> This is no longer an issue in the current age as tapscript enforces a maximum stack element size of 520 Bytes.
I don't think there's a new limit related to tapscript? In the very beginning there was no limit, but a 5k limit was put into place, then 520 the same commit that OP_CAT was disabled: 4bd188c4383d6e614e18f79dc337fbabe8464c82 On Sat, Oct 21, 2023 at 1:09 AM 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 <ref>A. Poelstra, "CAT > and Schnorr Tricks II", 2021, > https://www.wpsoftware.net/andrew/blog/cat-and-schnorr-tricks-ii.html > </ref> > OP_CAT is sufficent to build vaults in Bitcoin. > * Replicating CheckSigFromStack <ref> A. Poelstra, "CAT and Schnorr > Tricks I", 2021, > https://medium.com/blockstream/cat-and-schnorr-tricks-i-faf1b59bd298 > </ref> 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 > <pre> > 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); > </pre> > > 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 list > bitcoin-dev@lists.linuxfoundation.org > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev >
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