Accidental confiscation can occur if a poorly designed soft fork permanently prevents a user from being able to get a transaction confirmed.

A soft fork is a restriction on the previous consensus rules. If a soft fork overly restricts the use of a feature that a user needs to spend some bitcoins they previously received, those bitcoins will become unspendable unless the soft fork behavior is changed in a later hard fork.

Such confiscation can be done deliberately, in which case the operators of economic full nodes who enforce the consensus rules need to decide if the benefits of a confiscatory soft fork outweigh its problems. But it can also happen accidentally if a soft fork is poorly designed or if there’s poor communication between consensus developers and developers of contract protocols. Three particular risks stand out:

  • ● Unknown use of upgrade features: some features of the consensus protocol, such as the OP_NOPx and OP_SUCCESSx opcodes are reserved for future soft forks. If anyone begins using them before a soft fork, they could become a victim of accidental confiscation. For that reason, Bitcoin Optech very strongly recommends anyone considering use of a feature reserved for soft fork upgrades publicly announce their plans to protocol developer communication channels and proactively monitor any discussion of new soft forks for changes to the features being used.

  • ● Presigned transactions: some contract protocols and uses of Bitcoin may require a user to generate a spending transaction, sign it with the necessary private keys, and then delete the private keys. This prevents them from being able to sign alternative versions of the transaction. If the Bitcoin consensus rules are changed to make the spending transaction invalid, the user’s funds are lost. Optech recommends that presigned transaction protocols be well documented, they be checked against standardness policy (see below), and that anyone using them monitor any discussion of new soft fork proposals for changes that would render those transactions invalid.

  • ● Committed scripts: P2SH, P2WSH, and P2TR scriptpaths are all scripts that do not appear on the blockchain when the user commits to potentially using them. Any soft fork restriction to the scripting language can make a previously satisfiable script into one that cannot be satisfied. Optech makes the same recommendation here as we do for presigned transactions.

Relationship to transaction standardness policy

Full nodes such as Bitcoin Core will not relay certain transactions, allow them into their mempools, or put them in block templates—even if those transactions are valid according to the current consensus rules. There are a variety of reasons for the transaction relay policies, but some of those policies exist to discourage anyone from using features reserved for future soft fork upgrades.

For example, if you create a transaction at present which executes OP_NOP9, that transaction will be considered non-standard and will not be relayed while it is unconfirmed. It’s still valid in a block and all full nodes must accept it in a block to remain in consensus.

The goal is for anyone who begins using a reserved feature to quickly discover that transactions using that feature are hard to get mined. They can then either switch to an alternative method of accomplishing the same thing or they can convince full node developers to change their relay policy, which hopefully serves as sufficient public notification that the feature is being used, preventing later accidental confiscation.

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2024

2019

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