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The Signer Interface

In Brane, all signing operations are abstracted through the Signer interface. This provides a unified way to sign both transactions and raw messages, whether the key is held locally (e.g., a private key) or remotely (e.g., KMS, HSM, or MPC). 
public interface Signer {
    /**
     * Signs a transaction.
     */
    Signature signTransaction(UnsignedTransaction tx, long chainId);

    /**
     * Signs a raw message (EIP-191).
     */
    Signature signMessage(byte[] message);

    /**
     * Returns the address associated with this signer.
     */
    Address address();
}

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PrivateKeySigner

The most common implementation is PrivateKeySigner, which uses a local private key. 
import io.brane.core.crypto.PrivateKeySigner;

// Initialize with a hex private key
var signer = new PrivateKeySigner("0x...");

// Get the address
System.out.println("Address: " + signer.address());

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Signing Transactions

When using WalletClient, the signer is handled automatically. However, you can also use it directly: 
var tx = UnsignedTransaction.builder()
    .to(recipient)
    .value(amount)
    .build();

Signature sig = signer.signTransaction(tx, 1L); // Chain ID 1 (Mainnet)

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Signing Messages

You can sign arbitrary messages, which is useful for authentication (e.g., “Sign in with Ethereum”) or off-chain approvals. 
String message = "Hello Brane!";
Signature sig = signer.signMessage(message.getBytes(StandardCharsets.UTF_8));

System.out.println("Signature: " + sig.toHex());