Dynamic Proxy

[AugustoL]

Instead of using a single proxy contract this PR propose changes for creating and destroying a proxy contract for each extra call executed by the ERC827 token. This has another advantage, allowing the receiver contract to verify that the proxy was created by the original msg.sender of the tokens.

The computation of the proxy address receives as input the original msg.sender, the address of the contract to be called by the proxy and a nonce.

bytes32 salt = keccak256(abi.encodePacked(msg.sender, _to, nonces[msg.sender]));
address proxy = Create2.deploy(salt, proxyBytecode);

ERC827 contract keep tracks of nonces mapped by senders that increment each time a call is executed successfully.

Since the Create2 opcode also takes in count the address of the contract that deploys the contract it allow the token receiver to verify that the call is coming from a proxy that was created by the msg.sender.
For this the receiver contract would need the token and sender address

  function receiveVerifiedTokens(address sender, ERC827 token) public {
    address proxy = Create2.computeAddress(
      address(token),
      keccak256(abi.encodePacked(sender, address(this), token.nonces(sender))),
      token.proxyBytecode()
    );
    require(msg.sender == proxy, "ERC827Receiver: Sender invalid");
  }

Note in the ERC827Receiver test fiel I execute tests proving how the sender verification works.