Since its launch, the Bitcoin blockchain has undergone several technological developments to expand Bitcoin’s utility. In this article, we look at a proposal to revive an idea that Bitcoin’s creator first implemented but later discarded. 

Keep reading to learn more about the recent OP_CAT proposal, what it did in the past, and how Bitcoin developers propose to use it in the future. 

What Are Bitcoin Improvement Proposals?

Bitcoin Improvement Proposals (BIPs) are suggestions or guidelines on improving and adding functionality to the Bitcoin protocol.

They start as discussions between Bitcoin developers and can comprise protocol changes, technical improvements, network upgrades, introduction of new features, and best practices.

Suggested proposals are put in a draft, which is then reviewed by the Bitcoin community. Once accepted, implementation on the blockchain ensues. 

Developers, node operators, and miners all have a say during the BIP process, making BIPs integral to keeping future developments of the Bitcoin network decentralized.

BIPs typically cover three areas: standards, processes, and information.

What Is OP_CAT?

OP_CAT (short for operation code concatenate) was an opcode introduced by Bitcoin’s creator, Satoshi Nakamoto, in the Bitcoin code stack. This code was part of Bitcoin’s initial scripting system and was utilized to concatenate two strings, but Satoshi disabled it in 2010. 

In October 2023, a new proposal emerged. Taproot Wizards aimed to reintroduce OP_CAT as an opcode for Tapscript (the language used to script Taproot-enabled Bitcoin transactions) by changing the existing opcode OP_SUCCESS126. 

Thus, OP_CAT is the proposed reintroduction of an opcode from the Satoshi era, which would entail a simple amendment to Bitcoin’s core code. The upgrade would occur through a soft fork to ensure the new Bitcoin version doesn’t conflict with the older version. To be implemented, however, it would require considerable stakeholder collaboration. 

Restoring OP_CAT would enable functionalities that couldn’t have been possible before. The proposed opcode would enable two values to merge, giving developers a unique tool to create more powerful smart contracts on Bitcoin.

What Did OP_CAT Do in Bitcoin?

OP_CAT increased Bitcoin transaction utility and expanded the variety of operations users could perform on the Bitcoin platform. 

An opcode is essentially a basic building block in Bitcoin’s scripting language. It joins with other blocks to create complex rules and conditions guiding operations regarding blockchain transactions. This maintains the network’s stability. The opcode also informs the Bitcoin network what to do with the data in its possession.

OP_CAT enabled Bitcoin users to combine (concatenate) two data points within a scripting stack and put the values atop the stack. This action meant these values became the first to be acted upon in a transaction. Concatenated values on a stack ensured the related bitcoin could only be utilized upon meeting specific conditions.

Satoshi removed the OP_CAT opcode and set up a stack size limit because he was concerned about potential vulnerabilities affecting the young Bitcoin network. He noted OP_CAT could be used to launch a denial-of-service (DoS) attack. 

Why Do (Some) Developers Want OP_CAT Back?

Some developers view OP_CAT as an upgrade that would introduce greater functionality into the Bitcoin network, enhancing its capabilities. They see it as a means to support diverse applications in complex data structures and advanced smart contracts and as trustless Layer 2 bridges.

As a foundational component, Bitcoin OP_CAT would empower developers to create more sophisticated Bitcoin scripts, such as spending requirements for Bitcoin transactions necessitating multiple signatures. This fosters the creation of diverse and more secure Bitcoin transactions.

OP_CAT also broadens the scope for scripting, offering developers a versatile and dynamic tool. Moreover, it would address Satoshi’s security concerns brought by merging large stack elements with its 520-byte limit.

OP_CAT’s Potential for Bitcoin

OP_CAT presents great potential for Bitcoin. Let’s take a look at what it could potentially contribute if implemented. 

  • Increased functionality: OP_CAT can widen Bitcoin’s functionalities by providing a valuable way to join or concatenate stack values. This could also help create and evaluate complex data structures within tapscript, e.g., hashed structures.
  • Improved efficiency: OP_CAT presents a universal technique for concatenating stack values, boosting efficiency by streamlining the process. For instance, a user can limit currency usage by specifying the specific transaction templates needed for a particular transaction. 
  • Script expressiveness: Bitcoin Tapscript doesn’t have an all-purpose mechanism for joining items on the stack. This hampers Tapscript’s power and expressiveness. OP_CAT provides a straightforward, versatile, and powerful opcode to concatenate stack values.
  • New use cases: OP_CAT launches an array of potential use cases, including facilitating decentralized file hosting, enabling non-equivocation contracts, tree signatures, post-quantum Bitcoin signatures (Lamport Signatures), wallet management techniques like covenants, and facilitating decentralized token exchange.
  • Enhancing the script capabilities of Bitcoin: This includes token creation and management, increasing payment channels, and defining the rules for accessing and spending bitcoin.

The Drawbacks of OP_CAT

While proponents of the reintroduction of OP_CAT are excited about the potential benefits it could bring to Bitcoin, the BIP also comes with some potential risks and challenges. 

  • Security concerns: Reimplementing OP_CAT could put the security of the Bitcoin network at risk as it would require further testing to ensure it doesn’t create any bugs in the code that could be exploited.
  • Fork risk: A soft fork to implement OP_CAT could result in a chain split if some node operators choose not to upgrade to the new software.
  • Adding complexity to Bitcoin: The implementation of OP_CAT could add additional complexity to Bitcoin, which could hinder mass adoption. 

Final Words

While activation discussions are ongoing, the Bitcoin community hasn’t come to a consensus yet on whether to implement the OP_CAT BIP.

If all parties agree to adopt the proposal, it could herald new developments in Bitcoin’s scripting ecosystem, potentially improving Bitcoin’s functionality and security while unlocking several use cases.