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BUIP058 Dynamic checkpoints
Proposer: freetrader
Submitted on: 2017-05-19
Status: draft

Summary

Add a feature which allows dynamic addition and removal of non-hardcoded checkpoints.

Checkpoints denote blocks that must be present in a valid chain. A checkpoint is a tuple (chain_height, block_hash) indicating that the block at that height must always have the given hash.

Currently, there are only some hardcoded checkpoints in src/chainparams.cpp (not frequently updated).

Motivation

To allow more flexible, persistent protection of a chain against re-organization [1], at the discretion of the operator.

This could be particularly useful in a hard fork scenario such as the very contentious "breaking of the 1MB barrier".

Having an advanced user option to add checkpoints saves the operator from having to use 'invalidateblock' as a way of fighting attacker re-org blocks, essentially saving effort and reducing chance of costly mistake.

Since additional checkpoints may act to block re-organizations to a longer valid chain with more proof-of-work, this feature should be considered as an expert tool, probably only to be used by operators of mining nodes.

Implementation

An RPC call interface shall be added to add and remove dynamic checkpoints.

Whenever a dynamic checkpoint is added or removed, the active chain shall be re-evaluated for compliance and if necessary, a re-organization performed in an attempt to achieve compliance with the stipulated checkpoints.

The dynamic checkpoints shall be recorded in a human-readable checkpoint data file (e.g. checkpoints.csv) whenever the list changes.

At startup, the client shall read this file, if present, and add any checkpoints from there to its list of built-in checkpoints used for chain processing.

The client shall abort startup if the checkpoint data file is malformed (see Notes). If the file is not present, it shall proceed using only the built-in (hardcoded) checkpoints.

The RPC interface does not need to be specially exposed through the GUI

  • it would be available through the Debug Console anyway.

It shall not be possible to dynamically remove built-in checkpoints.

Notes

A checkpoint data file is malformed if it is syntactically invalid, or if it contains the following semantic problems:

  • duplicate entries
  • entries with conflicting hashes for identical height
  • entries with conflicting height for identical hashes

There have been a number of suggestions for potential names of the RPC calls.

Adding: 'mandateblock', 'requireblock', 'addcheckpoint'

The removal call names could be formed by pre-prending 'un-', or substituting 'remove' for 'add'.

I have intentionally not included some form of automatic addition of checkpoints in the form discussed in [2], to keep this BUIP functionally simple.

References

[1] https://bitcointalk.org/index.php?topic=437

[2] https://bitco.in/forum/threads/buip...by-a-support-threshold.2111/page-2#post-39031