From: Brandon Black <freedom@reardencode•com>
To: bitcoindev@googlegroups.com
Cc: j@rubin•io
Subject: Re: [bitcoindev] BIP for OP_CHECKSIGFROMSTACK
Date: Tue, 14 May 2024 14:55:20 -0700 [thread overview]
Message-ID: <ZkPdyMzR3DKkUlQd@console> (raw)
In-Reply-To: <ZinmVPFt9VQn8QLF@console>
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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prev parent reply other threads:[~2024-05-15 16:15 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-25 5:12 Brandon Black
2024-04-25 11:44 ` Andrew Poelstra
2024-05-14 21:55 ` Brandon Black [this message]
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