CAT-21 Protocol Specification

Overview

CAT-21 is a new protocol developed to operate on top of the Bitcoin blockchain and leverages the Ordinals concept. It introduces a unique way of representing and transacting digital assets, specifically pixelated cat images, in a decentralized manner. CAT-21 transactions are identified using the nLockTime field in a Bitcoin transaction, with a specific focus on the value 21.

Protocol Rules

In this document, the terms "shall" and "should" are used to convey different levels of requirement:

• "Shall" is used to indicate requirements strictly to be followed in order to conform to the document and from which no deviation is permitted.
• "Should" is used to indicate that among several possibilities one is recommended as particularly suitable, without mentioning or excluding others, or that a certain course of action is preferred but not necessarily required.

Identification of a CAT-21 Mint Transaction

1. nLockTime Value: A transaction is recognized as a CAT-21 mint if its nLockTime value is set to 21. This is a hard requirement, meaning it shall be followed for the transaction to be acknowledged as a CAT-21 mint.
2. Transaction Type: A CAT-21 mint transaction should be a payment to a pay-to-taproot (P2TR) address (which starts with bc1p). This recommendation is made to ensure consistency within the protocol and to ensure the best compatibility with existing Ordinals-aware wallets. However, this is not a strict requirement. Other types of addresses may be used, but P2TR is preferred.
3. First Output Matters: For a mint transaction, only the first output is important. It defines the recipient of the CAT-21 mint. All other inputs and outputs are ignored. Therefore, every Bitcoin transaction can create exactly one cat.
4. Ownership: The ownership of a CAT-21 ordinal is determined by the entity controlling the first Satoshi in the transaction output, in line with the Ordinal Theory.
5. Image Association: Each CAT-21 ordinal is intrinsically linked to an image. This image is generated based on the transaction ID of the mint transaction, creating a unique pixelated cat image.

Transferring CAT-21 ordinals

1. CAT-21 can be transferred using standard Bitcoin transactions.
2. The new owner of a CAT-21 ordinal is the entity controlling the first Satoshi of the mint transaction output. This is the same concept that applies to Ordinals and Inscriptions – but for digital cat artifacts!

Immutability

1. Digital CAT-21 ordinals can't be destroyed, they are immutable on the Bitcoin blockchain.
2. If a CAT-21 ordinal is sent to a "burner" address, the cat still exists but no longer has an owner. (The cat is free!)

Library and Tools

Image Representation

Each CAT-21 ordinal is intrinsically linked to a generated image. The creation of generative cat art is a fundamental part of this protocol. The pixelated cat images are inspired by the famous Mooncat Algorithm, adapted to meet new requirements.

• Seed: The seed for generating the image and traits is retrieved from the concatenated transactionId and blockId, hashed using the SHA-256 algorithm. By using this method, the characteristics of each CAT-21 ordinal will only be determinable after the transaction is included in a block, thus preventing anyone from generating transactions until they get a desirable outcome. This approach ensures fairness and unpredictability in the distribution of rare traits.
• Format: The images should be generated in a web-friendly, scalable format, with SVG being preferred.
• Traits: The existing traits from the original Ethereum Mooncats are utilized, with additional new traits such as red, green, and blue laser eyes and an orange background.
• Dimensions: The dimensions of the generated images shall be squares.
• Storage: Images and Traits are not stored on the blockchain but are generated on-demand using the concatenated transactionId and blockId as a seed.
• Reference Implementation: The artwork is defined by the reference implementation in the ordpool-parser repository (the engine that parses digital artifacts on ordpool.space). All other implementations should result in the exact same visual appearance to maintain consistency across the protocol.

Locating CAT-21 Mint Transactions

Existing CAT-21 Mint Transactions can be discovered using Blockchair's search functionality. The following links provide pre-configured queries according to CAT-21 protocol rules:

1. Query for Mainnet
2. Query for Testnet

Existing CAT-21 Mint Transactions can also be located via the Bitquery Grapqhl API:

query txns {
  bitcoin {
    transactions(txLocktime: {is: 21}) {
      hash
      txLocktime
      block {
        height
      }
    }
  }
}

These queries are designed to filter transactions based the nLockTime set to 21.

The official indexer at https://backend.cat21.spaces uses the Blockchair API as a source of truth and enriches this information with sat ranges from the Ord API.

CAT-21 Ecosystem

• The official website is https://cat21.space
• CAT-21 Mint Transactions in the Mempool are displayed on https://ordpool.space
• Confirmed CAT-21 Mint Transactions are displayed on https://ordimint.com
• MORE LINKS WITH COMPATIBLE SERVICES HERE (PRs welcome!)

Future Development

Open Discussion

The protocol may be expanded to include additional features based on community feedback and technological advancements. Please make a pull request to the cat21 repository to participate in the open development.

The CAT-21 protocol introduces a novel way of digital asset representation and transaction on the Bitcoin blockchain, inviting marketplaces, wallets, and indexers to adopt and integrate this disruptive protocol in the broader ecosystem!

Roadmap

1. ✅ Development of a sound Protocol Specification (this document)
2. ✅ Reference implementation in the ordpool-parser repository
3. ✅ Open-Source Indexer for CAT-21 ordinals in the cat21-ord repository
4. ✅ Development of a client-side script that creates a PSBT to mint a CAT-21 ordinal (for Xverse, Leather & Unisat)
5. ✅ Public mint 🚀
6. Open invitation of the Ordinals ecosystem to adapt the protocol in their products! DM @HausHoppe for support!

Credits

• @1440000bytes had the initial idea to use the nLockTime field for data storage. See this this twitter conversation.
• @HausHoppe adapted the idea and created this repository to engage collaboration and discussions on the protocol. He also developed the parser, the indexer and the official website.
• @ethspresso designed the laser eyes and crown traits and gave very valuable feedback.
• @LightRider5 created the initial PSBT script and gave very valuable feedback.