BIP: 349
  Layer: Consensus (soft fork)
  Title: OP_INTERNALKEY
  Author: Brandon Black 
          Jeremy Rubin 
  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0349
  Status: Draft
  Type: Standards Track
  Created: 2023-12-22
  License: BSD-3-Clause
## Abstract This BIP describes a new tapscript opcode (`OP_INTERNALKEY`) which pushes the _taproot internal key_ to the stack. ## Specification When verifying taproot script path spends having leaf version `0xc0` (as defined in [BIP 342]), `OP_INTERNALKEY` replaces `OP_SUCCESS203` (0xcb). `OP_INTERNALKEY` pushes the 32-byte x-only representation of the _taproot internal key_ (referred to as _p_), as defined in [BIP 341], to the stack. ## Motivation ### Key spend with additional conditions When building taproot outputs, especially those secured by an aggregate key representing more than one signer, 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 8 vBytes. ### Mitigated control block overhead for scripts using hash locks In cases where key 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 8 vBytes. Note: The internal key must be the X coordinate of a point on the SECP256K1 curve, so any such hash must be checked and modified until it is such an X coordinate. This will typically take approximately 2 attempts. ## Reference Implementation A reference implementation is provided here: https://github.com/bitcoin/bitcoin/pull/29269 ## 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_SUCCESS203`, `OP_INTERNALKEY` would invalidate their spend. ## Deployment TBD ## Credits TODO ## Copyright This document is licensed under the 3-clause BSD license. [BIP 341]: https://github.com/bitcoin/bips/blob/master/bip-0341.mediawiki [BIP 342]: https://github.com/bitcoin/bips/blob/master/bip-0342.mediawiki