L2 Signature

How To Sign Message

Overview

In Layer 2 trading systems, to ensure transaction security and prevent unauthorized operations, critical operations (such as limit orders, transfers, withdrawals, etc.) require users to provide digital signatures to prove the legitimacy of the operations. This signature is passed through the l2Signature parameter.

How To GET Your L2 Private Key

To generate l2Signature, you first need to obtain your L2 private key. This key is used to generate signatures that authorize various actions on the platform.

⚠️ Security Warning:
Keep your private key secure and never share it with anyone
Anyone with access to your private key can sign messages and execute operations on your behalf

Signature Algorithm

Two algorithms are used in the signature process:

ECDSA (Elliptic Curve Digital Signature Algorithm) is used to generate and verify signatures.
Pedersen Hash which consumes more CPU resources compared to regular Ethereum signatures, is used for message hashing.

L2Signature generation follows these standard steps:

Collect Parameters: Gather all required parameters for the operation
Calculate Hash: Use Pedersen Hash to compute the message hash
Generate Signature: Use ECDSA algorithm and L2 private key to sign the hash
Format Output: Format the signature according to API requirements

Python L2Signature Demo

Golang L2Signature Demo

Java Script L2Signature Demo

Java L2Signature Example

Below is a Java implementation of the Ecdsa signature algorithm. This example demonstrates how to sign a message using a private key.


    public static CreateOrderRequest signOrder(
            CreateOrderRequest request,
            Contract contract,
            Coin quotelCoin,
            PrivateKey privateKey) {
        BigInteger msgHash = L2SignUtil.hashLimitOrder(
                request.getSide() == OrderSide.BUY,
                BigIntUtil.toBigInt(quotelCoin.getStarkExAssetId()),
                BigIntUtil.toBigInt(contract.getStarkExSyntheticAssetId()),
                BigIntUtil.toBigInt(quotelCoin.getStarkExAssetId()),
                UnsignedLong.valueOf(new BigDecimal(request.getL2Value())
                        .multiply(new BigDecimal(BigIntUtil.toBigInt(quotelCoin.getStarkExResolution())))
                        .toBigIntegerExact()),
                UnsignedLong.valueOf(new BigDecimal(request.getL2Size())
                        .multiply(new BigDecimal(BigIntUtil.toBigInt(contract.getStarkExResolution())))
                        .toBigIntegerExact()),
                UnsignedLong.valueOf(new BigDecimal(request.getL2LimitFee())
                        .multiply(new BigDecimal(BigIntUtil.toBigInt(quotelCoin.getStarkExResolution())))
                        .toBigIntegerExact()),
                UnsignedLong.fromLongBits(request.getAccountId()),
                UnsignedInteger.valueOf(request.getL2Nonce()),
                UnsignedInteger.valueOf(request.getL2ExpireTime() / (60 * 60 * 1000L)));
        Signature signature = Ecdsa.sign(msgHash, privateKey);
        return request.toBuilder()
                .setL2Signature(L2Signature.newBuilder()
                        .setR(BigIntUtil.toHexStr(signature.r))
                        .setS(BigIntUtil.toHexStr(signature.s))
                        .build())
                .build();
    }

    public static BigInteger hashLimitOrder(
            boolean isBuyingSynthetic,
            BigInteger assetIdCollateral,
            BigInteger assetIdSynthetic,
            BigInteger assetIdFee,
            UnsignedLong amountCollateral,
            UnsignedLong amountSynthetic,
            UnsignedLong maxAmountFee,
            UnsignedLong positionId,
            UnsignedInteger nonce,
            UnsignedInteger expirationTimestamp) {
        BigInteger assetIdSell;
        BigInteger assetIdBuy;
        UnsignedLong amountSell;
        UnsignedLong amountBuy;
        if (isBuyingSynthetic) {
            assetIdSell = assetIdCollateral;
            assetIdBuy = assetIdSynthetic;
            amountSell = amountCollateral;
            amountBuy = amountSynthetic;
        } else {
            assetIdSell = assetIdSynthetic;
            assetIdBuy = assetIdCollateral;
            amountSell = amountSynthetic;
            amountBuy = amountCollateral;
        }
        BigInteger packedMessage0 = amountSell.bigIntegerValue();
        packedMessage0 = packedMessage0.shiftLeft(64).add(amountBuy.bigIntegerValue());
        packedMessage0 = packedMessage0.shiftLeft(64).add(maxAmountFee.bigIntegerValue());
        packedMessage0 = packedMessage0.shiftLeft(32).add(nonce.bigIntegerValue());

        BigInteger packedMessage1 = BigInteger.valueOf(3);
        packedMessage1 = packedMessage1.shiftLeft(64).add(positionId.bigIntegerValue());
        packedMessage1 = packedMessage1.shiftLeft(64).add(positionId.bigIntegerValue());
        packedMessage1 = packedMessage1.shiftLeft(64).add(positionId.bigIntegerValue());
        packedMessage1 = packedMessage1.shiftLeft(32).add(expirationTimestamp.bigIntegerValue());
        packedMessage1 = packedMessage1.shiftLeft(17);

        BigInteger msg = pedersenHash(assetIdSell, assetIdBuy);
        msg = pedersenHash(msg, assetIdFee);
        msg = pedersenHash(msg, packedMessage0);
        msg = pedersenHash(msg, packedMessage1);
        return msg;
    }

    public static BigInteger pedersenHash(BigInteger... input) {
        BigInteger[][] points = PEDERSEN_POINTS;
        Point shiftPoint = new Point(points[0][0], points[0][1]);
        for (int i = 0; i < input.length; i++) {
            BigInteger x = input[i];
            for (int j = 0; j < 252; j++) {
                int pos = 2 + i * 252 + j;
                Point pt = new Point(points[pos][0], points[pos][1]);
                if (x.and(BigInteger.ONE).intValue() != 0) {
                    shiftPoint = EcMath.add(shiftPoint, pt, Curve.secp256k1.A, Curve.secp256k1.P);
                }
                x = x.shiftRight(1);
            }
        }
        return shiftPoint.x;
    }

    public static Signature sign(BigInteger msgHash, PrivateKey privateKey) {
        Curve curve = privateKey.curve;
        BigInteger randNum = new BigInteger(curve.N.toByteArray().length * 8 - 1, new SecureRandom()).abs().add(BigInteger.ONE);
        Point randomSignPoint = EcMath.multiply(curve.G, randNum, curve.N, curve.A, curve.P);
        BigInteger r = randomSignPoint.x.mod(curve.N);
        BigInteger s = ((msgHash.add(r.multiply(privateKey.secret))).multiply(EcMath.inv(randNum, curve.N))).mod(curve.N);
        return Signature.create(r, s);
    }

Signature Construction Guide

This section provides detailed instructions for constructing signatures for various actions on the platform. Each operation has specific message formats and parameter requirements.

Withdrawal Signature

Used to authorize withdrawing assets from Layer 2 to an Ethereum address.

Parameters

assetIdCollateral - Asset ID for the collateral token from meta_data.coinList.starkExAssetId
positionId - User's account ID in Layer 2
ethAddress - Destination Ethereum address for withdrawal
nonce - Unique transaction identifier to prevent replay attacks
expirationTimestamp - Unix timestamp when signature expires
amount - Amount to withdraw in base units

Calculation

The following TypeScript function constructs the withdrawal message for signing:

// Construct withdrawal message for signing
function getWithdrawalToAddressMsg({
  assetIdCollateral,
  positionId,
  ethAddress, 
  nonce,
  expirationTimestamp,
  amount
}) {
  // Pack parameters into 256-bit words
  const w1 = assetIdCollateral;
  let w5 = BigInt(withdrawalToAddress); // Constant identifier
  w5 = (w5 << 64) + BigInt(positionId);
  w5 = (w5 << 32) + BigInt(nonce); 
  w5 = (w5 << 64) + BigInt(amount);
  w5 = (w5 << 32) + BigInt(expirationTimestamp);
  w5 = w5 << 49;

  // Calculate Pedersen hash
  return pedersen([
    pedersen([w1, ethAddress]),
    w5.toString(16)
  ]);
}

Limit Order Signature

Used to authorize a limit order for perpetual trading.

Parameters

assetIdSynthetic - Synthetic asset ID from meta_data.contractList.starkExSyntheticAssetId
assetIdCollateral - Collateral asset ID from meta_data.coinList.starkExAssetId
isBuyingSynthetic - true for buy orders, false for sell orders
assetIdFee - Fee token asset ID from meta_data.coinList.starkExAssetId
amountSynthetic - Amount of synthetic asset
amountCollateral - Amount of collateral asset
maxAmountFee - Maximum fee amount allowed
nonce - Unique order identifier
positionId - User's position ID
expirationTimestamp - Unix timestamp when order expires

Calculation

The following TypeScript function constructs the limit order message for signing:

function getLimitOrderMsg({
  assetIdSynthetic,
  assetIdCollateral,
  isBuyingSynthetic,
  assetIdFee,
  amountSynthetic,
  amountCollateral,
  maxAmountFee,
  nonce,
  positionId,
  expirationTimestamp
}) {
  // Determine sell/buy assets based on order side
  const [assetIdSell, assetIdBuy] = isBuyingSynthetic 
    ? [assetIdCollateral, assetIdSynthetic]
    : [assetIdSynthetic, assetIdCollateral];
  const [amountSell, amountBuy] = isBuyingSynthetic
    ? [amountCollateral, amountSynthetic] 
    : [amountSynthetic, amountCollateral];

  // Pack order data into 256-bit words
  const w1 = assetIdSell;
  const w2 = assetIdBuy;
  const w3 = assetIdFee;

  // Calculate message hash
  let msg = pedersen([w1, w2]);
  msg = pedersen([msg, w3]);

  let w4 = BigInt(amountSell);
  w4 = (w4 << 64) + BigInt(amountBuy);
  w4 = (w4 << 64) + BigInt(maxAmountFee);
  w4 = (w4 << 32) + BigInt(nonce);
  msg = pedersen([msg, w4.toString(16)]);

  let w5 = BigInt(limitOrderWithFees); // Constant identifier
  w5 = (w5 << 64) + BigInt(positionId);
  w5 = (w5 << 64) + BigInt(positionId);
  w5 = (w5 << 64) + BigInt(positionId);
  w5 = (w5 << 32) + BigInt(expirationTimestamp);
  w5 = w5 << 17;

  return pedersen([msg, w5.toString(16)]);
}

Transfer Signature

Used to authorize transfers between Layer 2 accounts.

Parameters

assetId - Asset ID being transferred
receiverPublicKey - Recipient's public key
senderPositionId - Sender's position ID
receiverPositionId - Recipient's position ID
srcFeePositionId - Fee source position ID
nonce - Unique transfer identifier
amount - Transfer amount
expirationTimestamp - Unix timestamp when transfer expires
assetIdFee - Fee token asset ID (optional, default '0')
maxAmountFee - Maximum fee amount (optional, default '0')

Calculation

The following TypeScript function constructs the transfer message for signing:

function getTransferMsg({
  assetId,
  receiverPublicKey,
  senderPositionId,
  receiverPositionId,
  srcFeePositionId,
  nonce,
  amount,
  expirationTimestamp,
  assetIdFee = '0',
  maxAmountFee = '0'
}) {
  // Pack transfer data into 256-bit words
  const w1 = assetId;
  const w2 = assetIdFee;
  const w3 = receiverPublicKey;

  let w4 = BigInt(senderPositionId);
  w4 = (w4 << 64) + BigInt(receiverPositionId);
  w4 = (w4 << 64) + BigInt(srcFeePositionId); 
  w4 = (w4 << 32) + BigInt(nonce);

  let w5 = BigInt(transfer); // Constant identifier
  w5 = (w5 << 64) + BigInt(amount);
  w5 = (w5 << 64) + BigInt(maxAmountFee);
  w5 = (w5 << 32) + BigInt(expirationTimestamp);
  w5 = w5 << 81;

  // Calculate message hash
  let msg = pedersen([w1, w2]);
  msg = pedersen([msg, w3]);
  msg = pedersen([msg, w4.toString(16)]);
  return pedersen([msg, w5.toString(16)]);
}

For more details on the signature construction, see the StarkEx documentation.

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