Predicate Based Asset IDs

[SilentCicero]

Abstract

Right now, our asset ID construction allows for contracts to mint assets with specific sub IDs. This is very useful for enabling rich multiple native assets. One sticking point with the UTXO model is that enabling blacklisting is nearly impossible for native assets. Many popular stable coins such as Tether and USDC require this feature to ensure that can take appropriate steps to blacklist bad actors from their system. While Fuel should encourage permissionless token design philosophically, not having reasonable facility for blacklisting on minted assets would be a net negative for the protocol. In this proposal, I suggest we consider changing the sub ID we currently have to a predicate ID root, in which, any actor can override existing predicates with this particular logic. This would enable system wide black listing, while retaining parallelism and minimally altering our existing asset system.

Current Asset ID

sha256(CONTRACT_ID ++ SUB_IDENTIFIER)

Proposed Asset ID

sha256(CONTRACT_ID ++ PREDICATE_IDENTIFIER)

Proposed Predicate Validation Logic

The asset predicate takes priority over the "owner" predicate and allow overriding. So the predicate logic would have to be able to both check if the asset based predicate is presented first, then if not, would check the owner predicate. This only introduces an additional check in the logic itself.

Minimal Changes

All existing behavior of multiple native assets can be retained, as token developers can still mint assets with a random predicate identifier or one that is clearly definable by the developer (e.g. "no_predicate").

Use-Cases

• Global blacklisting of assets (no-matter who or where they are held)
• Meta-transactional services, whereby additional actions can be triggered by third parties (but still retain the original owner)

UX Hurdles

Users would have to be made aware of these additional predicate conditions on their assets, but this can be done easily off-chain.

[dmihal]

Very supportive of this idea, however we need to preserve the ability to have SubIDs as well (not simply replacing them with predicate IDs)

How about including the Predicate ID within an additional hash?

sha256(CONTRACT_ID ++ sha256(PREDICATE_IDENTIFIER ++ SUB_IDENTIFIER))

[dmihal]

As for how this could be executed in practice, I believe the easiest implementation would be to add a new input-type, which would overwrite the normal ownership.

InputPredicateCoin

name	type	description
txID	byte[32]	Hash of transaction.
outputIndex	uint8	Index of transaction output.
owner	byte[32]	Owning address or predicate root.
amount	uint64	Amount of coins.
asset_id	byte[32]	Asset ID of the coins.
sub_id	byte[32]	The sub-ID of the asset, used for generating the asset ID.
txPointer	TXPointer	Points to the TX whose output is being spent.
maturity	uint32	UTXO being spent must have been created at least this many blocks ago.
predicateGasUsed	uint64	Gas used by predicate.
predicateLength	uint16	Length of predicate, in instructions.
predicateDataLength	uint16	Length of predicate input data, in bytes.
predicate	byte[]	Predicate bytecode.
predicateData	byte[]	Predicate input data (parameters).