BIP 349: OP_INTERNALKEY
Also known as: BIP349, BIP-0349.
In plain English: Adds a small script instruction that places a Taproot output's 'internal key', the main key behind the output, onto the script's working data. It saves space by avoiding repeating that key inside scripts, and is designed to complement other proposed instructions for advanced contracts.
- Status
- Draft
- Type
- Specification
- Layer
- Consensus (soft fork)
- Authors
- Brandon Black, Jeremy Rubin
- Assigned
- Last changed
- Topics
- Wallets
- Source
- bip-0349.md on GitHub · Markdown
Full specification
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.
Re-Keying with Merkle Root Preservation
Consider a program such CTV <X> CSFS <S+1> CLTV. Such fragments are useful for LN-Symmetry applications.
Such a program would be embedded within a Taproot script path, such as TR(X, {CTV <X> CSFS <S+1> CLTV}).
Were the internal key to be updated from X to Y, the resulting program would be: TR(Y, {CTV <X> CSFS <S+1> CLTV}).
The key in the leaf and the key-path would be mismatched. Were OP_INTERNALKEY to be used,
the leaf would automatically re-key.
E.g., TR(X, {CTV OP_INTERNALKEY CSFS <S+1> CLTV}) is equivalent to TR(X, {CTV <X> CSFS <S+1> CLTV})
and TR(Y, {CTV OP_INTERNALKEY CSFS <S+1> CLTV}) is equivalent to TR(Y, {CTV <Y> CSFS <S+1> CLTV}).
While this particular example is contrived, the general technique of using OP_INTERNALKEY
as updatable across an entire script tree is a helpful covenant primitive when it is desirable to
invalidate signatures from prior states. For example, the theoretical OP_TAPLEAFUPDATEVERIFY opcode
modifies the internal key directly to remove or add a participant, and OP_INTERNALKEY would ensure
that the tweaked key is used from all script paths where desired.
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
The concept for INTERNALKEY first arose in a discussion between Russell O'Connor and Jeremy Rubin in Bitcoin Wizards IRC, inspired by BIP-0118's key punning technique for the internal key. It was later drafted into this BIP by Brandon Black.
Copyright
This document is licensed under the 3-clause BSD license.