Unique Cell

A unique cell can be created on the Nervos CKB through TypeID which makes sure the unique cell cannot be updated or destroyed.

The unique cell data can store any format data, such as XUDT token information and the contract will not check the content and format of the cell data.

Unique Type Script

unique type script:
  code_hash: 
    unique_type
  args:
    type_id[0..20]  // The following will provide a detailed explanation on how to generate the type_id

How to generate unique type script args

https://github.com/nervosnetwork/rfcs/blob/master/rfcs/0022-transaction-structure/0022-transaction-structure.md#type-id

There are two ways to create a new cell with a specific type id.

1. Create a transaction which uses any out point as tx.inputs[0] and has a output cell whose type script is Type ID. The output cell's type script args is the hash of tx.inputs[0] and its output index. Because any out point can only be used once as an input, tx.inputs[0] and thus the new type id must be different in each creation transaction.
2. Destroy an old cell with a specific type id and create a new cell with the same type id in the same transaction.

Implementation can be found in generator-example/src/index.ts

const generateUniqueTypeArgs = (firstInput: CKBComponents.CellInput, firstOutputIndex: number) => {
  const input = hexToBytes(serializeInput(firstInput));
  const s = blake2b(32, null, null, PERSONAL);
  s.update(input);
  s.update(hexToBytes(`0x${u64ToLe(BigInt(firstOutputIndex))}`));
  return `0x${s.digest("hex").slice(0, 40)}`;
};

generator-example/src/lumos.ts

function generateUniqueTypeArgs(input: Input, index: number) {
  const hasher = new utils.CKBHasher();
  hasher.update(blockchain.CellInput.pack(input));
  hasher.update(Uint64.pack(index));
  return hasher.digestHex().slice(0, 42);
}

XUDT Information

The following is the info data format recommended by XUDT：

# general

# The number of decimals the XUDT token uses - e.g. 8, means to divide the token amount by 100000000 to get its user representation. 
# 0xF0-0xFF reserved
decimal: uint8  

# The length of the name in bytes
name_len: uint8

# The name of the XUDT token - e.g. USDT, means to convert 'Tether USDT' to hex form '0x5465746865722055534454'
name: variable max 255

# The length of the symbol in bytes
symbol_len: uint8

# The symbol of the XUDT token - e.g. USDT, means to convert 'USDT' to hex form '0x55534454'
symbol: variable max 255

# optional
...

Notice: The xUDT information format is NOT a key-value structure.

You can find the code about how to construct the xUDT information data in generator-example/src/lumos.ts

const coin = {
  decimal: 6,
  name: "UNIQUE COIN",
  symbol: "UNC",
};

const data = bytes.hexify(bytes.concat(
  Uint8.pack(coin.decimal),
  Uint8.pack(coin.name.length),
  new TextEncoder().encode(coin.name),
  Uint8.pack(coin.symbol.length),
  new TextEncoder().encode(coin.symbol),
));

Development

Build contracts:

make build

Run tests:

make test