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docs(rfc): non-evm chain memo design standard (#2963)
* upload non-evm chain memo design doc * add changelog entry and proposal date * change field name abortMessage to abortAddress * reorgs * reorgs 2 * better format doc and table; change terminology 'lite-data' to 'compact' * extend header to 4 bytes for extension purpose; update doc and diagram; add example code * correct the flag positions of CallOnRevert and AbortAddress * make receiver optional * rename --------- Co-authored-by: lumtis <[email protected]>
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# A proposed standard memo format for non-EVM chains | ||
<br> | ||
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## The goal | ||
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This doc is aimed to introduce a standard format of ZetaChain inbound `memo` across non-EVM chains. The `memo` in an inbound transaction is to carry both instructions and data, by which the cross-chain transaction to be processed as expected in the `zetacore`. A standard memo format helps to ensure consistent developer experience across EVM and non-EVM chains. | ||
<br><br> | ||
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## The EVM gateway inbound events | ||
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We want to mimic below `Deposit` and `Called` events in the EVM `gateway` contract for non-EVM chains. Typically, we at least want the following 3 pieces at the time of writing. | ||
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- The `receiver` address in ZEVM. | ||
- The `payload` of the call. | ||
- The `RevertOptions`. | ||
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``` | ||
event Deposited( | ||
address indexed sender, | ||
address indexed receiver, | ||
uint256 amount, | ||
address asset, | ||
bytes payload, | ||
RevertOptions revertOptions | ||
); | ||
event Called( | ||
address indexed sender, | ||
address indexed receiver, | ||
bytes payload, | ||
RevertOptions revertOptions | ||
); | ||
``` | ||
<br><br> | ||
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## 1. The Memo Diagram | ||
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![](images/001_layout.png)<br><br> | ||
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## 2. The memo structure | ||
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The raw-byte representation of a standard non-EVM memo contains `9` sections (2 mandatory + 7 optional). | ||
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`Section 0` is the `header`, which controls how the memo should be interpreted. | ||
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`Section 1~8` are data sections that carry actual data attached to the memo. | ||
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| | Section 0 | Section 1 | Section 2 | Section 3 | Section 4 | Section 5 | Section 6 | Section 7 | Section 8 | | ||
|-----------------|------------|-------------|-------------|----------------|--------------|--------------|---------------|-------------|-------------| | ||
| **Name** | Header | Receiver | Payload | RevertAddress | CallOnRevert | AbortAddress | RevertMessage | reserved | reserved | | ||
| **Size (byte)** | 4 | 20 | variable | variable | 1 | 20 | variable | -- | -- | | ||
| **Type** | [4]byte | ZEVM address| bytes | non-EVM address| bool | ZEVM address | bytes | -- | -- | | ||
| **Optional** | No | Yes | Yes | Yes | Yes | Yes | Yes | -- | -- | | ||
<br><br> | ||
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## 3. The header | ||
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The `4-byte` header is a self-contained declaration of the memo properties and rules. The proposed header structure is designed in a flexible manner. The data fields are all declared as optional. With `reserved` flags, new features and fields can be added in the future without breaking compatibility. | ||
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### `byte-0`: Memo Identifier | ||
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The ASCII code `0x5A` of letter `'Z'` is used as the identifier of the standard ZetaChain memo format. Once legacy memo is phased out, zetaclient will only accept memos beginning with `0x5A`. | ||
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### `byte-1`: Control Byte | ||
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| | bit 4 ~ 7 | bit 0 ~ 3 | | ||
|-----------------|------------------------------------------------|--------------------------------------| | ||
| **Name** | version # | encoding format | | ||
| **Optional** | No (0b0000 for now) | No (0b0000 or 0b0001 or 0b0010) | | ||
<br> | ||
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### `byte-2`: Control Byte | ||
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| | bit 4 ~ 7 | bit 0 ~ 3 | | ||
|-----------------|------------------------------------------------|--------------------------------| | ||
| **Name** | operation code | control bytes reserved | | ||
| **Optional** | No (0b0000 or 0001 or 0010 for now) | -- | | ||
<br> | ||
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### `byte-3`: Data Flags | ||
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| | bit 7 | bit 6 | bit 5 | bit 4 | bit 3 | bit 2 | bit 1 | bit 0 | | ||
|-----------------|---------------|--------------|---------------|-------------------|------------------|-------------------|-----------------|------------------| | ||
| **Name** | flag reserved | flag reserved| flag reserved | flag CallOnRevert | flag AbortAddress| flag RevertAddress| flag Payload | flag Receiver | | ||
| **Optional** | -- | -- | -- | -- | Yes | Yes | Yes | Yes | | ||
<br> | ||
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Supported `operation codes` : | ||
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The `operation code` is designed for the developers to explicitly specify the intention of an inbound transaction in non-contract chains (e.g. Bitcoin, Doge, …). | ||
The operation code is not necessary for non-contract chains, because the method name tells the intention of the transaction. | ||
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A 4-bit operation code would allow `16` operation codes at maximum. | ||
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| 0b0000 | 0b0001 | 0b0010 | | ||
|-------------|-------------------|-------------| | ||
| deposit | deposit_and_call | call | | ||
<br> | ||
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Supported `encoding flags`: | ||
This flag is to tell `zetaclient` how the memo fields should be decoded. A 2-bit weight flag seems good enough and won’t change in future.<br> | ||
Note: the `compact *` encoding is to minimizes length of memo data (compared to ABI) and will be explained in following sections. | ||
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| 0b00 00 | 0b000 | 0b0010 | | ||
|--------------|----------------|--------------| | ||
| ABI encoded | compact short | compact long | | ||
<br><br> | ||
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## 4. The types | ||
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To decode/encode the memo fields with `compact *` format, we've defined a few types below. | ||
To decode/encode the memo fields with `ABI encoded` format, just follow the [ABI Spec](https://docs.soliditylang.org/en/develop/abi-spec.html). | ||
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| Field | Size (byte) | Description | | ||
|-----------------------------|--------------|---------------------------------------------------------------------------------------------------------------------------------| | ||
| **ZEVM address** | fixed, 20 | The raw bytes representation of a ZEVM address. ZEVM address is the same as standard Ethereum address. | | ||
| **non-EVM address** | variable | The raw bytes representation of a non-EVM external chain address. Different addresses have different lengths, e.g., Bitcoin vs Solana; Bitcoin Segwit vs Legacy. | | ||
| **bytes** | variable | The raw bytes of arbitrary data. | | ||
<br><br> | ||
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## 5. Byte layout of variable-length types | ||
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There are two ways to encode your the memo fields into raw bytes. The `ABI encoded` and `compact *` encoded. | ||
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| Decoding flag | Format | Layout | Description | Rule | | ||
|---------------|---------------|-----------------------------------------|-----------------------------------------------|----------------------------------------------------------| | ||
| 0b0000 | ABI encoded | ABI encoded types | Types are packed/unpacked with ABI standard | [ABI Spec](https://docs.soliditylang.org/en/develop/abi-spec.html) | | ||
| 0b0001 | compact short | [1-byte length] + [data] | Carry up to 255 bytes of data. | A valid length (>= 0) must match data length. | | ||
| 0b0010 | compact long | [2-byte length, little-endian] + [data] | Carry up to 65535 bytes (64KB) of data. | A valid length (>= 0) must match data length. | | ||
| | ||
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## 6. How to pack your inbound memo | ||
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### Step-1: prepare your memo header | ||
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```go | ||
func MakeHead() []byte { | ||
header := make([]byte, 4) | ||
header[0] = 'Z' | ||
header[1] = 0<<4 | 0b0010 // version + encoding format (e.g. compact long) | ||
header[2] = 0b0001 << 4 // operation code (e.g. DepositAndCall) | ||
header[3] = 0b00000111 // receiver, payload and revertAddress are set | ||
return header | ||
} | ||
``` | ||
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### Step-2: pack your memo data | ||
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For use cases that requires compact transaction data, choose encoding format of `compact short` or `compact long`. | ||
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```go | ||
func CompactPack(receiver common.Address, payload []byte, revertAddress string) []byte { | ||
data := make([]byte, 0) | ||
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// pack receiver (fixed 20 bytes) | ||
data = append(data, receiver.Bytes()...) | ||
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// pack payload (dynamic) | ||
// encode length with 1 byte if 'compact short' is your choice | ||
lenthPayload := make([]byte, 2) | ||
binary.LittleEndian.PutUint16(lenthPayload, uint16(len(payload))) | ||
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data = append(data, lenthPayload...) // write length | ||
data = append(data, payload...) // write payload | ||
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// pack revert address (dynamic). | ||
// encode length with 1 byte if 'compact short' is your choice | ||
lengthRevertAddr := make([]byte, 2) | ||
binary.LittleEndian.PutUint16(lengthRevertAddr, uint16(len([]byte(revertAddress)))) | ||
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data = append(data, lengthRevertAddr...) // write length | ||
data = append(data, []byte(revertAddress)...) // write revert address | ||
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return data | ||
} | ||
``` | ||
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Also, ABI encoding format is another option to consider. | ||
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```go | ||
func ABIPack(receiver common.Address, payload []byte, revertAddress string) ([]byte, error) { | ||
// define the ABI for encoding the types: address, bytes, string | ||
abiString := `[{"type":"function","name":"encode","inputs":[ | ||
{"type":"address"}, | ||
{"type":"bytes"}, | ||
{"type":"string"}], | ||
"outputs":[]}]` | ||
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// parse the ABI | ||
parsedABI, err := abi.JSON(strings.NewReader(abiString)) | ||
if err != nil { | ||
return nil, err | ||
} | ||
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// pack the values using the ABI | ||
data, err := parsedABI.Pack("encode", receiver, payload, revertAddress) | ||
if err != nil { | ||
return nil, err | ||
} | ||
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// remove the 4-byte selector | ||
return data[4:], nil | ||
} | ||
``` |
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