Subject: [bitcoindev] BIP for OP_CHECKSIGFROMSTACK

From: Brandon Black <[email-masked]>
Date: Wed, 24 Apr 2024 22:12:52 -0700

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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