BIP draft: Hard fork opt-in mechanism for SPV nodes: https://github.com/bitcoin/bips/pull/320 This is a supplement, instead of a replacement, of the hardfork bit BIP: https://github.com/bitcoin/bips/pull/317 They solves different problems:
The hardfork bit tells full and SPV that a planned hardfork (instead of a softfork) has happened.
This BIP makes sure SPV nodes won't lose any money in a hardfork, even if they do not check the hardfork bit.
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BIP: ? Title: Hard fork opt-in mechanism for SPV nodes Author: Johnson Lau <jl2012@xbt.hk> Status: Draft Type: Standard Track Created: 2016-02-05
This document specifies a new algorithm for the transaction commitment in block header, to ensure that SPV nodes will not automatically follow a planned hard fork without explicit opt-in consent.
A hard fork in Bitcoin is a consensus rule change where previously invalid blocks become valid. For the operators of fully validating nodes, migration to the new fork requires conscious actions. However, this may not be true for SPV node, as many consensus rules are transparent to them. SPV nodes may follow the chain with most proof-of-work, even if the operators do not agree with the economical or ideological properties of the chain.
By specifying a new algorithm for the transaction commitment in block header, migration to the new fork requires explicit opt-in consent for SPV nodes. It is expected that this proposal will be implemented with other backward-incompatible consensus rule changes at the same time.
The calculation of Merkle root remains unchanged. Instead of directly committing the Merkle root to the header, we commit
Double-SHA256(zero|merkle_root|zero)
where zero is 0x0000....0000 with 32 bytes.
Since the header structure is not changed, non-upgraded SPV nodes will still be able to verify the proof-of-work of the new chain, and they will follow the new chain if it has most proof-of-work. However, they will not be able to the accept any incoming transactions on the new chain since they cannot verify them with the new commitment format. At the same time, SPV nodes will not accept any new transactions on the old chain, as they find it has less proof-of-work. Effectively, SPV nodes stop accepting any transactions, until their operators take further actions.
Zero-padding is applied before and after the merkle_root, so it is not possible to circumvent the rule change with any current implementations, even for faulty ones.
A future hard fork should change the padding value to stop non-upgraded SPV nodes from processing new transactions.
Hard forks may sometimes be totally uncontroversial and make barely noticeable change (BIP50, for example). In such cases, changing the padding value may not be needed as it may cause unnecessary disruption. The risk and benefit should be evaluated case-by-case.
As a mechanism to indicate hard fork deployment, this BIP breaks backward compatibility intentionally. However, without further changes in the block header format, non-upgraded full nodes and SPV nodes could still verify the proof-of-work of upgraded blocks.
Interaction with fraud proof system A fraud proof system is full nodes that will generate compact proofs to testify invalid blocks on the blockchain, verifiable by SPV nodes. Hard forks without any malicious intention may also be considered as a “fraud” among non-upgraded nodes. This may not be desirable, as the SPV node may accept devalued tokens on the old chain with less proof-of-work. With this BIP, non-upgraded SPV nodes will always believe the new chain is valid (since they cannot verify any fraud proof), while cannot be defrauded as they will not see any incoming transactions.
This document is placed in the public domain.