BIP 0325

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This page describes a BIP (Bitcoin Improvement Proposal).
Please see BIP 2 for more information about BIPs and creating them. Please do not just create a wiki page.

Please do not modify this page. This is a mirror of the BIP from the source Git repository here.

  BIP: 325
  Layer: Applications
  Title: Signet
  Author: Karl-Johan Alm <karljohan-alm@garage.co.jp>
  Comments-Summary: No comments yet.
  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0325
  Status: Draft
  Type: Standards Track
  Created: 2019-03-20
  License: CC0-1.0

Abstract

A new type of test network where signatures are used in addition to proof of work for block progress, enabling much better coordination and robustness (be reliably unreliable), for persistent, longer-term testing scenarios involving multiple independent parties.

Motivation

Testnet is a great place to try out new things without risking real money, but it is notoriously unreliable. Huge block reorgs, long gaps in between blocks being mined or sudden bursts of blocks in rapid succession mean that realistic testing of software, especially involving multiple independent parties running software over an extended period of time, becomes infeasible in practice.

A new type of test network would be more suitable for integration testing by organizations such as exchanges, or testing of next generation Layer-2 protocols like Eltoo or sidechain pegs. The goal is not to be perfectly reliable but rather to have a predictable amount of unreliability. You want a test network to behave like mainnet (i.e. no thousands of block reorgs) while also making it easier to trigger expected but rare events like a 6-block reorg. Regtest is not suitable for longer-term scenarios involving multiple independent parties because creating blocks costs nothing, so any party can completely control the test network.


Specification

A new type of network ("signet"), which takes an additional consensus parameter called the challenge (scriptPubKey). The challenge can be a simple pubkey (P2PKH style), or a k-of-n multisig, or any other script you would want.

The witness commitment of the coinbase transaction is extended to include a secondary commitment (the signature/solution) of either:

   1-4 bytes - Push the following (4 + x + y) bytes
   4 bytes - Signet scriptSig header (0xecc7daa2)
   x bytes - scriptSig
   y bytes - scriptWitness

The scriptSig is serialized by first encoding its length as CompactSize. If the scriptWitness is empty, it is encoded as 0 bytes, otherwise it is encoded in the usual way (see BIP 141 "witness" encoding).

Any push operations that do not start with the 4 byte Signet header are ignored. Multiple push operations with the 4 byte Signet header are ignored except for the first instance of the header.

To sign the block or verify a block signature, two virtual transactions, each with a single input and output are constructed from the block as follows.

The "to_spend" transaction is:

   nVersion = 0
   nLockTime = 0
   vin[0].prevout.hash = 0000...000
   vin[0].prevout.n = 0xFFFFFFFF
   vin[0].nSequence = 0
   vin[0].scriptSig = OP_0 PUSH72[ block_data ]
   vin[0].scriptWitness = []
   vout[0].nValue = 0
   vout[0].scriptPubKey = signet_challenge

where block_data is the serialization of the block's nVersion, hashPrevBlock, signet_merkle_root, and nTime. The signet_merkle_root is obtained by generating the merkle root of the block transactions, after modifying the coinbase witness commitment by replacing the signet solution with an empty solution (that is, the witness commitment includes a four byte push of the Signet header with no additional solution data, and no prior pushes beginning with the Signet header). This means the merkle root of the block is different from the merkle root in the signet commitment. This is needed, because the signature can never be included in the very message (in this case, a block) that is being signed.

The "to_sign" transaction is:

   nVersion = 0
   nLockTime = 0
   vin[0].prevout.hash = to_spend.txid
   vin[0].prevout.n = 0
   vin[0].nSequence = 0
   vout[0].nValue = 0
   vout[0].scriptPubKey = signet_challenge

The scriptSig and/or scriptWitness for vin[0] are filled in from the Signet header push above.

To simplify block generation (mining), the signature also does not commit to the the block nonce value, so that rolling the nonce to generate proof-of-work does not also require regenerating signatures. When grinding proof of work, the extended nonce cannot be used as it would invalidate the signature. Instead, simply resigning the same (or an updated) block will give a new search space.

A block is considered fully validated only if the to_sign transaction is a valid spend of the to_spend transaction. It is recommended that this verification is done directly before or after the witness commitment verification, as the data required to do both is approximately the same.

Genesis Block and Message Start

The genesis block is the same for all signet networks, whereas the message start is defined as the first four bytes of the sha256d of the challenge script as a single data push (see below).

Genesis Block

  • Time stamp: 1534313275
  • Nonce: 100123
  • Difficulty: 1e2adc28

The resulting genesis block hash is 0000032d7f67af9ec7b7152aea0fe7c95b9804ff973265e252f245e0ae61799d, and the block hex is 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.

Message Start

The message start is defined as the first four bytes of the sha256d of the challenge script, as a single push (i.e. prefixed with the challenge script length). Example:

  • Challenge script = 512103ad5e0edad18cb1f0fc0d28a3d4f1f3e445640337489abb10404f2d1e086be43051ae
  • Sha256d(len || challenge script) = sha256d(25512103ad...51ae) = 7ec653a59b1912f9db10da2c461ed827d48f9404d5ef0346a6c94aadd4203646
  • First four bytes = the message start = 7ec653a5

Compatibility

This specification is backwards compatible in the sense that existing software can use Signet out of the box.

Simply by adding the network parameters for signet (magic number, etc), a client can connect to and use any signet network without further modifications. The block headers have valid proof of work, so clients can trivially check that blocks are "probably" valid.

However, anyone can mine blocks that are accepted by the client for any given signet network. These blocks do not contain the required signatures, however, so any fully validating node will promptly reject them. As such, clients need to either validate the block signature inside the coinbase transaction, or connect to trusted peers.

Other software need not add block signature validation code that they will not use in production. This is adequate for non-production test purposes where the goal is to have a network behave as much like mainnet as possible.

Reference implementation

Pull request at https://github.com/bitcoin/bitcoin/pull/18267

Acknowledgements

TODO

References

  1. Original mailing list thread: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2019-March/016734.html
  2. Bitcoin Wiki entry: https://en.bitcoin.it/wiki/Signet

Copyright

This document is licensed under the Creative Commons CC0 1.0 Universal license.