Bitcoin Upgrade OP_CAT Isn’t All It’s Cracked Up To Be, Argues BitVM Creator

Bitcoin’s developer community has spent several months singing the praises of OP_CAT – a proposed soft fork change that many believe is Bitcoin’s next evolution.

BitVM creator Robin Linus has his reservations, however, publishing an article on Thursday debunking perceived misconceptions about the widely touted change.

Is OP_CAT the Future of Bitcoin?

The developer’s post – titled “Prevent the CATastrophe” – argued that OP_CAT can introduce risks to Bitcoin that proponents tend to downplay. “My goal isn’t to argue for or against op_cat, but to debunk these misconceptions and steer the discussion toward a more fact-based, rational analysis,” he wrote.

One includes the introduction of new programs that could greatly alter the incentives of Bitcoin miners, and the security of the entire network.

“The design space for op_cat-based DeFi applications on Bitcoin is largely unexplored, and we cannot foresee all the innovations that might emerge and how they affect MEV (maximal extractable value),” said Linus.

OP_CAT is an old Bitcoin opcode that was initially created to combine – or “concatenate” – two pieces of data. It was removed by Satoshi Nakamoto in 2010 due to security concerns, but was reintroduced in 2023 through BIP 347 as a way to let it handle more complex data structures, like Merkle trees, and enhance its functionality substantially.

On the other hand, many value Bitcoin’s simple code and resistance to change as a beacon of its security and purity. As such, OP_CAT supporters often point to how the change only involves 10 lines of new code, and simply reintroduces old tech – thus minimizing its attack vectors.

“The fact that you might not immediately exploit these 10 lines does not mean that op_cat won’t interfere with the broader system and its incentive structures,” countered Linus. “Since it’s impossible to predict all the consequences of activating op_cat, we cannot confidently assert its safety.”

Covenants on Bitcoin?

One of OP_CATs most touted benefits is its ability to introduce covenants (conditional payments) to Bitcoin. Linus argued that OP_CAT-based covenants would be “inefficient in terms of block space and transaction fees,” and that other opcode proposals would be better suited to this purpose.

In a message to CryptoPotato, popular OP_CAT supporter Bob Bodily – co-founder and CEO of Bioniq – agreed with Linus on his particular criticism regarding covenants. “There are better covenants proposals (CTV) that got violently shut down in the past few years, whereas CAT has broader Bitcoin ecosystem support,” he added.

One of the most popular proposed covenants use cases would be to enable trustless Bitcoin bridges, letting Bitcoin moving to other blockchains and L2 networks in a truly decentralized way. Rather than depending upon this, Linus and several colleagues recently published the “BitVM2” whitepaper, which outlines a method for creating trust-minimized Bitcoin bridges without any changes to Bitcoin Core.

According to Bodily, however, OP_CAT is a “lower level primitive” offering far more than just covenants – including functionality that would help Linus’s creation.

“Take the BitVM model (or BitVM2) and CAT makes it significantly better,” he wrote. “More efficient. Cheaper. More flexible. I don’t know why Robin is coming out openly against CAT like this because it would make everything he is doing better.”

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