Add bip-internalkey

bip-internalkey.mediawiki

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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 (<code>OP_INTERNALKEY</code>) which
+pushes the taproot internal key to the stack.
+
+==Specification==
+
+When verifying taproot script spends having leaf version 0xc0 (as defined in
+BIP342), <code>OP_INTERNALKEY</code> replaces <code>OP_SUCCESS203</code>
+(0xcb). <code>OP_INTERNALKEY</code> pushes the taproot internal key, as defined
+in BIP341, to the stack.
+
+==Motivation==
+
+===Key spend with additional conditions===
+
+When building taproot outputs, especially those secured by an aggregate key
+representing more than 1 signer; the parties may wish to collaborate on signing
+with the taproot internal key, but only with additional script restrictions. In
+this case, <code>OP_INTERNALKEY</code> saves 32 bytes of witness space.
+
+===Mitigated control block overhead for scripts using hash locks===
+
+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 here:
+
+https://github.com/brandonblack/bitcoin/commit/985e8c788d5e0f91802200044d31f9b86500255c
+
+==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 <code>OP_SUCCESS203</code>,
+<code>OP_INTERNALKEY</code> would invalidate their spend.
+
+==Deployment==
+
+TBD
+
+==Credits==
+
+Jeremy Rubin described most of the ideas for <code>OP_INTERNALKEY</code> 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.