Levin Protocol

This is a document explaining the current design of the Levin protocol, as used by Shekyl. The protocol is largely inherited from cryptonote, but has undergone some changes.

This document also may differ from the struct bucket_head2 in Shekyl's code slightly - the spec here is slightly more strict to allow for extensibility.

One of the goals of this document is to clearly indicate what is being sent "on the wire" to identify metadata that could de-anonymize users over I2P/Tor. These issues will be addressed as they are found. See ANONYMITY_NETWORKS.md in the docs folder for any outstanding issues.

PQC note:

• the transport header described here is unchanged by the planned PQC rollout
• however, reboot-only TransactionV3 objects with hybrid signatures will make transaction-bearing protocol messages materially larger
• the canonical transaction/authentication format is specified in POST_QUANTUM_CRYPTOGRAPHY.md

This document does not currently list all data being sent by the Shekyl protocol, that portion is a work-in-progress. Please take the time to do it if interested in learning about Shekyl p2p traffic!

Header

This header is sent for every Shekyl p2p message.

 0               1               2               3
 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      0x01     |      0x21     |      0x01     |      0x01     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      0x01     |      0x01     |      0x01     |      0x01     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             Length                            |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  E. Response  |                   Command
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                |                 Return Code
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                |Q|S|B|E|               Reserved
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                |      0x01     |      0x00     |      0x00     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     0x00      |
+-+-+-+-+-+-+-+-+

Signature

The first 8 bytes are the "signature" which helps identify the protocol (in case someone connected to the wrong port, etc). The comments indicate that byte sequence is from "benders nightmare".

This also can be used by deep packet inspection (DPI) engines to identify Shekyl when the link is not encrypted. SSL has been proposed as a means to mitigate this issue, but BIP-151 or the Noise protocol should also be considered.

Length

The length is an unsigned 64-bit little endian integer. The length does not include the header.

The implementation currently rejects received messages that exceed 100 MB (base 10) by default.

Expect Response

A zero-byte if no response is expected from the peer, and a non-zero byte if a response is expected from the peer. Peers must respond to requests with this flag in the same order that they were received, however, other messages can be sent between responses.

There are some commands in the cryptonote protocol where a response is expected from the peer, but this flag is not set. Those responses are returned as notify messages and can be sent in any order by the peer.

Command

An unsigned 32-bit little endian integer representing the Shekyl-specific command being invoked.

Return Code

A signed 32-bit little endian integer representing the response from the peer from the last command that was invoked. This is 0 for request messages.

Flags

• Q - Bit is set if the message is a request.
• S - Bit is set if the message is a response.
• B - Bit is set if this is a the beginning of a fragmented message.
• E - Bit is set if this is the end of a fragmented message.

Version

A fixed value of 1 as an unsigned 32-bit little endian integer.

Message Flow

The protocol can be subdivided into: (1) notifications, (2) requests, (3) responses, (4) fragmented messages, and (5) dummy messages. Response messages must be sent in the same order that a peer issued a request message. A peer does not have to send a response immediately following a request - any other message type can be sent instead.

Notifications

Notifications are one-way messages that can be sent at any time without an expectation of a response from the peer. The Q bit must be set, the S, B and E bits must be unset, and the Expect Response field must be zeroed.

Some notifications must be in response to other notifications. This is not part of the levin messaging layer, and is described in the commands section.

Requests

Requests are the basis of the admin protocol for Shekyl. The Q bit must be set, the S, B and E bits must be unset, and the Expect Response field must be non-zero. The peer is expected to send a response message with the same command number.

Responses

Response message can only be sent after a peer first issues a request message. Responses must have the S bit set, the Q, B and E bits unset, and have a zeroed Expect Response field. The Command field must be the same value that was sent in the request message. The Return Code is specific to the Command being issued (see [commands])(#commands)).

Fragmented

Fragmented messages were introduced for the "white noise" feature for i2p/tor. A transaction can be sent in fragments to conceal when "real" data is being sent instead of dummy messages. Only one fragmented message can be sent at a time, and bits B and E are never set at the same time (see dummy messages). The re-constructed message must contain a levin header for a different (non-fragment) message type.

The Q and S bits are never set and the Expect Response field must always be zero. The first fragment has the B bit set, neither B nor E is set for "middle" fragments, and E is set for the last fragment.

Dummy

Dummy messages have the B and E bits set, the Q and S bits unset, and the Expect Response field zeroed. When a message of this type is received, the contents can be safely ignored.

Commands

This document currently focuses on the Levin framing layer and high-level command inventory. It does not yet enumerate every field in every transaction- bearing payload.

For the rebooted chain:

• commands that relay transactions or blocks will eventually carry TransactionV3 payloads
• TransactionV3 introduces dedicated PQ authentication fields outside tx_extra
• downstream tooling must not assume pre-PQC transaction sizes

P2P (Admin) Commands

(1001 Request) Handshake

(1001 Response) Handshake

(1002 Request) Timed Sync

(1002 Response) Timed Sync

(1003 Request) Ping

(1003 Response) Ping

(1004 Request) Stat Info

(1004 Response) Stat Info

(1005 Request) Network State

(1005 Response) Network State

(1006 Request) Peer ID

(1006 Response) Peer ID

(1007 Request) Support Flags

(1007 Response) Support Flags

Cryptonote Protocol Commands

(2001 Notification) New Block

Carries a full serialized block. Post-v3, user transactions within the block include pqc_auth material (~5.4 KB per user tx). Miner coinbase remains excluded from pqc_auth.

(2002 Notification) New Transactions

Carries one or more serialized transactions for mempool relay. This is the primary message type affected by v3 PQC sizing: each TransactionV3 user transaction adds ~5,385 bytes of hybrid authentication data.

Over anonymity networks, this message is the most fingerprinting-sensitive: it is sent shortly after a wallet constructs a transaction and is the strongest timing signal linking a peer to a spend event.

(2003 Notification) Request Get Objects

(2004 Notification) Response Get Objects

Response carries requested block/transaction objects. Payload size is variable and scales linearly with the number of v3 transactions in the requested range.

(2006 Notification) Request Chain

(2007 Notification) Response Chain Entry

Carries block hashes for chain synchronization. Not affected by v3 PQC sizing (block headers do not contain pqc_auth).

(2008 Notification) New Fluffy Block

Carries block header plus transaction hashes (not full transactions). The receiving peer requests missing transactions via 2009. Not directly affected by PQC sizing, but the follow-up 2002/2009 exchange is.

(2009 Notification) Request Fluffy Missing TX

Requests specific transactions by hash. The response contains full serialized v3 transactions including pqc_auth.

Wire Data Privacy Summary

Command	PQC size impact	Anonymity sensitivity	Notes
1001 Handshake	None	Low	Peer identity exchange
1002 Timed Sync	None	Medium	Timestamp fingerprinting risk
2001 New Block	Proportional to tx count	Low	Broadcast, not origin-attributable
2002 New Transactions	+5.4 KB per user tx	High	Origin-attributable timing signal
2003/2004 Get Objects	Proportional to tx count	Low	Sync protocol
2006/2007 Chain Entry	None	None	Hash-only
2008 Fluffy Block	Minimal	Low	Header + hashes
2009 Missing TX	+5.4 KB per requested tx	Medium	Follow-up to fluffy block