Add bip-internalkey

bip-internalkey.md

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+| BIP-XXXX   | `OP_INTERNALKEY`
+| :--------- | :---------------
+| Layer      | Consensus (soft fork)
+| Author     | Brandon Black `<freedom@reardencode.com>`
+| Discussion | https://delvingbitcoin.org/t/lnhance-bips-and-implementation/376/9
+| 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 spends having leaf version `0xc0` (as defined in
+[BIP 342]), `OP_INTERNALKEY` replaces `OP_SUCCESS203` (0xcb). `OP_INTERNALKEY`
+pushes the taproot internal key, 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 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 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/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 `OP_SUCCESS203`, `OP_INTERNALKEY` would
+invalidate their spend.
+
+## Deployment
+
+TBD
+
+## Credits
+
+Jeremy Rubin described most of the ideas for `OP_INTERNALKEY` to me.
+
+## 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