预编译合约

Posted 2021-08-29 mutourend

1. 引言

ethereum virtual machine (EVM) 提供了丰富的图灵完备环境，用于运行智能合约。可以运行所有程序并不代表可高效运行。不论是公链、联盟链、私链，都需要 可将EVM中的特定操作扩展为可高效运行。

EVM的扩展方式有2种：

• 增加opcodes：类似于传统CPU的方式，向EVM增加额外的指令来支持新的操作——如addition cryptographic primitives。
  与向传统CPU添加指令类似，相应的不足为：为了充分利用新操作的优势，需修改编译器以使用这些指令（否则必须使用汇编）。
  此外，指令编码具有有限的指令空间（对于EVM，其指令空间为1个字节，对应只有256个指令）。同时，对于公链和私链，可能会对相同的opcode赋予不同的操作，使得相同的bytecode在不同的网络上表现完全不同。
• 预编译合约：采用Ethereum client的native language来编写合约（而不是EVM bytecode）的方式来添加操作。这些操作以合约的方式实现，不需要对编译器进行更新。预编译合约也有与opcodes类似的地址空间限制，但是预编译合约的地址空间要更大，可支持更多的操作。
  这种方式仍然有捕获合约地址的开销，如：unpacking parameters、calling the native code以及repacking the results into the EVM。同时，由于预编译合约是共识过程的一部分，要求预编译合约必须为deterministic的。
  借助预编译合约，可解决：
  allow complex cryptographic computations to be used in the EVM without having to deal with EVM overhead。

2. 以太坊的预编译合约

根据 https://github.com/ethereum/go-ethereum/blob/master/core/vm/contracts.go，以太坊中支持的预编译合约主要有：

// PrecompiledContractsBerlin contains the default set of pre-compiled Ethereum
// contracts used in the Berlin release.
var PrecompiledContractsBerlin = map[common.Address]PrecompiledContract{
	common.BytesToAddress([]byte{1}): &ecrecover{},
	common.BytesToAddress([]byte{2}): &sha256hash{},
	common.BytesToAddress([]byte{3}): &ripemd160hash{},
	common.BytesToAddress([]byte{4}): &dataCopy{},
	common.BytesToAddress([]byte{5}): &bigModExp{eip2565: true},
	common.BytesToAddress([]byte{6}): &bn256AddIstanbul{},
	common.BytesToAddress([]byte{7}): &bn256ScalarMulIstanbul{},
	common.BytesToAddress([]byte{8}): &bn256PairingIstanbul{},
	common.BytesToAddress([]byte{9}): &blake2F{},
}

// PrecompiledContractsBLS contains the set of pre-compiled Ethereum
// contracts specified in EIP-2537. These are exported for testing purposes.
var PrecompiledContractsBLS = map[common.Address]PrecompiledContract{
	common.BytesToAddress([]byte{10}): &bls12381G1Add{},
	common.BytesToAddress([]byte{11}): &bls12381G1Mul{},
	common.BytesToAddress([]byte{12}): &bls12381G1MultiExp{},
	common.BytesToAddress([]byte{13}): &bls12381G2Add{},
	common.BytesToAddress([]byte{14}): &bls12381G2Mul{},
	common.BytesToAddress([]byte{15}): &bls12381G2MultiExp{},
	common.BytesToAddress([]byte{16}): &bls12381Pairing{},
	common.BytesToAddress([]byte{17}): &bls12381MapG1{},
	common.BytesToAddress([]byte{18}): &bls12381MapG2{},
}

根据How to add custom precompiled contract?可知，以太坊的预编译合约中仅需实现2个接口：

• RequiredGas(): to define how many gas user need to pay when calling the pre-compiled contract。
• Run(): write your code to do the thing that you want the pre-compiled contract will do。

3. Tron的预编译合约

而对于Tron，根据https://github.com/tronprotocol/java-tron/blob/develop/actuator/src/main/java/org/tron/core/vm/PrecompiledContracts.java，其支持的预编译有：

  private static final DataWord ecRecoverAddr = new DataWord(
      "0000000000000000000000000000000000000000000000000000000000000001");
  private static final DataWord sha256Addr = new DataWord(
      "0000000000000000000000000000000000000000000000000000000000000002");
  private static final DataWord ripempd160Addr = new DataWord(
      "0000000000000000000000000000000000000000000000000000000000000003");
  private static final DataWord identityAddr = new DataWord(
      "0000000000000000000000000000000000000000000000000000000000000004");
  private static final DataWord modExpAddr = new DataWord(
      "0000000000000000000000000000000000000000000000000000000000000005");
  private static final DataWord altBN128AddAddr = new DataWord(
      "0000000000000000000000000000000000000000000000000000000000000006");
  private static final DataWord altBN128MulAddr = new DataWord(
      "0000000000000000000000000000000000000000000000000000000000000007");
  private static final DataWord altBN128PairingAddr = new DataWord(
      "0000000000000000000000000000000000000000000000000000000000000008");
  private static final DataWord batchValidateSignAddr = new DataWord(
      "0000000000000000000000000000000000000000000000000000000000000009");
  private static final DataWord validateMultiSignAddr = new DataWord(
      "000000000000000000000000000000000000000000000000000000000000000a");
  private static final DataWord verifyMintProofAddr = new DataWord(
      "0000000000000000000000000000000000000000000000000000000001000001");
  private static final DataWord verifyTransferProofAddr = new DataWord(
      "0000000000000000000000000000000000000000000000000000000001000002");
  private static final DataWord verifyBurnProofAddr = new DataWord(
      "0000000000000000000000000000000000000000000000000000000001000003");
  private static final DataWord merkleHashAddr = new DataWord(
      "0000000000000000000000000000000000000000000000000000000001000004");

Tron的预编译合约中仅需实现2个接口：

• getEnergyForData(): 定义调用该预编译合约所需的能量值。
• execute(): write your code to do the thing that you want the pre-compiled contract will do。

合约与地址绑定关系在Solidity中设定：

case FunctionType::Kind::ECRecover:
		case FunctionType::Kind::SHA256:
		case FunctionType::Kind::RIPEMD160:
		case FunctionType::Kind::ValidateMultiSign:
        case FunctionType::Kind::BatchValidateSign:
        case FunctionType::Kind::verifyBurnProof:
        case FunctionType::Kind::verifyTransferProof:
        case FunctionType::Kind::verifyMintProof:
        case FunctionType::Kind::pedersenHash:
		{
			_functionCall.expression().accept(*this);
			static map<FunctionType::Kind, u256> const contractAddresses{
				{FunctionType::Kind::ECRecover, 1},
				{FunctionType::Kind::SHA256, 2},
				{FunctionType::Kind::RIPEMD160, 3},
				{FunctionType::Kind::BatchValidateSign, 9},
                {FunctionType::Kind::ValidateMultiSign, 10},
                {FunctionType::Kind::verifyMintProof, 16777217}, //1000001
                {FunctionType::Kind::verifyTransferProof, 16777218}, //1000002
                {FunctionType::Kind::verifyBurnProof, 16777219}, //1000003
                {FunctionType::Kind::pedersenHash, 16777220} //1000004
			};
			m_context << contractAddresses.at(function.kind());
			for (unsigned i = function.sizeOnStack(); i > 0; --i)
				m_context << swapInstruction(i);
			appendExternalFunctionCall(function, arguments);
			break;
		}

参考资料

[1] What’s a precompiled contract and how are they different from native opcodes?
[2] A Prehistory of the Ethereum Protocol
[3] 2018年4月博客 Extending the EVM
[4] 2020年9月博客 Edgeware EVM Precompiles: A deeper dive