BRC-201: Cross-Chain Extension Protocol to BRC-20 Tokens

Introduction

BRC-201 proposes an extension protocol to the BRC-20 standard. It is aimed to be backward compatible with BRC-20, to improve scalability, especially to bridge BRC-20 tokens to chains with smart contract support, such as Ethereum, BNB Chain, Near, etc.

Ordinals and BRC-20

Ordinal theory imbues satoshis with numismatic value, allowing them to be collected and traded as curios. Individual satoshis can be inscribed with arbitrary content, creating unique Bitcoin-native digital artifacts that can be held in Bitcoin wallets and transferred using Bitcoin transactions.

BRC-20 is experimental token standard by Domo. Tokens can be deployed, minted, and transferred using Ordinal inscriptions.

Definition

BRC-201 is an extension of the BRC-20 token standard that enhances the Transfer operation. This extension remains compatible with the BRC-20 protocol.

BRC-20 token transfer looks like this:

{ 
  "p": "brc-20",
  "op": "transfer",
  "tick": "ordi",
  "amt": "100"
}

BRC-201 extend operation looks like this:

{ 
  "p": "brc-20",
  "op": "transfer",
  "tick": "ordi",
  "amt": "100",
  "chain": "eth",
  "ext": "bridge-out/in",
  "ref": "address/txhash"
}

BRC-201 introduces three additional fields: "chain", "ext", and "ref".

If an indexer does not support the BRC-201 protocol, the transaction will be treated as a regular transfer operation.

However, an enhanced indexer that supports BRC-201 can parse additional extension operations.

Currently, the BRC-201 specification defines "Bridge In" and "Bridge Out" as extension operations.

Bridge Out

example:

{ 
  "p": "brc-20",
  "op": "transfer",
  "tick": "ordi",
  "amt": "100",
  "ext": "bridge-out",
  "chain": "eth",
  "ref": "0x7a9ce9ea715c32cc2b04821e8aab6a4a364d7f97"
}

Key	Required?	Description
p	Yes	Protocol: Helps other systems identify and process brc-20 events
op	Yes	Operation: Type of event (Deploy, Mint, Transfer)
tick	Yes	Ticker: 4 letter identifier of the brc-20
amt	Yes	Amount to transfer: States the amount of the brc-20 to bridge-out.
ext	Yes	Extend operation: Type of extend event(Bridge-in, Bridge-out)
chain	Yes	Chain: Identifier specified the destination chain.
ref	Yes	Reference: reference content based on extend operation. Here is the receiver address on the destination chain.

1. Inscribe “Bridge-out” extend operation on Bitcoin, specifying chain and address as ref.

2. Submit the transaction proof ("SPV proof") to smart contract on the destination chain and call the BTC light client to verify it.

3. If the verification is successful, the contract mint no more than amt wrapped BRC-20 token to the account specified in the ref field. Meanwhile, the contract emits a "Mint" event.

4. Indexers MUST verify that the "Mint" event emitted by the contract matches theinscription id, and amt, chain, reffields of the bridge-out operation.

5. Indexers MUST mark the bridged-out tokens as "bridged-out" and track the bridged-out token amount.

Bridge In

example:

{ 
  "p": "brc-20",
  "op": "transfer",
  "tick": "ordi",
  "amt": "100",
  "ext": "bridge-in",
  "chain": "eth",
  "ref": "0x857ab26790d5926de3aa3972230dd8b926b1e6d57c6b7a077d649ae3bea393bc"
}

Key	Required?	Description
p	Yes	Protocol: Helps other systems identify and process brc-20 events
op	Yes	Operation: Type of event (Deploy, Mint, Transfer)
tick	Yes	Ticker: 4 letter identifier of the brc-20
amt	Yes	Amount to transfer: States the amount of the brc-20 to bridge-in.
ext	Yes	Extend operation: Type of extend event(Bridge-in, Bridge-out)
chain	Yes	Chain: Identifier specified the source chain.
ref	Yes	Reference: reference content based on extend operation. Here is the txhash on the source chain.

1. Submit the transaction to smart contract on the chain to bridge out the wrapped BRC-20 token, specifying the receiver address on BTC. The contract emits a "Burn" event.

2. Inscribe "Bridge-in" extend operation on Bitcoin, specifying chain and the transaction hash on source chain as ref.

3. Indexers MUST verify that the "Burn" event emitted by the contract match the amt and receiver of the bridge-in operation.

4. Indexers MUST reduce the tracked bridged-out token amount.