Add note about deploying your own paymaster, and a signature generation example

Author: mdehoog

README.md

@@ -89,3 +89,9 @@ If an error is returned or the result is empty, the paymaster is not available f
 8. Sign the user operation, and submit to your bundler of choice.
 
 Note that the `paymasterAndData` returned in step 6 contains a signature of the provided userOp, so any modification of the userOp post step 6 (except for the `sig` field) will result in the paymaster rejecting the operation.
+
+## Deploying and using your own paymaster
+
+If you want to deploy your own paymaster, you can deploy [paymaster.sol](./src/paymaster.sol) to your chain of choice.
+In order to generate signatures for your paymaster, there's a Golang package in the [signer](./signer) directory that allows you to sign userOperations that will be accepted by the paymaster (call `UserOperation.PaymasterSign`).
+Of course you can also rewrite this in your language of choice; the Golang package will provide a good example.

go.mod

@@ -0,0 +1,23 @@
+module github.com/base-org/paymaster
+
+go 1.21.1
+
+require github.com/ethereum/go-ethereum v1.13.5
+
+require (
+	github.com/bits-and-blooms/bitset v1.7.0 // indirect
+	github.com/btcsuite/btcd/btcec/v2 v2.2.0 // indirect
+	github.com/consensys/bavard v0.1.13 // indirect
+	github.com/consensys/gnark-crypto v0.12.1 // indirect
+	github.com/crate-crypto/go-kzg-4844 v0.7.0 // indirect
+	github.com/decred/dcrd/dcrec/secp256k1/v4 v4.0.1 // indirect
+	github.com/ethereum/c-kzg-4844 v0.4.0 // indirect
+	github.com/go-stack/stack v1.8.1 // indirect
+	github.com/holiman/uint256 v1.2.3 // indirect
+	github.com/mmcloughlin/addchain v0.4.0 // indirect
+	github.com/supranational/blst v0.3.11 // indirect
+	golang.org/x/crypto v0.14.0 // indirect
+	golang.org/x/sync v0.3.0 // indirect
+	golang.org/x/sys v0.13.0 // indirect
+	rsc.io/tmplfunc v0.0.3 // indirect
+)

go.sum

@@ -0,0 +1,122 @@
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signer/signer.go

@@ -0,0 +1,24 @@
+package signer
+
+import (
+	"crypto/ecdsa"
+
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/crypto"
+)
+
+type Signer interface {
+	SignHash(h common.Hash) ([]byte, error)
+}
+
+type PrivateKeySigner struct {
+	*ecdsa.PrivateKey
+}
+
+func (s *PrivateKeySigner) SignHash(h common.Hash) ([]byte, error) {
+	signature, err := crypto.Sign(h[:], s.PrivateKey)
+	if err == nil {
+		signature[64] += 27
+	}
+	return signature, err
+}

signer/user_operation.go

@@ -0,0 +1,75 @@
+package signer
+
+import (
+	"math/big"
+
+	"github.com/ethereum/go-ethereum/accounts"
+	"github.com/ethereum/go-ethereum/accounts/abi"
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/common/hexutil"
+	"github.com/ethereum/go-ethereum/crypto"
+)
+
+type UserOperation struct {
+	Sender               common.Address `json:"sender"               mapstructure:"sender"               validate:"required"`
+	Nonce                *hexutil.Big   `json:"nonce"                mapstructure:"nonce"                validate:"required"`
+	InitCode             hexutil.Bytes  `json:"initCode"             mapstructure:"initCode"             validate:"required"`
+	CallData             hexutil.Bytes  `json:"callData"             mapstructure:"callData"             validate:"required"`
+	CallGasLimit         *hexutil.Big   `json:"callGasLimit"         mapstructure:"callGasLimit"         validate:"required"`
+	VerificationGasLimit *hexutil.Big   `json:"verificationGasLimit" mapstructure:"verificationGasLimit" validate:"required"`
+	PreVerificationGas   *hexutil.Big   `json:"preVerificationGas"   mapstructure:"preVerificationGas"   validate:"required"`
+	MaxFeePerGas         *hexutil.Big   `json:"maxFeePerGas"         mapstructure:"maxFeePerGas"         validate:"required"`
+	MaxPriorityFeePerGas *hexutil.Big   `json:"maxPriorityFeePerGas" mapstructure:"maxPriorityFeePerGas" validate:"required"`
+	PaymasterAndData     hexutil.Bytes  `json:"paymasterAndData"     mapstructure:"paymasterAndData"     validate:"required"`
+	Signature            hexutil.Bytes  `json:"signature"            mapstructure:"signature"            validate:"required"`
+}
+
+func (op *UserOperation) PaymasterSign(paymaster common.Address, chainID, validUntil, validAfter *big.Int, signer Signer) ([]byte, error) {
+	hash, err := op.PaymasterHash(paymaster, chainID, validUntil, validAfter)
+	if err != nil {
+		return nil, err
+	}
+	h := common.BytesToHash(accounts.TextHash(hash[:]))
+	return signer.SignHash(h)
+}
+
+func (op *UserOperation) PaymasterHash(paymaster common.Address, chainID, validUntil, validAfter *big.Int) (common.Hash, error) {
+	address, _ := abi.NewType("address", "", nil)
+	uint256, _ := abi.NewType("uint256", "", nil)
+	uint48, _ := abi.NewType("uint48", "", nil)
+	bytes32, _ := abi.NewType("bytes32", "", nil)
+	args := abi.Arguments{
+		{Name: "sender", Type: address},
+		{Name: "nonce", Type: uint256},
+		{Name: "hashInitCode", Type: bytes32},
+		{Name: "hashCallData", Type: bytes32},
+		{Name: "callGasLimit", Type: uint256},
+		{Name: "verificationGasLimit", Type: uint256},
+		{Name: "preVerificationGas", Type: uint256},
+		{Name: "maxFeePerGas", Type: uint256},
+		{Name: "maxPriorityFeePerGas", Type: uint256},
+		{Name: "chainId", Type: uint256},
+		{Name: "paymaster", Type: address},
+		{Name: "validUntil", Type: uint48},
+		{Name: "validAfter", Type: uint48},
+	}
+	packed, err := args.Pack(
+		op.Sender,
+		op.Nonce.ToInt(),
+		crypto.Keccak256Hash(op.InitCode),
+		crypto.Keccak256Hash(op.CallData),
+		op.CallGasLimit.ToInt(),
+		op.VerificationGasLimit.ToInt(),
+		op.PreVerificationGas.ToInt(),
+		op.MaxFeePerGas.ToInt(),
+		op.MaxPriorityFeePerGas.ToInt(),
+		chainID,
+		paymaster,
+		validUntil,
+		validAfter,
+	)
+	if err != nil {
+		return common.Hash{}, err
+	}
+	return crypto.Keccak256Hash(packed), nil
+}