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Drivechain Documentation

Paul Sztorc 
August 15, 2017
Document 1 of 3
v0.2

Note from Paul: Double brackets "{{" and "}}" surround parts that are "unfinished". ???'s indicate free parameters that could be anything, and have yet to be determined.

Two Concepts

For clarity, I've broken the drivechain idea into two modular concepts:

  1. The "hashrate escrow" which allows users to make their transactions vulnerable to reduced-security SPV proofs.
  2. The "blind merged mining" idea which is basically a modern version of merged-mining that is specialized in order to make asymmetric sidechains as tame as possible.

Also for clarity, this documentation intentionally does not cover anything done on any sidechain. It focuses completely on the mainchain.

Each of the two concepts [a] tracks data that is elsewhere in the block, and rearranges it, and [b] has its own transaction type. The new transaction-types are comparable to similar soft forks: to old nodes they are always valid spends, and upon this canvas we instruct new nodes to invalid these tx-types unless a very strict set of rules are followed.

A total of three hash-commitments are required, arranged in the following way:

           ** Storage of Hash Commitments **   

   Hashrate Escrow Items  ----> (mandatory) inserted in Coinbase at TxOut index 0
      /           \            
     /             \                       
Sidechain_DB   Withdrawal_DB      

Blind Merged-Mining Data  ----> (optional) inserted in Coinbase at some TxOut, (identified by a 4-byte header)

This first commitment must always be present. This is to prevent free-riding. However, it is not very burdensome -- if miners don't want to participate, they can easily calculate a default valid hash-values. These do not depend on any information they would not already have.

If the second commitment is not present, sidechain nodes will simply assume that no side:block was found in the last 10 minutes. There is no possibility of free-riding here, as the commitment does not affect anything that is happening across multiple blocks.

(An earlier version of this document described a way of combining the two ideas into a single hash. I decided that it was confusing people, and have removed it.)

Links

  1. Link to 1st BIP -- Hashrate Escrow
  2. Link to 2nd BIP -- Blind Merged-Mining

=== END OF DOCUMENTATION ===

TODO (meta)

Plans for this document:

  1. Edit for clarity.
  2. Flesh out the "soft" sections ie "Motivation" etc
  3. Somehow reconcile this with more detailed descriptions (ie txdb.cpp [1] ?)

[1] https://github.com/bitcoin/bitcoin/blob/master/src/txdb.cpp