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Date: Tue, 14 May 2024 14:55:20 -0700
From: Brandon Black <freedom@reardencode.com>
To: bitcoindev@googlegroups.com
Cc: j@rubin.io
Subject: Re: [bitcoindev] BIP for OP_CHECKSIGFROMSTACK
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Hello list,

Two considerations for this BIP that I'd love thoughts on:

# Should this include an implementation of CHECKSIGFROMSTACKADD?

I had been inclined to leave it out since it can be implemented using a
few opcodes and the altstack, but upon seeing some of the advanced
miniscripting that Rob Hamilton showed in his talk in ATX, I can see
that CSFSA would potentially be more commonly used than I had previously
realized.

I'm inclined to add CSFSA and use another SUCCESS.

# Should CHECKSIGFROMSTACKVERIFY be switched to tapscript only?

Given that I intended this partly to be used with BIP119 and CTV is a
NOP upgrade and available in legacy scripts, I had included CSFSV as a
NOP, but I'm curious to hear other opinions on this.

Thanks kindly,

--Brandon

On 2024-04-24 (Wed) at 22:12:52 -0700, Brandon Black wrote:
> Hello list,
> 
> Back in 2021, Jeremy wrote[0] about bringing OP_CHECKSIGFROMSTACK (or
> OP_CHECKDATASIG) to bitcoin. That email proposed adopting the
> specification from Bitcoin Cash for Bitcoin, but it is not directly
> suitable, as it verifies DER encoded ECDSA signatures and not R||S
> encoded BIP340 Schnorr signatures. The BIP here included, and proposed
> for the BIPs repository[2] is a bitcoin-specific design for
> OP_CHECKSIGFROMSTACK and OP_CHECKSIGFROMSTACKVERIFY. It further differs
> from Jeremy's email by specifying the repurposing of a NOP (NOP5) for
> OP_CHECKSIGFROMSTACKVERIFY to bring data signature verification to all
> script types, not only tapscript (although this is subject to
> change)[1].
> 
> -----------
> ## Abstract
> 
> This BIP describes two new opcodes for the purpose of checking
> cryptographic signatures in bitcoin scripts against data other than
> bitcoin transactions.
> 
> ## Summary
> 
> We propose replacing `OP_NOP5` (0xb4) in bitcoin script with
> `OP_CHECKSIGFROMSTACKVERIFY`. When verifying taproot script spends
> having leaf version 0xc0 (as defined in [BIP 342]), we propose
> `OP_CHECKSIGFROMSTACK` to replace `OP_SUCCESS204` (0xcc).
> 
> `OP_CHECKSIGFROMSTACK` and `OP_CHECKSIGFROMSTACKVERIFY` have semantics
> similar to `OP_CHECKSIG` and `OP_CHECKSIGVERIFY` respectively, as
> specified below.
> 
> Only 32-byte keys are constrained. Similar to [BIP 341] unknown key
> types, for other key lengths no signature verification is performed.
> 
> ## Specification
> 
> * If fewer than 3 elements are on the stack, the script MUST fail and
>   terminate immediately.
> * The public key (top element), message (second to top element), and
>   signature (third from top element) are read from the stack.
> * For `OP_CHECKSIGFROMSTACK` the top three elements are popped from the
>   stack.
> * If the public key size is zero, the script MUST fail and terminate
>   immediately.
> * If the public key size is 32 bytes, it is considered to be a public
>   key as described in [BIP 340]:
>     * If the signature is not the empty vector, the signature is
>       validated against the public key and message according to [BIP
>       340]. Validation failure in this case immediately terminates
>       script execution with failure.
> * If the public key size is not zero, and it is not a [BIP 340] public
>   key; the public key is of an unknown public key type, and no actual
>   signature verification is applied. During script execution of
>   signature opcodes they behave exactly as known public key types except
>   that signature validation is considered to be successful.
> * If the script did not fail and terminate before this step, regardless
>   of the public key type:
>     * If the signature is the empty vector:
>         * For `OP_CHECKSIGFROMSTACKVERIFY`, the script MUST fail and
>           terminate immediately.
>         * For `OP_CHECKSIGFROMSTACK`, an empty vector is pushed onto the
>           stack, and execution continues with the next opcode.
>     * If the signature is not the empty vector:
>         * For tapscript 0xc0, the opcode is counted towards the sigops
>           budget as described in [BIP 342].
>         * For legacy and segwit v0, the opcode is counted towards the
>           sigops limit, as described in [BIP 141]
>         * For `OP_CHECKSIGFROMSTACKVERIFY`, execution continues without
>           any further changes to the stack.
>         * For `OP_CHECKSIGFROMSTACK`, a 1-byte value 0x01 is pushed onto
>           the stack.
> 
> ## Design Considerations
> 
> 1. Message hashing: [BIP 340] is compatible with any size of message and
>    does not require it to be a securely hashed input, so the message is
>    not hashed prior to [BIP 340] verification.
> 2. Verify NOP upgrade: To bring stack signature verification to legacy
>    and segwitv0 bitcoin script, a NOP upgrade path was chosen for
>    `OP_CHECKSIGFROMSTACKVERIFY`. This necessarily means leaving the 3
>    arguments on the stack when executing `OP_CHECKSIGFROMSTACKVERIFY`.
>    Scripts will need to drop or otherwise manage these stack elements.
> 3. Add/multisig: No concession is made to `OP_CHECKMULTISIG` or
>    `OP_CHECKSIGADD` semantics with `OP_CHECKSIGFROMSTACK(VERIFY)`. In
>    Tapscript, add semantics can be implemented with 1 additional vByte
>    per key (`OP_TOALTSTACK OP_CHECKSIGFROMSTACK OP_FROMALTSTACK
>    OP_ADD`).
> 4. Splitting R/S on the stack: Implementing split/separate signatures is
>    left as an exercise for other bitcoin upgrades, such as `OP_CAT`.
> 5. [BIP 118]-style Taproot internal key: Rather than introducing an
>    additional key type in this change, we suggest implementing
>    OP_INTERNALKEY or separately introducing that key type for all
>    Tapscript signature checking operations in a separate change.
> 6. Unknown key lengths: The semantics of other signature checking
>    opcodes in their respective script types (legacy, segwit-v0,
>    tapscript-c0) are applied.
> 
> ## Resource Limits
> 
> These opcodes are treated identically to other signature checking
> opcodes and count against the various sigops limits and budgets in their
> respective script types.
> 
> ## Motivation
> 
> ### LN Symmetry
> 
> When combined with [BIP 119] (`OP_CHECKTEMPLATEVERIFY`/CTV),
> `OP_CHECKSIGFROMSTACK` (CSFS) can be used in Lightning Symmetry
> channels.  The construction `OP_CHECKTEMPLATEVERIFY <pubkey>
> OP_CHECKSIGFROMSTACK` with a spend stack containing the CTV hash and a
> signature for it is logically equivalent to `<bip118_pubkey>
> OP_CHECKSIG` and a signature over
> `SIGHASH_ALL|SIGHASH_ANYPREVOUTANYSCRIPT`. The `OP_CHECKSIGFROMSTACK`
> construction is 8 vBytes larger.
> 
> ### Delegation
> 
> Using a script like: `<pubkey> SWAP IF 2 PICK SWAP CSFSV ENDIF CHECKSIG`
> either direct verification or delegation can be achieved by the
> following unlock stacks: `<sig> 0` or `<dsig> <dpubkey> <sig> 1`
> 
> ## Reference Implementation
> 
> A reference implementation is provided in provided here:
> 
> https://github.com/bitcoin/bitcoin/pull/29270
> 
> ## Backward Compatibility
> 
> By constraining the behavior of an OP_SUCCESS opcode and an OP_NOP
> 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_SUCCESS204`, `OP_CHECKSIGFROMSTACK` would invalidate their spend.
> 
> ## Deployment
> 
> TBD
> 
> ## Credits
> 
> Reference implementation was made with reference to the implementation
> in Elements and started by moonsettler.
> 
> ## Copyright
> 
> This document is licensed under the 3-clause BSD license.
> 
> [BIP 119]: https://github.com/bitcoin/bips/blob/master/bip-0119.mediawiki
> 
> [BIP 118]: https://github.com/bitcoin/bips/blob/master/bip-0118.mediawiki
> 
> [BIP 340]: https://github.com/bitcoin/bips/blob/master/bip-0340.mediawiki
> 
> [BIP 341]: https://github.com/bitcoin/bips/blob/master/bip-0341.mediawiki
> 
> [BIP 342]: https://github.com/bitcoin/bips/blob/master/bip-0342.mediawiki
> 
> [OP_CAT]: https://github.com/EthanHeilman/op_cat_draft/blob/main/cat.mediawiki
> 
> -----------
> 
> [0]: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-July/019192.html
> [1]: https://github.com/bitcoin/bips/pull/1535#discussion_r1578562450
> [2]: https://github.com/bitcoin/bips/pull/1535
> 
> 
> -- 
> --Brandon
> 
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