Re: [bitcoin-dev] Proposed BIP for OP_CAT

[Steven Roose <steven@roose.io>]

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I agree that there is no reason not to use OP_SUCCESS126, i.e. the 
original OP_CAT opcode 0x7e. In many codebases, for example in Core, 
there might be two OP_CAT constants than which might be confusing.

On 10/22/23 09:58, vjudeu via bitcoin-dev wrote:
> > This opcode would be activated via a soft fork by redefining the 
> opcode OP_SUCCESS80.
> Why OP_SUCCESS80, and not OP_SUCCESS126? When there is some existing 
> opcode, it should be reused. And if OP_RESERVED will ever be 
> re-enabled, I think it should behave in the same way, as in 
> pre-Taproot, so it should "Mark transaction as invalid unless occuring 
> in an unexecuted OP_IF branch". Which means, "<condition> OP_VERIFY" 
> should be equivalent to "<condition> OP_NOTIF OP_RESERVED OP_ENDIF".
>
>
> On 2023-10-21 07:09:13 user Ethan Heilman via bitcoin-dev 
> <bitcoin-dev@lists.linuxfoundation.org> wrote:
>
>     Hi everyone,
>
>     We've posted a draft BIP to propose enabling OP_CAT as Tapscript opcode.
>     https://github.com/EthanHeilman/op_cat_draft/blob/main/cat.mediawiki
>
>     OP_CAT was available in early versions of Bitcoin. It was disabled as
>     it allowed the construction of a script whose evaluation could create
>     stack elements exponential in the size of the script. This is no
>     longer an issue in the current age as tapscript enforces a maximum
>     stack element size of 520 Bytes.
>
>     Thanks,
>     Ethan
>
>     ==Abstract==
>
>     This BIP defines OP_CAT a new tapscript opcode which allows the
>     concatenation of two values on the stack. This opcode would be
>     activated via a soft fork by redefining the opcode OP_SUCCESS80.
>
>     When evaluated the OP_CAT instruction:
>     # Pops the top two values off the stack,
>     # concatenate the popped values together,
>     # and then pushes the concatenated value on the top of the stack.
>
>     OP_CAT fails if there are less than two values on the stack or if a
>     concatenated value would have a combined size of greater than the
>     maximum script element size of 520 Bytes.
>
>     ==Motivation==
>     Bitcoin tapscript lacks a general purpose way of combining objects on
>     the stack restricting the expressiveness and power of tapscript. For
>     instance this prevents among many other things the ability to
>     construct and evaluate merkle trees and other hashed data structures
>     in tapscript. OP_CAT by adding a general purpose way to concatenate
>     stack values would overcome this limitation and greatly increase the
>     functionality of tapscript.
>
>     OP_CAT aims to expand the toolbox of the tapscript developer with a
>     simple, modular and useful opcode in the spirit of Unix[1]. To
>     demonstrate the usefulness of OP_CAT below we provide a non-exhaustive
>     list of some usecases that OP_CAT would enable:
>
>     * Tree Signatures provide a multisignature script whose size can be
>     logarithmic in the number of public keys and can encode spend
>     conditions beyond n-of-m. For instance a transaction less than 1KB in
>     size could support tree signatures with a thousand public keys. This
>     also enables generalized logical spend conditions. [2]
>     * Post-Quantum Lamport Signatures in Bitcoin transactions. Lamport
>     signatures merely requires the ability to hash and concatenate values
>     on the stack. [3]
>     * Non-equivocation contracts [4] in tapscript provide a mechanism to
>     punish equivocation/double spending in Bitcoin payment channels.
>     OP_CAT enables this by enforcing rules on the spending transaction's
>     nonce. The capability is a useful building block for payment channels
>     and other Bitcoin protocols.
>     * Vaults [5] which are a specialized covenant that allows a user to
>     block a malicious party who has compromised the user's secret key from
>     stealing the funds in that output. As shown in A. Poelstra, "CAT
>     and Schnorr Tricks II", 2021,
>     https://www.wpsoftware.net/andrew/blog/cat-and-schnorr-tricks-ii.html
>     OP_CAT is sufficent to build vaults in Bitcoin.
>     * Replicating CheckSigFromStack  A. Poelstra, "CAT and Schnorr
>     Tricks I", 2021,
>     https://medium.com/blockstream/cat-and-schnorr-tricks-i-faf1b59bd298
>       which would allow the creation of simple covenants and other
>     advanced contracts without having to presign spending transactions,
>     possibly reducing complexity and the amount of data that needs to be
>     stored. Originally shown to work with Schnorr signatures, this result
>     has been extended to ECDSA signatures. [6]
>
>     The opcode OP_CAT was available in early versions of Bitcoin. However
>     OP_CAT was removed because it enabled the construction of a script for
>     which an evaluation could have memory usage exponential in the size of
>     the script.
>     For instance a script which pushed an 1 Byte value on the stack then
>     repeated the opcodes OP_DUP, OP_CAT 40 times would result in a stack
>     value whose size was greater than 1 Terabyte. This is no longer an
>     issue because tapscript enforces a maximum stack element size of 520
>     Bytes.
>
>     ==Specification==
>
>     Implementation
>
>        if (stack.size() < 2)
>          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
>        valtype vch1 = stacktop(-2);
>        valtype vch2 = stacktop(-1);
>
>        if (vch1.size() + vch2.size() > MAX_SCRIPT_ELEMENT_SIZE)
>            return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
>
>        valtype vch3;
>        vch3.reserve(vch1.size() + vch2.size());
>        vch3.insert(vch3.end(), vch1.begin(), vch1.end());
>        vch3.insert(vch3.end(), vch2.begin(), vch2.end());
>
>        popstack(stack);
>        popstack(stack);
>        stack.push_back(vch3);
>
>     The value of MAX_SCRIPT_ELEMENT_SIZE is 520 Bytes == Reference Implementation == [Elements](https://github.com/ElementsProject/elements/blob/master/src/script/interpreter.cpp#L1043) ==References== [1]: R. Pike and B. Kernighan, "Program design in the UNIX environment", 1983,https://harmful.cat-v.org/cat-v/unix_prog_design.pdf  [2]: P. Wuille, "Multisig on steroids using tree signatures", 2015,https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-July/019233.html  [3]: J. Rubin, "[bitcoin-dev] OP_CAT Makes Bitcoin Quantum Secure [was CheckSigFromStack for Arithmetic Values]", 2021,https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-July/019233.html  [4]: T. Ruffing, A. Kate, D. Schröder, "Liar, Liar, Coins on Fire: Penalizing Equivocation by Loss of Bitcoins", 2015,https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.727.6262&rep=rep1&type=pdf  [5]: M. Moser, I. Eyal, and E. G. Sirer, Bitcoin Covenants,http://fc16.ifca.ai/bitcoin/papers/MES16.pdf  [6]: R. Linus, "Covenants with CAT and ECDSA", 2023,https://gist.github.com/RobinLinus/9a69f5552be94d13170ec79bf34d5e85#file-covenants_cat_ecdsa-md  _______________________________________________ bitcoin-dev mailing listbitcoin-dev@lists.linuxfoundation.org  https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>
>
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