Earlier this year, I was writing "13 Lines of Code to Help BTC Implement Smart Contract? When reading OP_CAT soft fork, OP_CAT is still an unfamiliar term to many people. If it weren’t for the BTC Non-fungible Token project Taproot Wizards to launch the Quantum Cat series Non-fungible Token with the help of meme culture to build momentum for OP_CAT, few people would probably know about this seemingly boring technical concept.

But just half a year later, OP_CAT is already hot and is even regarded as the next “Lighting Network”. Fractal Bitcoin, the recently popular fractal network, is actually the code implementation of OP_CAT in BTC, and Token FB has tripled or quadrupled within a few days of its launch. Therefore, in the BTC ecosystem, a number of OP_CAT-based protocols were born in just one day, such as CAT20 on the fractal network, which also focuses on the concept of OP_CAT, and the GAS of the parting network was once pulled to more than 5000 in the days of mint, and the current price of a CAT over-the-counter is around 5.5 dollars, and there are even very few makers with a price, and the early mint people have achieved 5 to 20 times the increase. And Quantum Cat has always been able to maintain a price of 0.25BTC and become a blue chip of BTC Non-fungible Token.

It seems that this thing can pump as long as it is stained with OP_CAT.

Not only is OP_CAT a source of ‘self-happiness’ in the community, but it is also widely discussed among BTC developers. I have reviewed this year’s BTC Optech newsletter and BTC developer meetings, and OP_CAT frequently appears in the discussions as a regular participant.

Currently, the OP_CAT BTC BIP draft jointly released by Bitcoin Core developers Ethan Heilman and Armin Sabouri has been officially named as BIP347. Many heavyweights in the BTC field, such as Lighting NetworkWhite Paper author Tadge Dryja, Lighting Network’s CTO Olaoluwa Osuntokun, and Blockstream research director Andrew Poelstra, have shown great support for OP_CAT.

Why do Lighting Network developers favor OP_CAT?

To understand this phenomenon, we have to start with the Lighting Network. In 2011, Satoshi Nakamoto mentioned the embryonic form of the Lighting Network in an email, as a key solution for BTC scalability and payment speed improvement. From the day of its birth, the Lighting Network has attracted a large number of developers’ follow.

In the past two years, the concept of “BTC ecosystem” has been gaining momentum. Various BTC expansion solutions such as Sidechains and Virtual Machine have emerged one after another, attracting many investors. However, they are not recognized by senior BTC developers and are difficult to enter the mainstream. After all, Lighting Network is currently the most outstanding “payment channel” in BTC in the eyes of the public, and it is one of the few “legitimate children” left by Satoshi Nakamoto and the “legitimate son” in the BTC community spirit. Related reading: “Seven years of forging a sword, the origins and challenges of the legitimacy of the Lighting Network”.

However, there has been a sudden change in the past year. Several developers have announced their withdrawal from work related to the Lighting Network, and voices denying the Lighting Network have been emerging, especially from some senior developers. Fiatjaf, the founder of Nostr, is particularly blunt: ‘The Lighting Network has been deceiving BTC users’ time, energy, and money for as long as 6 years.’

Against this background, some developers are looking for the next ‘Lighting Network’. OP_CAT, on the other hand, is considered by many BTC developers to be the next major breakthrough for BTC after the Lighting Network. Due to the technical nature of OP_CAT, let us first get to know those BTC developers who favor OP_CAT before discussing what OP_CAT is and what OP_CAT can do.

BTC developers supporting OP_CAT

Lighting Network White Paper Author

Tadge Dryja is one of the authors of the Lighting NetworkWhite Paper, and in 2015 Tadge Dryja and Joseph Poon, another author of the Lighting NetworkWhite Paper, led the lead by Elizabeth Stark, created Lightning Labs. The biggest difference between Lightning Labs and BlockStream, another BTCLighting Network company, is that Lightning Labs uses the Go programming language, while Blockstream uses the C programming language.

However, at the end of 2016, Tadge Dryja had a disagreement and argument with the Lightning Labs team, so when Lightning Labs was only one year old, Tadge Dryja chose to leave and instead joined the Digital Currency Initiative (DCI) at the Massachusetts Institute of Technology (MIT), continuing his research on the Lighting Network. At MIT DCI, he participated in multiple research projects, mainly focusing on the scalability and interoperability of Cryptocurrency and Smart Contracts. This includes the development of the Utreexo project, which is a new BTC scalability technology aimed at making BTC Nodes smaller and faster. In 2022, Tadge Dryja joined Lightspark as a senior research scientist. At Lightspark, he continues to address scalability issues with BTC and blockchain, leveraging his expertise as a co-inventor of the Lighting Network.

It is precisely because of his deep knowledge of BTCSmart contract and scalability technology that Tadge Dryja also recognized the potential of OP_CAT early on, and has been supporting testing OP_CAT on the BTC testnet, and encouraging developers to try to “break” OP_CAT to find potential problems. **

Lightning Labs CTO

Olaoluwa Osuntokun (Roasbeef) is another co-founder and CTO of Lightning Labs, and also an outstanding BTC developer who has made important contributions to the development of the Lighting Network. It can be said that the early team of Lightning Labs is no less competitive than Blockstream.

When it comes to Olaoluwa Osuntokun, it is impossible not to mention the ‘gentleman’s dispute’ between him and Tadge Dryja. Interestingly, one of the main reasons why Tadge Dryja left Lightning Labs was because of Olaoluwa. When Tadge Dryja was working at Lightning Labs, he developed the first version of the protocol named LIT, which was not compatible with BOLT developed by Blockstream. However, the protocol developed by Olaoluwa was compatible, gradually gaining recognition and favor from more developers. This ultimately led to Olaoluwa surpassing Dryja to some extent and accelerated Dryja’s departure.

However, history is always full of drama. Now, these two “old enemies” from Lightning Labs are currently supporting OP_CAT. **

Blockstream Research Director

There seems to have always been a rumor among the public: Blockstream is the “behind-the-scenes player” of BTC. This rumor is not groundless. In 2014, “the father of PoW” Adam Back gathered many well-known early BTC developers such as Matt Corallo, Greg Maxwell, Pieter Wuille, etc., to establish the Blockstream company, and clearly took a stand and promoted the Lighting Network in the subsequent BTC scaling war, giving Lighting Network its current status.

And for now, Andrew Poelstra, Blockstream’s head of research, is a guy who can never get around the OP_CAT discussion. Andrew Poelstra is the Director of Research at Blockstream and a veteran BTC cryptographic scripting developer with a strong presence in the industry. He discussed OP_CAT back in an article titled “CAT and Schnorr Tricks I” on January 30, 2021, pointing out that using OP_CAT in conjunction with CHECKSIGFROMSTACK can provide a clever way to introspect transactions.

Although Blockstream currently does not create a separate blockchain for OP_CAT, they have provided support for OP_CAT testing through Bitcoin Inquisition (a test network for evaluating new BTC features). It’s like building a secure “training ground” for OP_CAT, allowing researchers to carefully observe its performance under real-world conditions.

Andrew Poelstra also compared and discussed other proposals such as OP_CAT and OP_CTV, pointing out that OP_CAT provides greater flexibility while OP_CTV focuses more narrowly on non-recursive smart contracts.

Author of BIP 347

In the push of OP_CAT, BTC developers Ethan Heilman and Armin Sabouri contributed no small amount, and they co-authored and advanced OP_CAT’s proposal, which has been officially designated as BIP 347. This proposal aims to redefine the OP_SUCCESS126 of the Operation Code through Soft Fork in order to re-enable the OP_CAT.

Armin Sabouri has previously focused on improving BTC scripts and consensus layers. He believes that although OP_CAT is not the ultimate solution, it provides the possibility to implement various new functions and is an important step in improving BTC scripts.

And Ethan Heilman is also the person who influenced Andrew Poelstra’s thinking. It was their private communication in the autumn of 2019 that changed Andrew Poelstra’s original opposition to the so-called BTCSmart Contract functionality. Ethan Heilman pointed out that although people have concerns about the so-called BTCSmart Contract functionality, these so-called harmful Smart Contracts can actually be implemented through CHECKMULTISIG. To prove this point, Ethan Heilman launched a challenge on social media, encouraging people to propose feasible ‘dark’ Smart Contracts, but so far no one has succeeded.

StarkWare

StarkWare is a company specializing in the development of blockchain scaling technology, particularly adept at using Zero-Knowledge Proofs (ZKPs) to enhance the privacy and efficiency of transactions. The company is committed to outsourcing complex computations from the mainchain (such as Ethereum) to secondary networks without sacrificing security and the characteristics of decentralization. In this way, StarkWare solves the scalability problem of blockchain while maintaining transaction transparency and verifiability.

StarkWare’s drive to promote the activity of OP_CAT stems from its interest in scaling the BTC network and Smart Contract capabilities. OP_CAT is a BTC opcode that allows for more complex transactions and contracts to be executed on the BTC network through the concatenation of data. By enabling OP_CAT, StarkWare aims to achieve Smart Contract capabilities similar to Ethereum on BTC, thereby expanding the use cases and enhancing the functionality of BTC.

In July 2022, StarkWare launched a $1 million research fund specifically for researching the pros and cons of enabling OP_CAT on BTC. The fund aims to support researchers and developers who have innovative ideas on how to safely and effectively implement OP_CAT on BTC.

In addition, StarkWare has also demonstrated the potential of using OP_CAT to implement Zero-Knowledge Proof in the BTC test environment, especially on Signet. These works demonstrate how OP_CAT can be used to achieve more complex operations on the BTC network, such as the application of Zero-Knowledge Proof, which is a key part of Smart Contract functionality. Recently, StarkWare has also established a partnership with sCrypt to explore the design of a PoC (Proof of Concept) bridge based on OP_CAT and ZK magic. It can be seen that StarkWare may also launch a protocol related to OP_CAT in the near future.

Others

Other developers who are not so well-known, let’s put them together.

Salvatore Ingala has been deeply researching the BTC payment pool and smart contracts. He proposed a solution to optimize the payment pool exit process using OP_CAT, believing that this can significantly reduce on-chain data and operational costs. Ingala sees OP_CAT as a potential plugin for other BTC scaling strategies, such as Arc and Coinpools, and even future Optimistic rollups for BTC, to enhance efficiency and security.

Anthony Towns is one of the main developers of the Bitcoin Inquisition, which is a more flexible tool similar to a test network for testing BTC protocol changes that are not yet widely accepted. Towns has advocated for the activation of OP_CAT on the Inquisition platform, providing a secure “experimental field” for OP_CAT. Although Towns recognizes the importance of OP_CAT in testing new features and exploring the programmability of BTC scripts, he remains cautious about adding too much programmability to BTC, fearing that it may increase the risk of BTC being scrutinized or controlled.

Robin Linus, the creator of BitVM, believes that the reintroduction of OP_CAT provides a powerful tool for BTC, especially to support projects like BitVM that make it easier and more efficient to verify arbitrary computation on BTC. With the execution of OP_CAT, the BTC ecosystem is able to create more versatile and expressive Smart Contracts, facilitating a more functional and practical blockchain environment. Related Reading: What Do Veteran Developers Say About BitVM to Calculate Anything on BTC? 》

Why is the development of the Lighting Network so follow?

Developers of the Lighting Network have shown such a strong interest in OP_CAT, what is the mystery behind this? To understand this, we need to first understand the essence of OP_CAT.

What is OP_CAT?

OP_CAT is an opcode in BTC script, used to concatenate two data segments on the stack into a larger element. Although its function seems simple, it can provide more Smart Contract capabilities for the BTC network, allowing developers to create and handle more complex data and transaction logic.

OP_CAT is not a completely new opcode, it first existed in earlier versions of BTC, but it was disabled in later versions due to security and complexity concerns. However, as BTC continues to grow, more and more developers believe that it is time to re-enable OP_CAT. For more information, see: 13 lines of code to help BTC implement Smart Contract? Understanding OP_CAT Soft Fork》

But why would such a seemingly simple operation provoke such a big discussion among BTC developers? One key reason is that it can significantly enhance the Smart Contract capability of the BTC network, enabling developers to implement functionalities that are complex or even impossible to achieve in the current BTC script.

The Potential Application of OP_CAT in Payments

OP_CAT has shown great potential in the payment system, especially in off-chain protocol and payment channel networks. Its reactivation will greatly improve the operational efficiency of these systems and reduce the burden of on-chain transactions. The main functions after the implementation of OP_CAT are:

Multi-signature (Multisig) Optimization: In the Multi-signature scenario, OP_CAT can help users merge multiple signatures, combining them into a single data block, reducing the number of signatures that need to be submitted. This not only saves on-chain space, but also reduces Money Laundering. Multi-signature is very important for payment security and shared account management in BTC, especially in applications such as Lighting Network, where OP_CAT can make this process more efficient.

State Contracts: OP_CAT can also be used for state contracts. This kind of contract is a form of Smart Contract that can maintain the state across multiple transactions, and through the OP_CAT, developers can stitch together the state information of different transactions to achieve more complex contract logic on BTC. For example, some complex payment protocols or distributed applications (such as lotteries, lotteries, or other complex financial products) need to maintain a certain state between multiple on-chain transactions, which can be achieved through OP_CAT.

Scalability of payment channel networks: Another important use case for OP_CAT is a payment channel network similar to the Lighting Network. In the payment channel network, users usually make a large number of micropayments on the off-chain, and only on the chain will they be on the chain when the final settlement is made. The splicing function of OP_CAT can make the intermediate transactions in the payment channel more effectively managed and verified. By stitching together different payment requests, users can perform more complex payment operations without increasing the burden on the on-chain. In this way, the throughput and efficiency of the payment channel can be significantly improved.

With the advancement of BIP347 proposal, and more developers and researchers joining the exploration of OP_CAT, we have reason to believe that this once shelved opcode will bring new vitality to the BTC network. Just as Lighting Network went from concept to mainstream, the reactivation of OP_CAT may also become the next key milestone for BTC scalability and payment innovation.