CoTA Protocol Overview
The CoTA (aka. Compact Token Aggregator) protocol is a layer-1.5 account based fungible / non-fungible token and k-v data managment solution on Nervos CKB. The basic idea of CoTA is to manage and verify enormous number of data by a Sparse Merkle Tree (SMT) with a constant size of on-chain storage space, like 32bytes. The draft version of the CoTA protocol was published on Nervos Talk at 2021. Since then, the CoTA protocol has evolved rapidly and extended to more user scenarios, including unique CCID, 3rd-party data storage, JoyID, etc.
Basic data structure
The CoTA Cell simply consists of two necessary parts, the SMT root in the cell data field, and the CoTA typescript in the cell type field. They stand for the personal data and operation verification logic separately.
# CoTA cell data structure
data:
version: byte
smt_root: byte32
type:
code_hash: cota_type
args: lockscript_hash[0..20] # must match self.lockscript
lock:
--
Two-step operation design pattern
One of the major pain points in the application scenarios of sudt protocol is the token transfer operation requires an update to both the sender's and receiver's cell simultaneously. The receiver has to use an anyone-can-pay style lock to their cells, which introduces security risks, composability issues and inconvenient user experiences.
To solve this problem, we introduce a two-step operation design pattern. The principle is simple, the sender first minus token and leaves a proof in their cell, and then the receiver restores the token in their cell according to the mathematical proof. With this method, everyone operates their cells only. And the receiver can immediately confirm the token transfer as soon as the sender completes the first step.
Decentralized global registry
There should be only one CoTA cell for one address to prevent double-claim issues. Otherwise, multiple CoTA cells with the same address could repeatedly claim tokens according to the same proof. So a global registry must exist to ensure the CoTA cell's uniqueness.
FT, NFT, and account model on UTXO chain
CoTA protocol builds in standard token define, mint, and transfer operations, covering fungible and non-fungible tokens. The CoTA cell stores the token issuance, distribution, and holding records under one address. So it's an account model layer over the UTXO model chain, and this feature makes the programming and achieving composability much easier on CKB.
K-V structure for CoTA SMT
Key type
The data.smt_root is the essential data field in the design. Data stored in the SMT is managed by key-value pair, and several kinds of data are stored and compressed in this hash root. The CoTA typescript (smart contract) processes the transaction logic according to its value. We use a two-bytes variable smt_root.key.smt_type to identify different key types.
| type name | 1st byte | 2nd byte | description |
|---|---|---|---|
| cota-NFT-define | 0x81 (NFT) | 0x00 | NFT issuance definition |
| cota-NFT-hold | 0x81 (NFT) | 0x01 | current hold NFT |
| cota-NFT-withdrawal | 0x81 (NFT) | 0x02 | NFT withdrawal record |
| cota-NFT-claim | 0x81 (NFT) | 0x03 | NFT claim record |
| cota-FT-define | 0x82 (FT) | 0x00 | FT issuance definition |
| cota-FT-hold | 0x82 (FT) | 0x01 | current hold FT |
| cota-FT-withdrawal | 0x82 (FT) | 0x02 | FT withdrawal record |
| cota-FT-claim | 0x82 (FT) | 0x03 | FT claim record |
| extension-data | 0xF0 | 0x00 | user-defined data |
| reserved | other | -- | -- |