Intermediate

Precompiled Contracts

Welcome to our tutorial on precompiled contracts on the Klaytn blockchain network. In Klaytn, precompiled contracts are a special type of smart contract that is already compiled and deployed on the blockchain, providing a set of predefined functionality that can be called by other contracts. These precompiled contracts are optimized for specific operations, such as elliptic curve operations, and can be used to improve the performance and efficiency of smart contracts. In this tutorial, we will explore the basics of precompiled contracts, including the different types of precompiled contracts available on the Klaytn network, the process of calling and interacting with precompiled contracts, and best practices for using them in your smart contract development. Whether you are a developer, researcher, or simply a blockchain enthusiast, this tutorial will provide valuable insights and hands-on experience in using precompiled contracts on the Klaytn network.

Address 0x01: ecrecover(hash, v, r, s)

The address 0x01 implements ecrecover. It returns the address from the given signature by calculating a recovery function of ECDSA. Its function prototype is as follows:

1function ecrecover(bytes32 hash, bytes8 v, bytes32 r, bytes32 s) returns (address);

Address 0x02: sha256(data)

The address 0x02 implements SHA256 hash. It returns a SHA256 hash from the given data. Its function prototype is as follows:

1function sha256(bytes data) returns (bytes32);

Address 0x03: ripemd160(data)

The address 0x03 implements RIPEMD160 hash. It returns a RIPEMD160 hash from the given data. Its function prototype is as follows:

1function ripemd160(bytes data) returns (bytes32);

Address 0x04: datacopy(data)

The address 0x04 implements datacopy (i.e., identity function). It returns the input data directly without any modification. This precompiled contract is not supported by the Solidity compiler. The following code with inline assembly can be used to call this precompiled contract.

function callDatacopy(bytes memory data) public returns (bytes memory) {
    bytes memory ret = new bytes(data.length);
    assembly {
        let len := mload(data)
        if iszero(call(gas, 0x04, 0, add(data, 0x20), len, add(ret,0x20), len)) {
            invalid()
        }
    }

    return ret;
}     

Address 0x05: bigModExp(base, exp, mod)

The address 0x05 implements the formula base**exp % mod. It returns the result from the given data. This precompiled contract is not supported by the Solidity compiler. The following code can be used to call this precompiled contract. Note that although this precompiled contract supports an arbitrary length of inputs, the below code uses a fixed length of inputs as an example.

function callBigModExp(bytes32 base, bytes32 exponent, bytes32 modulus) public returns (bytes32 result) {
    assembly {
        // free memory pointer
        let memPtr := mload(0x40)

        // length of base, exponent, modulus
        mstore(memPtr, 0x20)
        mstore(add(memPtr, 0x20), 0x20)
        mstore(add(memPtr, 0x40), 0x20)

        // assign base, exponent, modulus
        mstore(add(memPtr, 0x60), base)
        mstore(add(memPtr, 0x80), exponent)
        mstore(add(memPtr, 0xa0), modulus)

        // call the precompiled contract BigModExp (0x05)
        let success := call(gas, 0x05, 0x0, memPtr, 0xc0, memPtr, 0x20)
        switch success
        case 0 {
            revert(0x0, 0x0)
        } default {
            result := mload(memPtr)
        }
    }
}

Address 0x06: bn256Add(ax, ay, bx, by)

The address 0x06 implements a native elliptic curve point addition. It returns an elliptic curve point representing (ax, ay) + (bx, by) such that (ax, ay) and (bx, by) are valid points on the curve bn256. This precompiled contract is not supported by the Solidity compiler. The following code can be used to call this precompiled contract.

function callBn256Add(bytes32 ax, bytes32 ay, bytes32 bx, bytes32 by) public returns (bytes32[2] memory result) {
    bytes32[4] memory input;
    input[0] = ax;
    input[1] = ay;
    input[2] = bx;
    input[3] = by;
    assembly {
        let success := call(gas, 0x06, 0, input, 0x80, result, 0x40)
        switch success
        case 0 {
            revert(0,0)
        }
    }
}

Address 0x07: bn256ScalarMul(x, y, scalar)

The address 0x07 implements a native elliptic curve multiplication with a scalar value. It returns an elliptic curve point representing scalar * (x, y) such that (x, y) is a valid curve point on the curve bn256. This precompiled contract is not supported by the Solidity compiler. The following code can be used to call this precompiled contract.

function callBn256ScalarMul(bytes32 x, bytes32 y, bytes32 scalar) public returns (bytes32[2] memory result) {
    bytes32[3] memory input;
    input[0] = x;
    input[1] = y;
    input[2] = scalar;
    assembly {
        let success := call(gas, 0x07, 0, input, 0x60, result, 0x40)
        switch success
        case 0 {
            revert(0,0)
        }
    }
}

Address 0x08: bn256Pairing(a1, b1, a2, b2, a3, b3, …, ak, bk)

The address 0x08 implements an elliptic curve paring operation to perform zkSNARK verification. For more information, see EIP-197. This precompiled contract is not supported by the Solidity compiler. The following code can be used to call this precompiled contract.

function callBn256Pairing(bytes memory input) public returns (bytes32 result) {
    // input is a serialized bytes stream of (a1, b1, a2, b2, ..., ak, bk) from (G_1 x G_2)^k
    uint256 len = input.length;
    require(len % 192 == 0);
    assembly {
        let memPtr := mload(0x40)
        let success := call(gas, 0x08, 0, add(input, 0x20), len, memPtr, 0x20)
        switch success
        case 0 {
            revert(0,0)
        } default {
            result := mload(memPtr)
        }
    }
}

Address 0x09: blake2F(rounds, h, m, t, f)

The address 0x09 implements BLAKE2b F compression function. For more information, see EIP-152. This precompiled contract is not supported by the Solidity compiler. The following code can be used to call this precompiled contract.

function callBlake2F(uint32 rounds, bytes32[2] memory h, bytes32[4] memory m, bytes8[2] memory t, bool f) public view returns (bytes32[2] memory) {
    bytes32[2] memory output;

    bytes memory args = abi.encodePacked(rounds, h[0], h[1], m[0], m[1], m[2], m[3], t[0], t[1], f);

    assembly {
        if iszero(staticcall(not(0), 0x09, add(args, 32), 0xd5, output, 0x40)) {
            revert(0, 0)
        }
    }

    return output;
}

Address 0x3fd: vmLog(str)

The address 0x3FD prints the specified string str to a specific file or passes it to the logger module. For more information, see debug_setVMLogTarget. Note that this precompiled contract should be used only for debugging purposes, and it is required to enable the --vmlog option when the Klaytn node starts. Also, the log level of the Klaytn node should be 4 or more to see the output of vmLog. This precompiled contract is not supported by the Solidity compiler. The following code can be used to call this precompiled contract.

function callVmLog(bytes memory str) public {
address(0x3fd).call(str);
}

Address 0x3fe: feePayer()

The address 0x3FE returns a fee payer of the executing transaction. This precompiled contract is not supported by the Solidity compiler. The following code can be used to call this precompiled contract.

function feePayer() internal returns (address addr) {
    assembly {
        let freemem := mload(0x40)
        let start_addr := add(freemem, 12)
        if iszero(call(gas, 0x3fe, 0, 0, 0, start_addr, 20)) {
          invalid()
        }
        addr := mload(freemem)
    }
}

Address 0x3ff: validateSender()

The address 0x3FF validates the sender’s signature with the message. Since Klaytn decouples key pairs from addresses, it is required to validate that a signature is properly signed by the corresponding sender. To do that, this precompiled contract receives three parameters:

• The sender’s address to get the public keys

• The message hash that is used to generate the signature

• The signatures that are signed by the sender’s private keys with the given message hash

The precompiled contract validates that the given signature is properly signed by the sender’s private keys. Note that Klaytn natively supports multi signatures, which means there can be multiple signatures. The signature must be 65 bytes long.

function ValidateSender(address sender, bytes32 msgHash, bytes sigs) public returns (bool) {
    require(sigs.length % 65 == 0);
    bytes memory data = new bytes(20+32+sigs.length);
    uint idx = 0;
    uint i;
    for( i = 0; i < 20; i++) {
        data[idx++] = (bytes20)(sender)[i];
    }
    for( i = 0; i < 32; i++ ) {
        data[idx++] = msgHash[i];
    }
    for( i = 0; i < sigs.length; i++) {
        data[idx++] = sigs[i];
    }
    assembly {
        // skip length header.
        let ptr := add(data, 0x20)
        if iszero(call(gas, 0x3ff, 0, ptr, idx, 31, 1)) {
          invalid()
        }
        return(0, 32)
    }
}

With this, you complete this workshop successfully!!