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CashTokens

CashTokens are native tokens on Bitcoin Cash, meaning that they are validated by all full nodes on the network and their transaction rules checked by each miner when constructing new blocks. CashTokens added fungible and non-fungible token primitives. CashTokens was first proposed in February of 2022 and activated on Bitcoin Cash mainnet in May of 2023.

tip

You can read more about CashTokens on cashtokens.org which has the full specification as well as a list of usage examples.

CashTokens UTXO data​

To understand CashTokens it is helpful to start with the layout of the UTXO data. In the networkProvider data from the SDK, the token property contains the new CashTokens fields:

interface Utxo {
  txid: string;
  vout: number;
  satoshis: bigint;
  token?: TokenDetails;
}

interface TokenDetails {
  amount: bigint;
  category: string;
  nft?: {
    capability: 'none' | 'mutable' | 'minting';
    commitment: string;
  };
}

Fungible Tokens​

The amount field is the amount of fungible tokens on the UTXO, the category is the "tokenId" for the token on the UTXO. The maximum size for a fungible token amount is the max signed 64-bit integer or 9223372036854775807.

Non-Fungible Tokens​

The nft info on a UTXO will only be present if the UTXO contains an NFT. The nft object has 2 properties: the capability and the commitment. The commitment is the data field for the NFT which can is allowed to be up to 40 bytes.

Capability none then refers to an immutable NFT where the commitment cannot be changed. The mutable capability means the commitment field can change over time, usually to contain smart contract state. Lastly the minting capability means that the NFT can create new NFTs from the same category.

note

A UTXO can hold both an amount of fungible tokens as well as an nft, as long as both tokens have the same category. This is quite a common pattern for covenants which want to hold contract state and fungible tokens on the same UTXO.

CashTokens introspection data​

While CashTokens might seem overwhelming at first, realize that in contracts you will only use it through the following introspection details

  • bytes tx.inputs[i].tokenCategory - tokenCategory + tokenCapability of a specific input.
  • bytes tx.inputs[i].nftCommitment - NFT commitment data of a specific input.
  • int tx.inputs[i].tokenAmount - Amount of fungible tokens of a specific input.

and their equivalent for outputs:

  • bytes tx.outputs[i].tokenCategory - tokenCategory + tokenCapability of a specific output. (see below).
  • bytes tx.outputs[i].nftCommitment - NFT commitment data of a specific output
  • int tx.outputs[i].tokenAmount - Amount of fungible tokens of a specific output.

CashTokens Gotchas​

There are a few important "gotchas" to be aware of when developing with CashTokens in smart contracts for the first time.

1) tokenCategory contains the nft-capability​

bytes tx.inputs[i].tokenCategory

When accessing the tokenCategory through introspection the result returns 0x (empty byte string) when that specific item does not contain tokens. If the item does have tokens it returns the bytes32 tokenCategory. When the item contains an NFT with a capability, the 32-byte tokenCategory is concatenated together with 0x01 for a mutable NFT and 0x02 for a minting NFT.

If you want to check for an NFT using introspection, you have either split the tokenCategory from the capability or check the concatenation of the tokenCategory and capability.

// Constructor parameters: providedCategory

// Extract the separate tokenCategory and capability
bytes32 tokenCategory, bytes capability = tx.inputs[0].tokenCategory.split(32);

// Check that the NFT is the correct category and has a "minting" capability
require(providedCategory == tokenCategory);
require(capability == 0x02);

// Alternatively:

// Check by concatenating the providedCategory and capability
require(tx.inputs[0].tokenCategory == providedCategory + 0x02);

2) tokenCategory encoding​

The tokenCategory introspection variable returns the tokenCategory in the original unreversed order, this is unlike wallets and explorers which use the reversed byte-order. So be careful about the byte-order of tokenCategory when working with BCH smart contracts.

// when using a standard encoded tokenId, reverse the hex before using it in your contract
const contract = new Contract(artifact, [reverseHex(tokenId)], { provider })

It is not recommended to do the byte-reversal in script, because this adds extra unnecessary overhead to the script.

  // NOT THIS
  require(tx.inputs[0].tokenCategory == providedTokenId.reverse());

3) "invisible" empty nfts​

Because the nft-capability has no separate introspection item, and nothing is appended to the tokenCategory in case of capability none, empty nfts can be "invisible" when combined with fungible tokens.

First let's consider the case where a UTXO only holds an empty NFT:

  // Input 0 has an empty nft (commitment 0x) with providedTokenId
  // If there was no empty nft the tokenCategory would return 0x
  require(tx.inputs[0].nftCommitment == 0x);
  require(tx.inputs[0].tokenAmount == 0);
  require(tx.inputs[0].tokenCategory == providedTokenId);

Contrast this with the scenario where a UTXO holds both an empty NFT and fungible tokens of the same category:

  // Input 0 might or might not have an empty nft (commitment 0x) with providedTokenId
  // Either way, the tokenCategory would return providedTokenId
  require(tx.inputs[0].nftCommitment == 0x);
  require(tx.inputs[0].tokenAmount == 10);
  require(tx.inputs[0].tokenCategory == providedTokenId);

The NFT introspection fields (nftCommitment and tokenCategory) of these UTXOs look the same to the smart contract in both of these scenarios.

This means that a covenant UTXO holding both a minting NFT and the fungible token supply for the same token category cannot prevent that empty nfts are created by users when they are allowed to create a fungible token output. The possibility of these "junk" empty NFTs should be taken into account so they do not present any security problems for the contract system.

tip

The easiest way to prevent issues with "junk" empty NFTs is to check that only NFTs with non-empty commitments can be interacted with in the contract system.

4) Explicit vs implicit burning​

CashTokens can be burned explicitly by sending them to an OP_RETURN output, which is provably unspendable. CashTokens can also be burned implicitly, by including them in the inputs but not the outputs of a transaction. Always be mindful when adding token-carrying inputs to not forget to add the tokens in the outputs, otherwise they will be considered as an implicit burn.

note

Signing for CashTokens inputs is designed in such a way that pre-CashTokens wallets - which only know how to send and receive Bitcoin Cash - cannot spend CashTokens inputs and thus can never accidentally burn CashTokens this way.

CashTokens Genesis transactions​

A CashTokens genesis transaction is a transaction which creates a new category of CashTokens. To create a CashTokens genesis transaction you need a vout0 UTXO because the txid of the UTXO will be you newly created category.

The requirement for a vout0 UTXO can mean that you might need to create a setup transaction "pre-genesis" which will create this output. The "pre-genesis" txid then is your token's category.

tip

CashTokens Creation is illustrated very nicely by transaction diagram in the specification document in the section on token categories.

CashTokens BCMR metadata​

Although not directly related to smart contracts, BCMR metadata is important for user-facing CashTokens. This way users can see your token name, icon, description and any relevant project links directly in their wallet. Many CashTokens wallets use the Paytaca BCMR indexer to fetch BCMR metadata info about CashTokens.

The Paytaca BCMR indexer listens for on-chain authchain transactions which publish metadata with an OP_RETURN publication output. These type of metadata updates are self-published on-chain identity claims. The zero-th output chain since the token genesis is the authchain. The UTXO at the "head" of this chain holds the authority to update the token's metadata.

tip

For easy creation of CashTokens with BCMR metadata there is the Paytaca CashTokens Studio or to programmatically publish on-chain BCMR authchain updates there is the AuthUpdate JS program.