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

prover/circuits/circuit.go

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.

Read more on testing here

prover/circuits/circuit_test.go

// ...

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

prover/cmd/main.go

proverService, err := prover.NewService(&AppCircuit{}, config)
// ...
err = proverService.Serve(33247)
// ...

Then, we run the main program to start up the prover

go run main.go

Console Output

>> compiling circuit
10:34:52 INF compiling circuit
ignoring uninitialized slice: Input_StorageSlots_Toggles []frontend.Variable
ignoring uninitialized slice: Input_Transactions_Toggles []frontend.Variable
10:34:52 INF parsed circuit inputs nbPublic=6 nbSecret=1089
ignoring uninitialized slice: Input_StorageSlots_Toggles []frontend.Variable
ignoring uninitialized slice: Input_Transactions_Toggles []frontend.Variable
ignoring uninitialized slice: Input_StorageSlots_Toggles []frontend.Variable
ignoring uninitialized slice: Input_Transactions_Toggles []frontend.Variable
commit output: rounds 1, data len 480, padded len 1088
10:34:52 INF building constraint builder nbConstraints=608462
circuit compiled in 408.001292ms, number constraints 608462
circuit digest 0x1dfdcf1616c15230b5bcd7555570e324647743ca9111e968a6519e33136b036e
trying to read setup from cache...
no setup matching circuit digest 0x0e299aa204fde71d5da9b94e7b905857ba1ce912ec37ad00f54f1bb2fdea0705 is found in /Users/xxx/circuitOut
>> setup
size system 608468
size lagrange 1048576
init SRS disk cache dir /Users/xxx/kzgsrs
fetching srs ignition from file
srs ignition not found in file
downloading file https://kzg-srs.s3.us-west-2.amazonaws.com/kzg_srs_100800000_bn254_MAIN_IGNITION
writing srs ignition file
srs iginition ready
setup done in 2.392068s
///////////////////////////////////////////////////////////////////////////////
// vk hash: 0x25d2751bfc09b1222b834f3043762dad7e1591671f8c03456996cabe53a95c71
///////////////////////////////////////////////////////////////////////////////

67143336 bytes written to /Users/xxx/circuitOut/0x1dfdcf1616c15230b5bcd7555570e324647743ca9111e968a6519e33136b036e/pk
34368 bytes written to /Users/xxx/circuitOut/0x1dfdcf1616c15230b5bcd7555570e324647743ca9111e968a6519e33136b036e/vk
>> scan local storage: /Users/xxx/circuitOut/input/input/data.json
>> finish scan local storage: /Users/xxx/circuitOut/input/input/data.json
>> serving prover REST API at port 33257
>> serving prover GRPC at port 33247

command 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.