Author: Biteye, Core Contributor Wilson Lee

Original Editor: Biteye, Core Contributor Crush

I. Introduction

It is well known that Bitcoin cannot achieve general computation, and many early public chains, including Ethereum, are committed to breaking through this limitation to bring general computation to the blockchain, while Bitcoin remains in the position of "digital gold".

After the rise of new Bitcoin assets such as inscriptions and runes, the market realized the huge potential for "digital gold" to expand, and various Bitcoin expansion solutions emerged, forming a prosperous situation. Among them, the most eye-catching is the return of the OP_CAT proposal.

With the introduction of OP_CAT, STARK technology will be able to help Bitcoin achieve zero-knowledge proof verification, thereby introducing true general computation capabilities to Bitcoin.

In July of this year, StarkWare initiated a $1 million OP_CAT research fund, aiming to promote research on the pros and cons of activating OP_CAT on Bitcoin. It is not difficult to see that StarkWare has great potential to demonstrate its strength in the Bitcoin OP_CAT era.

II. The Past and Present of OP_CAT

Departure of OP_CAT

OP_CAT is an opcode in the Bitcoin script, which functions to concatenate two elements in the stack into one. This is very useful when building complex transaction scripts and can increase the flexibility of the script.

Bitcoin script is a stack-based programming language, and opcodes are its underlying basic instructions. Bitcoin script uses these opcodes to perform functions such as conditional judgment and signature verification, but its computational capabilities are relatively limited.

Ethereum, by introducing the Ethereum Virtual Machine (EVM), has given the blockchain more powerful computational capabilities. EVM allows developers to write arbitrarily complex smart contracts. EVM also relies on opcodes to issue basic instructions to the computer, similar to Bitcoin's opcodes, but with a wider range of functions.

The key difference is that Bitcoin's opcodes are mainly used to verify the validity of transactions, while Ethereum's opcodes are used to execute more complex logic. This difference allows Ethereum to achieve general computation, while Bitcoin's computational capabilities are relatively limited.

The decentralized nature of blockchain means that computational resources are very valuable, so it is necessary to prevent malicious attacks (such as DDOS) from excessively consuming resources. Ethereum controls the computational resource consumption of each transaction through gas limits. When the gas is exhausted, the transaction stops executing, preventing the entire Ethereum network from being stuck in endless computation for a single transaction.

OP_CAT can introduce more logic for single computation by concatenating stack elements, which gives the Bitcoin script a certain degree of flexibility, but also faces the risk of DDOS attacks.

For security reasons, Satoshi Nakamoto removed the OP_CAT opcode in 2010 to reduce the attack surface, which also caused Bitcoin to lose a certain degree of script flexibility, especially when data concatenation is needed.

The Road to the Return of OP_CAT: Expansion and Controversy

With the expansion of the Bitcoin network and the emergence of more functional requirements, the community has begun to re-examine OP_CAT, believing that it may play an important role in expansion solutions.

In recent years, discussions on reintroducing OP_CAT have continued to heat up, especially in its potential connection with Bitcoin expansion and smart contracts. With the updates of Bitcoin protocols such as Taproot, concerns about security and memory usage have gradually been addressed, and the call to re-enable OP_CAT has begun to rise.

In October 2023, the OP_CAT proposal put forward by developers Ethan Heilman and Armin Sabouri received widespread attention.

The proposal aims to restore the OP_CAT opcode through a soft fork, which will significantly enhance the functionality of the Bitcoin script, especially in implementing complex contract functions in Tapscript (Bitcoin Taproot transaction script language).

With the rise of inscriptions and runes, discussions around OP_CAT have become more formal this year. With the community's push, the OP_CAT proposal officially received the designation BIP-420 (BIP stands for Bitcoin Improvement Proposal).

This designation was later changed to BIP-347. The main purpose of BIP-347 is to introduce more complex covenants, allowing for more complex smart contracts, cross-chain bridges, and on-chain transactions. The implementation of covenants will enable the introduction of functions such as "vault" style transactions, reversible payments, periodic payments, and complex financial instruments (such as custody and bonds) in Bitcoin.

OP_CAT can bring potential benefits but also faces some challenges. For example, implementing this improvement may increase the complexity of Bitcoin, bringing risks of security and network forks. In addition, some community members are concerned that new features may affect the simplicity and accessibility of Bitcoin.

Therefore, the impact of the return of OP_CAT still requires continuous discussion and exploration.

III. Why STARK

STARK is a zero-knowledge proof system developed by StarkWare. Similar to the well-known SNARK, STARK also achieves scalability by transforming the execution process of complex programs into easily verifiable zero-knowledge proofs. This approach can significantly compress the computational load of a large number of transactions and quickly verify their correctness.

Basic Idea of Zero-Knowledge Proof

The core idea of zero-knowledge proof technology is to transform the result of a complex computation into a simple and quickly verifiable "proposition," without the verifier needing to re-execute the computation process to confirm the correctness of the result.

For example, if a complex computation process takes several seconds or even minutes to complete, the most direct way for the verifier to verify the result is to repeat the calculation for several seconds or even minutes. However, by transforming the entire computation process into a zero-knowledge proof, the verification process can be reduced to milliseconds.

Technical Differences Between STARK and SNARK

The key difference between STARK and SNARK lies in the mathematical foundation they use for zero-knowledge proof computation and verification.

SNARK mainly relies on elliptic curve pairing operations. Although this operation can achieve concise zero-knowledge proofs, it does not involve hash operations. In addition, the operation of SNARK relies on the properties of elliptic curves, which may limit its application in certain scenarios.

In contrast to SNARK, STARK relies entirely on hash functions and polynomial commitments as its core operations. Hash functions are widely used cryptographic tools in blockchain systems such as Bitcoin. They map arbitrary-length inputs to fixed-length outputs, providing efficient computation and strong security.

Adaptability: STARK and Bitcoin

Since the Bitcoin system itself is built around hash computations, this makes the computation method of STARK highly compatible with Bitcoin's native computation method.

STARK's hash operations can be more directly combined with Bitcoin's existing hash computation logic. This adaptability means that STARK can more efficiently implement zero-knowledge proofs on the Bitcoin network without requiring major modifications to Bitcoin's existing computation mechanism.

Why is OP_CAT a Prerequisite?

The role of OP_CAT is to concatenate elements in the stack, which is crucial for building complex zero-knowledge proof verification scripts. Through OP_CAT, the Bitcoin script can more flexibly handle the combination of multiple data segments, thereby supporting more complex logical structures in the verification process. This makes the introduction of STARK possible because OP_CAT provides the necessary script capabilities to implement STARK proof verification.

Specifically, the introduction of OP_CAT allows Bitcoin to support the complex data operations required for STARK proofs, such as concatenation, verification, and iterative operations. These operations are indispensable in the generation and verification process of zero-knowledge proofs. Through OP_CAT, Bitcoin can maintain efficient verification and security without introducing Turing completeness, thereby enabling the application of STARK on the Bitcoin network.

IV. Summary and Outlook

Bitcoin's scalability, as a new focus of the industry, is crucial for the sustainable development of the industry. In this race, StarkWare has demonstrated strong innovative capabilities in the field of zero-knowledge proofs and scalability due to its leading position.

However, the successful application of STARK still depends on the further development of features such as OP_CAT. We look forward to the continuous efforts and exploration of various technical teams in this field, jointly promoting the evolution of the Bitcoin ecosystem.

Looking ahead, with the introduction of OP_CAT, StarkWare is expected to shine in this new era, further consolidating its leading position in blockchain scalability. We are optimistic about StarkWare's potential in promoting Bitcoin scalability and improving network efficiency, and look forward to their bringing more breakthroughs and innovations to blockchain technology.

Risk Warning: The above is for information sharing only and is not investment advice. Readers should comply with local laws and regulations.

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