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Add bip-internalkey
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reardencode committed Dec 27, 2023
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<pre>
BIP: TBD
Layer: Consensus (soft fork)
Title: INTERNALKEY
Author: Brandon Black <[email protected]>
Comments-Summary: No comments yet.
Status: Draft
Type: Standards Track
Created: 2023-12-22
License: PD
</pre>

==Abstract==

This BIP describes a new tapscript opcode (OP_INTERNALKEY) which pushes the
taproot internal key to the stack.

==Specification==

When verifying taproot script spends having leaf version 0xc0 (as defined in
BIP342), OP_INTERNALKEY replaces OP_SUCCESS187 (0xbb). OP_INTERNALKEY pushes
the taproot internal key, as defined in BIP341, to the stack.

==Motivation==

When building taproot output restrictions, especially those secured by an
aggregate key representing more than 1 signer, where the parties may wish to
collaborate on signing with the taproot internal key, but only with additional
script restrictions. In this case, OP_INTERNALKEY saves 32 bytes of witness
space.

In cases where script path spending is not desired, the internal key may be set
to a NUMS point whose bytes would otherwise be required in a tapscript. This
could be used with any hash locked transaction, for example, to save 32 witness
bytes.

==Reference Implementation==

A reference implementation is provided in provided here:

https://github.com/brandonblack/bitcoin/commit/aa94c2865badc9bef86c29c330d4da77f341c0f2

==Backward Compatibility==

By constraining the behavior of an OP_SUCCESS opcode, deployment of the BIP can
be done in a backwards compatible, soft-fork manner. If anyone were to rely on
the OP_SUCCESS behavior of OP_SUCCESS187, OP_INTERNALKEY would invalidate their
spend.

==Deployment==

TBD

==Credits==

Jeremy Rubin described most of the ideas for OP_INTERNALKEY to me.

==References==

[https://github.com/bitcoin/bips/blob/master/bip-0341.mediawiki BIP 341] Taproot

[https://github.com/bitcoin/bips/blob/master/bip-0342.mediawiki BIP 342] Tapscript

==Copyright==

This document is placed in the public domain.


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