Building the App Circuit
Our app's goal is to allow anyone to prove to an on-chain smart contract that an address made a USDC transfer with volume more than 500 USDC. We are going to implement this app step by step. You can find the finished version in this Github repo.
Writing the Circuit
Edit circuit.go and write our circuit
package circuits
import (
"github.com/brevis-network/brevis-sdk/sdk"
)
type AppCircuit struct{}
var USDCTokenAddr = sdk.ConstUint248("0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48")
var minimumVolume = sdk.ConstUint248(500000000) // minimum 500 USDC
var _ sdk.AppCircuit = &AppCircuit{}
func (c *AppCircuit) Allocate() (maxReceipts, maxStorage, maxTransactions int) {
// Our app is only ever going to use one storage data at a time so
// we can simply limit the max number of data for storage to 1 and
// 0 for all others
return 32, 0, 0
}
func (c *AppCircuit) Define(api *sdk.CircuitAPI, in sdk.DataInput) error {
receipts := sdk.NewDataStream(api, in.Receipts)
receipt := sdk.GetUnderlying(receipts, 0)
// Check logic
// The first field exports `from` parameter from Transfer Event
// It should use the second topic in Transfer Event log
api.Uint248.AssertIsEqual(receipt.Fields[0].Contract, USDCTokenAddr)
api.Uint248.AssertIsEqual(receipt.Fields[0].IsTopic, sdk.ConstUint248(1))
api.Uint248.AssertIsEqual(receipt.Fields[0].Index, sdk.ConstUint248(1))
// Make sure two fields uses the same log to make sure account address linking with correct volume
api.Uint32.AssertIsEqual(receipt.Fields[0].LogPos, receipt.Fields[1].LogPos)
// The second field exports `Volume` parameter from Transfer Event
// It should use Data in Transfer Event log
api.Uint248.AssertIsEqual(receipt.Fields[1].IsTopic, sdk.ConstUint248(0))
api.Uint248.AssertIsEqual(receipt.Fields[1].Index, sdk.ConstUint248(0))
// Make sure this transfer has minimum 500 USDC volume
api.Uint248.AssertIsLessOrEqual(minimumVolume, api.ToUint248(receipt.Fields[1].Value))
api.OutputUint(64, api.ToUint248(receipt.BlockNum))
api.OutputAddress(api.ToUint248(receipt.Fields[0].Value))
api.OutputBytes32(receipt.Fields[1].Value)
return nil
}Testing the Circuit
Edit circuit_test.go for circuit testing. First, we assign correct values to the circuit input. Then, we use test.ProverSucceeded to test if our circuit can successfully generate a proof using the correct input.
// ...
func TestCircuit(t *testing.T) {
rpc := "RPC_URL"
localDir := "$HOME/circuitOut/myBrevisApp"
app, err := sdk.NewBrevisApp(1, rpc, localDir)
check(err)
txHash := common.HexToHash(
"0x8a7fc50330533cd0adbf71e1cfb51b1b6bbe2170b4ce65c02678cf08c8b17737")
app.AddReceipt(sdk.ReceiptData{
TxHash: txHash,
Fields: []sdk.LogFieldData{
{
IsTopic: true,
LogPos: 0,
FieldIndex: 1,
},
{
IsTopic: false,
LogPos: 0,
FieldIndex: 0,
},
},
})
appCircuit := &AppCircuit{}
appCircuitAssignment := &AppCircuit{}
circuitInput, err := app.BuildCircuitInput(appCircuit)
check(err)
///////////////////////////////////////////////////////////////////////////////
// Testing
///////////////////////////////////////////////////////////////////////////////
test.ProverSucceeded(t, appCircuit, appCircuitAssignment, circuitInput)
} Spin Up a Prover for Your AppCircuit
proverService, err := prover.NewService(&AppCircuit{}, config)
// ...
err = proverService.Serve(33247)
// ...Then, we run the main program to start up the prover
go run main.gocommand line tool curl is required for downloading srs ignition file.
At the same time, you may download it through browser directly and put it into your srs file folder. Brevis sdk will use $HOME/kzgsrs/kzg_srs_100800000_bn254_MAIN_IGNITI as file path if srsDir configuration is $HOME/kzgsrs.
The VK Hash
Notice the highlighted log that look like this
///////////////////////////////////////////////////////////////////////////////
// vk hash: 0x25d2751bfc09b1222b834f3043762dad7e1591671f8c03456996cabe53a95c71
///////////////////////////////////////////////////////////////////////////////This is the hash of your circuit's verifying key. You must store this hash in your contract and check it when handling contract callbacks. More on this in later steps.
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