Deployment Schedule & Technique

There is no presently defined deployment schedule for BIP-119.

There has been discussion on the mailing list about soft fork activation techniques as an extension to existing methods, but it is still undecided how new upgrades should be proposed, rolled out, and activated or rejected.

OP_CHECKTEMPLATEVERIFY Deployment Options

There are a couple different implementations and deployment strategies for OP_CHECKTEMPLATEVERIFY:

• OP_NOP4 Softfork
• Taproot Tapscript Extension
• Templated Tapscript Extension

This page serves to mark the differences between these versions.

OP_CHECKTEMPLATEVERIFY is in some places (e.g., archives) referred to as OP_SECURETHEBAG or OP_CHECKOUTPUTSHASHVERIFY as working names for the technique while under development while under development.

BIP 119 OP_NOP4 Softfork

This is currently the favored implementation and deployment of OP_CHECKTEMPLATEVERIFY.

In this version, OP_CHECKTEMPLATEVERIFY is implemented as a soft-fork upgrade to OP_NOP much like OP_CHECKLOCKTIMEVERIFY. This would make OP_CHECKTEMPLATEVERIFY available for both SegWit and plain script outputs.

Note: P2SH is not compatible with OP_CHECKTEMPLATEVERIFY inehrently because of the hash cycle caused by putting the redeemScript on the scriptSig, which changes the TXID.

The implementation suggests to begin signaling for the soft-fork on versionbit 25 on January 1st, 2020 for 1 year, but this can be replaced with actual values once formally proposed.

Links

• BIP Draft
• Implementation

Taproot Tapscript Extension

This version of OP_CHECKTEMPLATEVERIFY builds on Tapscript’s proposed OP_SUCCESS{X} script upgrade mechanism. Therefore OP_CHECKTEMPLATEVERIFY is only available within Tapscript. There are use cases where bare scripts may be desirable.

Deployment would only be possible after or with Taproot’s deployment, which is presently uncertain.

For these reasons, the OP_NOP4 deployment strategy has been drafted.

Links

• BIP Draft
• Implementation

Templated Tapscript Extension

Because of the structure of Taproot, an OP_CHECKTEMPLATEVERIFY script has an additional 32 byte overhead compared to a bare OP_NOP4 style upgrade. By templating the Tapscripts to support a special case OP_CHECKTEMPLATEVERIFY, this overhead can be eliminated.

This technique is notable because it could be added to Taproot after the OP_NOP4 upgrade, but cannot be added to Taproot as a soft-fork if it is not natively supported in Tapscript.

Links

Implementation

In addition, there is a withdrawn version, OP_CHECKOUTPUTSHASHVERIFY, which was vulnerable to malleability issues.