feat: add ERC20 as gas fee (#15)

* add ERC20 as fee

* add feeToken to ERC20 as fee tutorial

* add more description

* address comments

* Update README.md

---------

Co-authored-by: Qi Zhou <[email protected]>

README.md

@@ -7,6 +7,7 @@ This repository provides a step-by-step walk through for builders interested in
 - [Delegate an account to a p256 key](./chapter1/delegate-p256/): Describes how EIP-7702+EIP-7212 provide the ability to sign a message with a P256 key
 - [BLS Multisig](./chapter1/bls-multisig/): In-depth walk-through how to implement a Multisig based on BLS signatures verified through precompiles from EIP-2537
 - [EOF](./chapter1/eof/): Instructions on how to deploy and inspect contracts in the new EOF format
+- [ERC20 Fee](./chapter1/erc20-fee/): Describes how EIP-7702 provides the ability to pay ERC20 as gas fee to the gas sponsor.
 
 ### Build & Test
 

chapter1/contracts/src/ERC20Fee.sol

@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.23;
+
+interface IERC20 {
+    function transfer(address _to, uint256 _value) external returns (bool success);
+}
+
+contract TestERC20 {
+    mapping (address => uint256) public balance;
+
+    function transfer(address _to, uint256 _value) public returns (bool success) {
+        balance[msg.sender] -= _value;
+        balance[_to] += _value;
+        return true;
+    }
+
+    function mint(address _to, uint256 _value) public {
+        balance[_to] += _value;
+    }
+}
+
+/// @notice Contract designed for being delegated to by EOAs to authorize an ERC20 transfer with ERC20 as fee.
+contract ERC20Fee {
+
+    /// @notice Internal nonce used for replay protection, must be tracked and included into prehashed message.
+    uint256 public nonce;
+
+    /// @notice Main entrypoint to send tx.
+    function execute(address to, bytes memory data, uint256 value, IERC20 feeToken, uint256 fee, uint8 v, bytes32 r, bytes32 s) public {
+        bytes32 digest = keccak256(abi.encode(nonce++, to, data, value, feeToken, fee));
+        address addr = ecrecover(digest, v, r, s);
+
+        require(addr == address(this));
+
+        (bool success,) = to.call{value: value}(data);
+        require(success, "call failed");
+        require(feeToken.transfer(msg.sender, fee));
+    }
+}

chapter1/erc20-fee/README.md

@@ -0,0 +1,77 @@
+# Simple 7702 demo to pay gas fee using ERC20
+
+This example demonstrates how EIP-7702 allows Alice to authorize a smart contract to execute an ERC20 transfer and pay fee in ERC20 to Bob, who sponsors the gas fees for a seamless experience.
+
+## Steps involved
+
+- Start local anvil node with Odyssey features enabled
+
+```bash
+anvil --odyssey
+```
+
+- Anvil comes with pre-funded developer accounts which we can use for the example going forward
+
+```bash
+# using anvil dev accounts 
+export ALICE_ADDRESS="0x70997970C51812dc3A010C7d01b50e0d17dc79C8"
+export ALICE_PK="0x59c6995e998f97a5a0044966f0945389dc9e86dae88c7a8412f4603b6b78690d"
+export BOB_PK="0x5de4111afa1a4b94908f83103eb1f1706367c2e68ca870fc3fb9a804cdab365a"
+export BOB_ADDRESS="0x3C44CdDdB6a900fa2b585dd299e03d12FA4293BC"
+export CHARLES_ADDRESS="0x90F79bf6EB2c4f870365E785982E1f101E93b906"
+```
+
+- Deploy ERC20 and mint tokens
+```bash
+forge create TestERC20 --private-key $BOB_PK
+export ERC20=<ERC20 addr>
+cast send $ERC20 'mint(address,uint256)' $ALICE_ADDRESS 10000000000000000000 --private-key $BOB_PK # 10E9 tokens
+cast call $ERC20 'balance(address)' $ALICE_ADDRESS
+```
+
+- We need to deploy a contract which verifies the user signature and execute ERC20 transfers.:
+
+```bash
+forge create ERC20Fee --private-key $BOB_PK
+
+export ERC20_FEE="<enter-contract-address>"
+```
+
+- Alice (delegator) can sign a message which will delegate all calls to her address to the bytecode of smart contract we've just deployed.
+
+First, let's verify that we don't have a smart contract yet associated to Alice's account, if that's the case the command below should return a `0x` response: 
+
+```bash
+$ cast code $ALICE_ADDRESS
+0x
+```
+
+
+- Alice can sign an EIP-7702 authorization using `cast wallet sign-auth` as follows:
+
+```bash
+SIGNED_AUTH=$(cast wallet sign-auth $ERC20_FEE --private-key $ALICE_PK)
+```
+
+- Alice can sign an off-chain data to authorize anyone to send ERC20 on behave of Alice in exchange of ERC20 fee
+
+```bash
+ERC20_TRANSFER_CALLDATA=$(cast calldata 'transfer(address,uint256)' $CHARLES_ADDRESS 1000000000000000000)
+SIGNED=$(cast wallet sign --no-hash $(cast keccak256 $(cast abi-encode 'f(uint256,address,bytes,uint256,address,uint256)' 0 $ERC20 $ERC20_TRANSFER_CALLDATA 0 $ERC20 1000)) --private-key $ALICE_PK)
+V=$(echo $SIGNED | cut -b 1-2,131-132)
+R=$(echo $SIGNED | cut -b 1-66)
+S=$(echo $SIGNED | cut -b 1-2,67-130)
+```
+
+- Bob (delegate) relays the transaction on Alice's behalf using his own private key and thereby paying gas fee from his account and get the ERC20 fee:
+
+```bash
+cast send $ALICE_ADDRESS "execute(address,bytes,uint256,address,uint256,uint8,bytes32,bytes32)" $ERC20 $ERC20_TRANSFER_CALLDATA 0 $ERC20 1000 $V $R $S  --private-key $BOB_PK --auth $SIGNED_AUTH
+```
+
+- Bob will receive the ERC20 token as the fee, and Charles will receive the ERC20 token
+```bash
+cast call $ERC20 "balance(address)" $BOB_ADDRESS
+cast call $ERC20 "balance(address)" $CHARLES_ADDRESS
+```
+