC of first encrypted frame is valid on both sides

Either side may disconnect if authentication of the first framed packet fails.

Handshake messages:

auth = auth-size || enc-auth-body
auth-size = size of enc-auth-body, encoded as a big-endian 16-bit integer
auth-vsn = 4
auth-body = [sig, initiator-pubk, initiator-nonce, auth-vsn, ...]
enc-auth-body = ecies.encrypt(recipient-pubk, auth-body || auth-padding, auth-size)
auth-padding = arbitrary data

ack = ack-size || enc-ack-body
ack-size = size of enc-ack-body, encoded as a big-endian 16-bit integer
ack-vsn = 4
ack-body = [recipient-ephemeral-pubk, recipient-nonce, ack-vsn, ...]
enc-ack-body = ecies.encrypt(initiator-pubk, ack-body || ack-padding, ack-size)
ack-padding = arbitrary data

Implementations must ignore any mismatches in auth-vsn and ack-vsn. Implementations must also ignore any additional list elements in auth-body and ack-body.

Secrets generated following the exchange of handshake messages:

static-shared-secret = ecdh.agree(privkey, remote-pubk)
ephemeral-key = ecdh.agree(ephemeral-privkey, remote-ephemeral-pubk)
shared-secret = keccak256(ephemeral-key || keccak256(nonce || initiator-nonce))
aes-secret = keccak256(ephemeral-key || shared-secret)
mac-secret = keccak256(ephemeral-key || aes-secret)

Framing

All messages following the initial handshake are framed. A frame carries a single encrypted message belonging to a capability.

The purpose of framing is multiplexing multiple capabilities over a single connection. Secondarily, as framed messages yield reasonable demarcation points for message authentication codes, supporting an encrypted and authenticated stream becomes straight-forward. Frames are encrypted and authenticated via key material generated during the handshake.

The frame header provides information about the size of the message and the message's source capability. Padding is used to prevent buffer starvation, such that frame components are byte-aligned to block size of cipher.

frame = header-ciphertext || header-mac || frame-ciphertext || frame-mac
header-ciphertext = aes(aes-secret, header)
header = frame-size || header-data || header-padding
header-data = [capability-id, context-id]
capability-id = integer, always zero
context-id = integer, always zero
header-padding = zero-fill header to 16-byte boundary
frame-ciphertext = aes(aes-secret, frame-data || frame-padding)
frame-padding = zero-fill frame-data to 16-byte boundary

See the Capability Messaging section for definitions of frame-data and frame-size.

MAC

Message authentication in RLPx uses two keccak256 states, one for each direction of communication. The egress-mac and ingress-mac keccak states are continuously updated with the ciphertext of bytes sent (egress) or received (ingress). Following the initial handshake, the MAC states are initialized as follows:

Initiator:

egress-mac = keccak256.init((mac-secret ^ recipient-nonce) || auth)
ingress-mac = keccak256.init((mac-secret ^ initiator-nonce) || ack)

Recipient:

egress-mac = keccak256.init((mac-secret ^ initiator-nonce) || ack)
ingress-mac = keccak256.init((mac-secret ^ recipient-nonce) || auth)

When a frame is sent, the corresponding MAC values are computed by updating the egress-mac state with the data to be sent. The update is performed by XORing the header with the encrypted output of its corresponding MAC. This is done to ensure uniform operations are performed for both plaintext MAC and ciphertext. All MACs are sent cleartext.

header-mac-seed = aes(mac-secret, keccak256.digest(egress-mac)[:16]) ^ header-ciphertext
egress-mac = keccak256.update(egress-mac, header-mac-seed)
header-mac = keccak256.digest(egress-mac)[:16]

Computing frame-mac:

egress-mac = keccak256.update(egress-mac, frame-ciphertext)
frame-mac-seed = aes(mac-secret, keccak256.digest(egress-mac)[:16]) ^ keccak256.digest(egress-mac)[:16]
egress-mac = keccak256.update(egress-mac, frame-mac-seed)
frame-mac = keccak256.digest(egress-mac)[:16]

Verifying the MAC on ingress frames is done by updating the ingress-mac state in the same way as egress-mac and comparing to the values of header-mac and frame-mac in the ingress frame. This should be done before decrypting header-ciphertext and frame-ciphertext.

Capability Messaging

All messages following the initial handshake are associated with a 'capability'. Any number of capabilities can be used concurrently on a single RLPx connection.

A capability is identified by a short ASCII name (max eight characters) and version number. The capabilities supported on either side of the connection are exchanged in the Hello message belonging to the 'p2p' capability which is required to be available on all connections.

Message Encoding

The initial Hello message is encoded as follows:

frame-data = msg-id || msg-data
frame-size = length of frame-data, encoded as a 24bit big-endian integer

where msg-id is an RLP-encoded integer identifying the message and msg-data is an RLP list containing the message data.

All messages following Hello are compressed using the Snappy algorithm.

frame-data = msg-id || snappyCompress(msg-data)
frame-size = length of frame-data encoded as a 24bit big-endian integer

Note that the frame-size of compressed messages refers to the compressed size of msg-data. Since compressed messages may inflate to a very large size after decompression, implementations should check for the uncompressed size of the data before decoding the message. This is possible because the snappy format contains a length header. Messages carrying uncompressed data larger than 16 MiB should be rejected by closing the connection.

Message ID-based Multiplexing

While the framing layer supports a capability-id, the current version of RLPx doesn't use that field for multiplexing between different capabilities. Instead, multiplexing relies purely on the message ID.

Each capability is given as much of the message-ID space as it needs. All such capabilities must statically specify how many message IDs they require. On connection and reception of the Hello message, both peers have equivalent information about what capabilities they share (including versions) and are able to form consensus over the composition of message ID space.

Message IDs are assumed to be compact from ID 0x10 onwards (0x00-0x0f is reserved for the "p2p" capability) and given to each shared (equal-version, equal-name) capability in alphabetic order. Capability names are case-sensitive. Capabilities which are not shared are ignored. If multiple versions are shared of the same (equal name) capability, the numerically highest wins, others are ignored.

"p2p" Capability

The "p2p" capability is present on all connections. After the initial handshake, both sides of the connection must send either Hello or a Disconnect message. Upon receiving the Hello message a session is active and any other message may be sent. Implementations must ignore any difference in protocol version for forward-compatibility reasons. When communicating with a peer of lower version, implementations should try to mimic that version.

At any time after protocol negotiation, a Disconnect message may be sent.

Hello (0x00)

[protocolVersion: P, clientId: B, capabilities, listenPort: P, nodeKey: B_64, ...]

First packet sent over the connection, and sent once by both sides. No other messages may be sent until a Hello is received. Implementations must ignore any additional list elements in Hello because they may be used by a future version.

  • protocolVersion the version of the "p2p" capability, 5.
  • clientId Specifies the client software identity, as a human-readable string (e.g. "Ethereum(++)/1.0.0").
  • capabilities is the list of supported capabilities and their versions: [[cap1, capVersion1], [cap2, capVersion2], ...].
  • listenPort (legacy) specifies the port that the client is listening on (on the interface that the present connection traverses). If 0 it indicates the client is not listening. This field should be ignored.
  • nodeId is the secp256k1 public key corresponding to the node's private key.

Disconnect (0x01)

[reason: P]

Inform the peer that a disconnection is imminent; if received, a peer should disconnect immediately. When sending, well-behaved hosts give their peers a fighting chance (read: wait 2 seconds) to disconnect to before disconnecting themselves.

reason is an optional integer specifying one of a number of reasons for disconnect:

Reason Meaning
0x00 Disconnect requested
0x01 TCP sub-system error
0x02 Breach of protocol, e.g. a malformed message, bad RLP, ...
0x03 Useless peer
0x04 Too many peers
0x05 Already connected
0x06 Incompatible P2P protocol version
0x07 Null node identity received - this is automatically invalid
0x08 Client quitting
0x09 Unexpected identity in handshake
0x0a Identity is the same as this node (i.e. connected to itself)
0x0b Ping timeout
0x10 Some other reason specific to a subprotocol

Ping (0x02)

[]

Requests an immediate reply of Pong from the peer.

Pong (0x03)

[]

Reply to the peer's Ping packet.

Change Log

Known Issues in the current version

  • The frame encryption/MAC scheme is considered 'broken' because aes-secret and mac-secret are reused for both reading and writing. The two sides of a RLPx connection generate two CTR streams from the same key, nonce and IV. If an attacker knows one plaintext, they can decrypt unknown plaintexts of the reused keystream.
  • General feedback from reviewers has been that the use of a keccak256 state as a MAC accumulator and the use of AES in the MAC algorithm is an uncommon and overly complex way to perform message authentication but can be considered safe.
  • The frame encoding provides capability-id and context-id fields for multiplexing purposes, but these fields are unused.

Version 5 (EIP-706, September 2017)

EIP-706 added Snappy message compression.

Version 4 (EIP-8, December 2015)

EIP-8 changed the encoding of auth-body and ack-body in the initial handshake to RLP, added a version number to the handshake and mandated that implementations should ignore additional list elements in handshake messages and Hello.

References

Copyright © 2014 Alex Leverington. This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.